I want to discharge a CRT through a 1M Ohm resistor so as not to damage the cascade rectifier that's possibly holding some of the charge.
I only have 1/2 watt resistors at 1M Ohm, however, and it's advised to use 3W or higher rated resistors.
It would make sense to me intuitively to be able to use several 1/2 watt resistors in parallel. This is because the watt rating describes current-times-voltage and running resistors in parallel would distribute the current across each parallel branch, bringing the required total power down and thus the required wattage rating.
I'd probably also need to get larger than 1M Ohm resistors to compensate for running them in parallel so that the total resistance comes out near 1M.
The problem is, I can't seem to find the set of equations that would prove my hunch. Can anyone shed some light on this for me?
Half-watt resistors in parallel?
In this case, the type of resistor is more important than the power rating.
I assume that you are talking about a voltage in the several kilovolt range, especially since you mention a voltage multiplier.
First, you want a resistor whose insulation is good enough and that is large enough that it won't arc over. Think in terms of inches, rather than fractions of an inch.
Second, you want a resistor that can handle high peak powers. Carbon film and metal film resistors are very poor in this respect. The resistive element is a long and narrow spiral cut in a thin carbon or metal film. It doesn't take much stress above the maximum rating to burn the element open and it doesn't take much energy, since the volume that has to be heated is very small. Carbon composition resistors are much better in this regard, since the resistive element occupies a large fraction of the bulk of the resistor.
I would suggest a string of 4 or 5 2 watt carbon composition resistors hooked in series and placed in a plastic tube for insulation. The resistors will still be momentarily overstressed, but they are rugged enough that they will probably not be significantly damaged. Don't expect accuracy or stability.
If the resistors are connected during normal operation (as a bleeder resistor), the power rating of the resistors will have to be increased. The power dissipated is equal to the square of the voltage divided by the resistance. For a 5000 volt supply and a 1 megohm bleeder, this comes out to 25 watts. If the resistance is increased to 5 megohms, the power is reduced to 5 watts, and this can be divided between five resistors, resulting in a dissipation of 1 watt in each resistor. If 2 watt resistors are used, there will be a large safety margin, at least as far as power is concerned. The voltage across each resistor is still 1000 volts, which is a little on the high side. More resistors would reduce the voltage stress on each resistor.
I assume that you are talking about a voltage in the several kilovolt range, especially since you mention a voltage multiplier.
First, you want a resistor whose insulation is good enough and that is large enough that it won't arc over. Think in terms of inches, rather than fractions of an inch.
Second, you want a resistor that can handle high peak powers. Carbon film and metal film resistors are very poor in this respect. The resistive element is a long and narrow spiral cut in a thin carbon or metal film. It doesn't take much stress above the maximum rating to burn the element open and it doesn't take much energy, since the volume that has to be heated is very small. Carbon composition resistors are much better in this regard, since the resistive element occupies a large fraction of the bulk of the resistor.
I would suggest a string of 4 or 5 2 watt carbon composition resistors hooked in series and placed in a plastic tube for insulation. The resistors will still be momentarily overstressed, but they are rugged enough that they will probably not be significantly damaged. Don't expect accuracy or stability.
If the resistors are connected during normal operation (as a bleeder resistor), the power rating of the resistors will have to be increased. The power dissipated is equal to the square of the voltage divided by the resistance. For a 5000 volt supply and a 1 megohm bleeder, this comes out to 25 watts. If the resistance is increased to 5 megohms, the power is reduced to 5 watts, and this can be divided between five resistors, resulting in a dissipation of 1 watt in each resistor. If 2 watt resistors are used, there will be a large safety margin, at least as far as power is concerned. The voltage across each resistor is still 1000 volts, which is a little on the high side. More resistors would reduce the voltage stress on each resistor.
-
dyarker
- Posts: 1917
- Joined: Fri Aug 22, 2003 1:01 am
- Location: Izmir, Turkiye; from Rochester, NY
- Contact:
Use seven 150K 1/2W resisters in series. That would make 1.05M, 3.5W.
Yes, for parallel, seven 7M resistors would be needed, but use series. The series setup is better for high voltage. (The shorter bodies of 1/2W resistors are more likely to allow arcing/leakage than higher wattage resistors (especially after sone dust has accumulated). Series divides the voltage across each resistor.
Cheers,
added: started typing before rshayes' post. I agree.
Yes, for parallel, seven 7M resistors would be needed, but use series. The series setup is better for high voltage. (The shorter bodies of 1/2W resistors are more likely to allow arcing/leakage than higher wattage resistors (especially after sone dust has accumulated). Series divides the voltage across each resistor.
Cheers,
added: started typing before rshayes' post. I agree.
Dale Y
Hi there,
I agree with the other posts in that a series string should definitely
be used rather than a set of parallel resistors. As mentioned in
the posts, the voltage distributes between the several resistors
so that each resistor only sees a fraction of the total voltage.
The only discrepancy here is that even though you use several
resistors in series and the voltage divides between the several
resistors, the voltage across each individual resistor may still be
higher than the voltage rating for each resistor.
Also, the max voltage could be much higher than 5000 volts, and
could be more towards 20,000 volts.
Taking this info into consideration, a good idea would be to find out
the voltage rating of the resistors you intend to purchase for this and
before calculating the resistance required calculate the number of
resistors needed to meet the max voltage spec first, and if you dont
know the actual total voltage then perhaps you should assume 20kv.
Good assumtions would be to assume 20kv, and assume each resistor can
take a max of 500 volts (actually 700v with a little safety margin).
Dividing 20000 by 500 means we need 40 resistors in series. Dividing
1M ohm by 40 means each resistor should be 25k ohms. The nearest
standard value would be 24k, so we would need 42 resistors in series.
In quantities of 100, carbon comp resistors might cost 10 cents per unit
while carbon film might cost 1.6 cents per unit, and the working voltage
and overvoltage specs are pretty much the same for both types.
Using carbon film resistors you could get 100 for $1.60 and that would
be enough resistors to make two big discharge resistors, each having a
rating of 21 watts (42 resistors in series).
To be even safer, use the max voltage spec of the resistor to calculate
the value and number required in the series string.
Using the max spec of 350v (either type of material) and max total
voltage of 20kv, 58 one-half watt resistor in series would be required.
The value would be 17.2k, but the nearest standard value seems to be
16k, so 62 resistors in series would be required. Total cost would
still be $1.60 for carbon film, $6.20 for carbon comp.
Working voltage for either type would be 21700 volts, surge rating
would be 43400 volts. That should handle just about any voltage you
come across without burning up or arcing over.
A little detail as to the construction...
The voltage applied end to end of the resulting resistor will still be
very high, maybe 20,000 volts or more. This of course means
the two ends have to be kept rather far apart. Also, since the
resistors will be wired end to end and the voltage across each
resistor could be 350v or higher, this means across any two
resistors will be 700v, and any three 1050v, etc. This means
it would not be a good idea to simply connect them all in series
and then fold over the construction several times because two
nodes that are quite different in voltage level might come too
close to each other, again causing arc over and possible damage
to the circuit and the newly constructed discharge resistor itself.
A possible way around this is to wire the resistors end to end,
but instead of connecting them in a straigh line, solder each
resistor at a right angle to the last one, and this will create a
square box like construction where every four resistors create
a square and all the squares are stacked one on top of the other,
similar to the way you would wind a coil of wire if you wound it
on a square coil form. The max voltage between any two nodes
along the outside of the box would be four times the voltage across
one resistor, which would be 1400 volts (350v across each resistor).
This should be managable with a spacing of 1/4 inch or so.
Since 62 resistors are required for the most safe construction at
350v per resistor, the whole construction could be made to under
4 inches long with the wires coming out at opposite ends.
I agree with the other posts in that a series string should definitely
be used rather than a set of parallel resistors. As mentioned in
the posts, the voltage distributes between the several resistors
so that each resistor only sees a fraction of the total voltage.
The only discrepancy here is that even though you use several
resistors in series and the voltage divides between the several
resistors, the voltage across each individual resistor may still be
higher than the voltage rating for each resistor.
Also, the max voltage could be much higher than 5000 volts, and
could be more towards 20,000 volts.
Taking this info into consideration, a good idea would be to find out
the voltage rating of the resistors you intend to purchase for this and
before calculating the resistance required calculate the number of
resistors needed to meet the max voltage spec first, and if you dont
know the actual total voltage then perhaps you should assume 20kv.
Good assumtions would be to assume 20kv, and assume each resistor can
take a max of 500 volts (actually 700v with a little safety margin).
Dividing 20000 by 500 means we need 40 resistors in series. Dividing
1M ohm by 40 means each resistor should be 25k ohms. The nearest
standard value would be 24k, so we would need 42 resistors in series.
In quantities of 100, carbon comp resistors might cost 10 cents per unit
while carbon film might cost 1.6 cents per unit, and the working voltage
and overvoltage specs are pretty much the same for both types.
Using carbon film resistors you could get 100 for $1.60 and that would
be enough resistors to make two big discharge resistors, each having a
rating of 21 watts (42 resistors in series).
To be even safer, use the max voltage spec of the resistor to calculate
the value and number required in the series string.
Using the max spec of 350v (either type of material) and max total
voltage of 20kv, 58 one-half watt resistor in series would be required.
The value would be 17.2k, but the nearest standard value seems to be
16k, so 62 resistors in series would be required. Total cost would
still be $1.60 for carbon film, $6.20 for carbon comp.
Working voltage for either type would be 21700 volts, surge rating
would be 43400 volts. That should handle just about any voltage you
come across without burning up or arcing over.
A little detail as to the construction...
The voltage applied end to end of the resulting resistor will still be
very high, maybe 20,000 volts or more. This of course means
the two ends have to be kept rather far apart. Also, since the
resistors will be wired end to end and the voltage across each
resistor could be 350v or higher, this means across any two
resistors will be 700v, and any three 1050v, etc. This means
it would not be a good idea to simply connect them all in series
and then fold over the construction several times because two
nodes that are quite different in voltage level might come too
close to each other, again causing arc over and possible damage
to the circuit and the newly constructed discharge resistor itself.
A possible way around this is to wire the resistors end to end,
but instead of connecting them in a straigh line, solder each
resistor at a right angle to the last one, and this will create a
square box like construction where every four resistors create
a square and all the squares are stacked one on top of the other,
similar to the way you would wind a coil of wire if you wound it
on a square coil form. The max voltage between any two nodes
along the outside of the box would be four times the voltage across
one resistor, which would be 1400 volts (350v across each resistor).
This should be managable with a spacing of 1/4 inch or so.
Since 62 resistors are required for the most safe construction at
350v per resistor, the whole construction could be made to under
4 inches long with the wires coming out at opposite ends.
LEDs vs Bulbs, LEDs are winning.
Great info but I think he just wants to discharge the CRT of any residual charge before working on it. In this case I would say 1M is just a ballpark recommendation so that if there is a significant charge, you will not get an arc. 500k to 10M would probably suffice.
For parallel resistors I ususlly just get lazy and google parallel resistor calculator and use one of the many thet pop up. Here is one with the formula prominant.
http://www.1728.com/resistrs.htm
For discharge purposes and if all I had were 1M resistors I would just use 4 of them. Two in parallel will give you 500k. Do that twice and put those pairs in series and you're back up to 1M and you have 4X the original wattage. Wrap it in tape and tape that to a pencil so one lead sticks out like a probe and wire the other to an alligator clip and you're good to go.
A good arc could damage some resistor types but then your out a few nickles, right. Carbon comp would be most resiliant and cheap. Metal film could be unreliable after arcing.
For parallel resistors I ususlly just get lazy and google parallel resistor calculator and use one of the many thet pop up. Here is one with the formula prominant.
http://www.1728.com/resistrs.htm
For discharge purposes and if all I had were 1M resistors I would just use 4 of them. Two in parallel will give you 500k. Do that twice and put those pairs in series and you're back up to 1M and you have 4X the original wattage. Wrap it in tape and tape that to a pencil so one lead sticks out like a probe and wire the other to an alligator clip and you're good to go.
A good arc could damage some resistor types but then your out a few nickles, right. Carbon comp would be most resiliant and cheap. Metal film could be unreliable after arcing.
It's interesting to me that resistors in series are preferred over in parallel. Is this primarily to prevent arcing over the (smaller wattage) resistors?
While the voltage IS very high (20kV is likely, on the top end) woiuldn't putting smaller wattage resistors in parallel dissipate current better and run LESS risk of damaging the resistors?
I don't understand why voltage is of primary concern, rather than current. With that many volts, the current flow is likely to be crazy high, and chopping the current down at any voltage would be likely to reduce the chances of creating a bomb rather than a discharge cable for the picture tube.
It sounds like, from what Mr. Al detailed, that buying a part for this is preferable than using a bunch of on-hand 1/2 watt resistors since it'll likely take so many. To build a large device like that, I would worry about accidentally making contact with myself, the friend I'm helping, some other equipment, the house cat, and so forth. I guess it sounds unwieldy.
I'm thinking I might be better served by just buying a high voltage resistor that can take as much as 40kV at like 15W and be done with it. It would allow me to build a discharge tool out of this resistor, some low gauge wire and some heavy alligator clips that I could give to my friend if he needed to perform this operation again.
While the voltage IS very high (20kV is likely, on the top end) woiuldn't putting smaller wattage resistors in parallel dissipate current better and run LESS risk of damaging the resistors?
I don't understand why voltage is of primary concern, rather than current. With that many volts, the current flow is likely to be crazy high, and chopping the current down at any voltage would be likely to reduce the chances of creating a bomb rather than a discharge cable for the picture tube.
It sounds like, from what Mr. Al detailed, that buying a part for this is preferable than using a bunch of on-hand 1/2 watt resistors since it'll likely take so many. To build a large device like that, I would worry about accidentally making contact with myself, the friend I'm helping, some other equipment, the house cat, and so forth. I guess it sounds unwieldy.
I'm thinking I might be better served by just buying a high voltage resistor that can take as much as 40kV at like 15W and be done with it. It would allow me to build a discharge tool out of this resistor, some low gauge wire and some heavy alligator clips that I could give to my friend if he needed to perform this operation again.
-
dyarker
- Posts: 1917
- Joined: Fri Aug 22, 2003 1:01 am
- Location: Izmir, Turkiye; from Rochester, NY
- Contact:
"It's interesting to me that resistors in series are preferred over in parallel. Is this primarily to prevent arcing over the (smaller wattage) resistors?"
Yes.
"While the voltage IS very high (20kV is likely, on the top end) woiuldn't putting smaller wattage resistors in parallel dissipate current better and run LESS risk of damaging the resistors?"
No. The power (P = I * E) is dissipated either way. The magnitude of the current is determined by voltage and total resistance (I = E/R). With higher resistance resistors in parallel each resistor has a smaller current but has entire voltage. With lower resistance resistors in series, each has the full current, but voltage is divided. The power is the same parallel or series.
Generally, for high voltage use series. For high current use parallel.
I hope he only wants to bleed residual charge. Continuous use at 20KV and 1M Ohm is 400W if my calculator is working right!
Cheers,
Yes.
"While the voltage IS very high (20kV is likely, on the top end) woiuldn't putting smaller wattage resistors in parallel dissipate current better and run LESS risk of damaging the resistors?"
No. The power (P = I * E) is dissipated either way. The magnitude of the current is determined by voltage and total resistance (I = E/R). With higher resistance resistors in parallel each resistor has a smaller current but has entire voltage. With lower resistance resistors in series, each has the full current, but voltage is divided. The power is the same parallel or series.
Generally, for high voltage use series. For high current use parallel.
I hope he only wants to bleed residual charge. Continuous use at 20KV and 1M Ohm is 400W if my calculator is working right!
Cheers,
Dale Y
-
Robert Reed
- Posts: 2277
- Joined: Wed Nov 24, 2004 1:01 am
- Location: ASHTABULA,OHIO
- Contact:
CRT voltages can vary any where between 1 Kv and 30KV. If the assumption is correct that you only want to discharge a residual charge of 20Kv - AFTER POWER HAS BEEN REMOVED FROM THE UNIT! - then your power dissapation and voltage reduction will follow a discharge curve consistent with the circuits time constants. Most hi-voltage supplys along with CRT parasitic capacitances will be on the order of several hundreds of PF. Given an extreme limit of 1000 PF and your 1 Meg resistor will produce a time constant of 1 millesecond. That means your 20Kv would be reduced to about 7 Kv in the first millisecond and the whole supply would be depleted in only 5 milliseconds, hardly enough time to create any significant heating of your resistive components! (Althogh for this high of a voltage, you might want to discharge it for one whole second to make sure the voltage is almost completely gone,and maybe even leaving a safety ground tied for the duration of your work).I would think that a few 1/4 watt resistors equalling your 1 Meg value would be all that is required here. Your main focus in construction should concern operator safety in regards to probe construction.
One followup note: If you had intended this circuit to be wired in permanently - forget it - as It would be doubtful if that CRT hi-volt. supply could handle that type of load (20 milliamps).
One followup note: If you had intended this circuit to be wired in permanently - forget it - as It would be doubtful if that CRT hi-volt. supply could handle that type of load (20 milliamps).
Hi again,
I thought i made it clear as to why a series arrangement is better,
but i guess i didnt. Let me state it a bit differently.
A resistor has a power rating, and it also has a max voltage rating.
There are actually two ratings, one is 'working voltage' and the
other is what i call 'surge voltage'. The surge rating is higher
and is not a voltage which you want to run your resistor at for
very long. The working voltage is the voltage that the resistor
can take for an indefinite period of time as long as other
parameters (such as power or temperature) are not exceeded.
Because each resistor rating is 350 volts, and that is much less
than the 20kv estimate we made, you should not use a single
resistor or any number of resistors in parallel because that would
subject the single resistor or the multiple resistors in parallel to
the same high voltage of 20kv, which is far beyond their working
voltage of 350 volts.
On the other hand, if you connect them in series, you can use enough
resistors so that each one only has 350v or less across them. This
is enough to ensure that none of the resistors will arc over and cause
a brief but still potentially damaging current surge.
Thus, connecting them in series means the resistors will not be subject
to a voltage that is above that at which they are rated, while
connecting them in parallel will subject each and every resistor to
a voltage that is way beyond what they are designed for, and this is
true regardless of how short the discharge time period is.
If you want to take a chance that is up to you, and perhaps a single
resistor will work out ok but you may not even know it if you do
some damage to the circuit which could manifest itself in the form of
causing premature failure at a later date.
OH yeah one more thing...
If you look at some of the resistor data sheets you will see that the
manufacturers also recommend a series connection when the voltage
exceeds the working voltage of a single device. You will also find
some high voltage probes (used for measuring high voltage with a meter)
made this way too. If i remember right, Popular Electronics had a
high voltage probe project one month many many moons ago
(possibly in the 60's or 70's).
If you are still not happy, look up some of the high voltage
resistors available...Super Mox by Ohmite perhaps...Check
the cost.
As a final note, this might help to understand what you are up against...
http://rimstar.org/equip/hv150kvprobe/hv150kvprobe.htm
Ok, this is really the final note now...
As Robert mentioned, connecting the resistor permanently is not an
option because it would eat up too much power from the driver.
There is another, more potentially dangerous reason for not doing this,
and also serves as another lesson in high voltage work. This is, that
if you do find a resistor value that works when permanently installed
(a much higher value) if that resistor fails you have no way of
knowing that, and you get a deadly shock when you go to work on
the appliance. The lesson to be learned there is not to trust the
resistor no matter how it was made. This of course means a secondary
procedure is also mandatory: after turning off the unit and waiting
maybe 10 minutes, use the resistor to discharge the HV lead.
Hold it there for a few seconds, then take a jumper lead and connect
one end to chassis ground and the other end to a screwdriver with
a nice thick rubber handle. Short the HV lead directly to chassis
ground by touching the screwdriver tip to the HV lead and check for
a spark. A spark means the resistor did not work and may be defective.
Also, before using the jumper check it with an ohmmeter to make sure
it is really a good jumper...some jumpers fail because the aligator
clip becomes detacted from the metal of the wire while the insulation
of the wire keep the aligator clip attached, making it *appear* to be
a good jumper, when it really acts as an open circuit. If you have
any doubts, use a second jumper too and short the HV again.
Also as mentioned, connect a permanent jumper to the HV to ground
while you are working on it, then remove it when you are ready to
power up again.
Another trick often used is to keep one hand in your pocket when you go
to discharge the HV lead. In the event that you do get a shock this
prevents the current from taking a path directly through your heart,
which will kill you. It also helps to stand on a rubber welcome mat or
even an old tire laid on it's side. I've read it only takes 3ma to kill you.
I thought i made it clear as to why a series arrangement is better,
but i guess i didnt. Let me state it a bit differently.
A resistor has a power rating, and it also has a max voltage rating.
There are actually two ratings, one is 'working voltage' and the
other is what i call 'surge voltage'. The surge rating is higher
and is not a voltage which you want to run your resistor at for
very long. The working voltage is the voltage that the resistor
can take for an indefinite period of time as long as other
parameters (such as power or temperature) are not exceeded.
Because each resistor rating is 350 volts, and that is much less
than the 20kv estimate we made, you should not use a single
resistor or any number of resistors in parallel because that would
subject the single resistor or the multiple resistors in parallel to
the same high voltage of 20kv, which is far beyond their working
voltage of 350 volts.
On the other hand, if you connect them in series, you can use enough
resistors so that each one only has 350v or less across them. This
is enough to ensure that none of the resistors will arc over and cause
a brief but still potentially damaging current surge.
Thus, connecting them in series means the resistors will not be subject
to a voltage that is above that at which they are rated, while
connecting them in parallel will subject each and every resistor to
a voltage that is way beyond what they are designed for, and this is
true regardless of how short the discharge time period is.
If you want to take a chance that is up to you, and perhaps a single
resistor will work out ok but you may not even know it if you do
some damage to the circuit which could manifest itself in the form of
causing premature failure at a later date.
OH yeah one more thing...
If you look at some of the resistor data sheets you will see that the
manufacturers also recommend a series connection when the voltage
exceeds the working voltage of a single device. You will also find
some high voltage probes (used for measuring high voltage with a meter)
made this way too. If i remember right, Popular Electronics had a
high voltage probe project one month many many moons ago
(possibly in the 60's or 70's).
If you are still not happy, look up some of the high voltage
resistors available...Super Mox by Ohmite perhaps...Check
the cost.
As a final note, this might help to understand what you are up against...
http://rimstar.org/equip/hv150kvprobe/hv150kvprobe.htm
Ok, this is really the final note now...
As Robert mentioned, connecting the resistor permanently is not an
option because it would eat up too much power from the driver.
There is another, more potentially dangerous reason for not doing this,
and also serves as another lesson in high voltage work. This is, that
if you do find a resistor value that works when permanently installed
(a much higher value) if that resistor fails you have no way of
knowing that, and you get a deadly shock when you go to work on
the appliance. The lesson to be learned there is not to trust the
resistor no matter how it was made. This of course means a secondary
procedure is also mandatory: after turning off the unit and waiting
maybe 10 minutes, use the resistor to discharge the HV lead.
Hold it there for a few seconds, then take a jumper lead and connect
one end to chassis ground and the other end to a screwdriver with
a nice thick rubber handle. Short the HV lead directly to chassis
ground by touching the screwdriver tip to the HV lead and check for
a spark. A spark means the resistor did not work and may be defective.
Also, before using the jumper check it with an ohmmeter to make sure
it is really a good jumper...some jumpers fail because the aligator
clip becomes detacted from the metal of the wire while the insulation
of the wire keep the aligator clip attached, making it *appear* to be
a good jumper, when it really acts as an open circuit. If you have
any doubts, use a second jumper too and short the HV again.
Also as mentioned, connect a permanent jumper to the HV to ground
while you are working on it, then remove it when you are ready to
power up again.
Another trick often used is to keep one hand in your pocket when you go
to discharge the HV lead. In the event that you do get a shock this
prevents the current from taking a path directly through your heart,
which will kill you. It also helps to stand on a rubber welcome mat or
even an old tire laid on it's side. I've read it only takes 3ma to kill you.
LEDs vs Bulbs, LEDs are winning.
-
Robert Reed
- Posts: 2277
- Joined: Wed Nov 24, 2004 1:01 am
- Location: ASHTABULA,OHIO
- Contact:
Smarrioti and MrAl
MrAl mentioned some good points there. Most GFI circuits will trip at 5ma, so I assume the 'brains' in charge of designing these things did their homework on this. This would indicate that 5 ma plus a comfortable margin would be life threatening. However, long before that happens, the relex action fom just a "harmless" shock could cause someone a world of pain. In my career,I have been shocked several times from hi-voltage and never suffered any damage at all from the shock. But when it comes to reflex action (involuntary muscle contraction) therein lies a problem - one in which I nearly poked my eye out while holding a pair of needle nose pliers (of which made the circuit between me and hi-voltage) and they came flying back at me when the accidental contact occurred, narrowly missing my eye,but puncturing the skin very nearby - involuntary muscle contraction. Needless to say, a lesson like that makes you tenfold cautious in the future. My point being that even before a lethal shock could occur, there is a very definate danger here for physical injury and its done to YOURSELF by YOURSELF!
Back to the topic at hand - I had a temporary circuit set up at one time while developing an automotive tachometer circuit that involved four 20Meg 1/2 watt carbon composition resistors in series from sparkplug lead to ground. This setup had a lot of accumalted test time on it and never saw any evidence of arcing or phsyical changes. The plug voltage was pulsed 10Kv which each pulse would be similar to one discharge pulse on your CRT. Also, years ago in the B&W TV area, it was common practice to discharge CRT hi- voltage (about 15Kv at that time)
by a manner in which MrAl described with screwdriver and ground lead.
Standard transmission line cable ( RG-58U) has a breakdown voltage of about 15 Kv and I have seen these cables used many times in H- voltage probes. For grounding application only, they were constructed by tieing the center conductor and sheild together at one end to a ground clip to chassis. The other end had a string of resistors tied to the center conductor. This assy. was enclosed in one of the larger plastic ball point pen cartridges. A point protruded from the far end in which to make circuit contact. These were used as continuous duty test probes and all the ones I had seen (and used) worked quite well. The cable sheild was run well into the cartridge and gave the operator a pretty good measure of protection.
MrAl mentioned some good points there. Most GFI circuits will trip at 5ma, so I assume the 'brains' in charge of designing these things did their homework on this. This would indicate that 5 ma plus a comfortable margin would be life threatening. However, long before that happens, the relex action fom just a "harmless" shock could cause someone a world of pain. In my career,I have been shocked several times from hi-voltage and never suffered any damage at all from the shock. But when it comes to reflex action (involuntary muscle contraction) therein lies a problem - one in which I nearly poked my eye out while holding a pair of needle nose pliers (of which made the circuit between me and hi-voltage) and they came flying back at me when the accidental contact occurred, narrowly missing my eye,but puncturing the skin very nearby - involuntary muscle contraction. Needless to say, a lesson like that makes you tenfold cautious in the future. My point being that even before a lethal shock could occur, there is a very definate danger here for physical injury and its done to YOURSELF by YOURSELF!
Back to the topic at hand - I had a temporary circuit set up at one time while developing an automotive tachometer circuit that involved four 20Meg 1/2 watt carbon composition resistors in series from sparkplug lead to ground. This setup had a lot of accumalted test time on it and never saw any evidence of arcing or phsyical changes. The plug voltage was pulsed 10Kv which each pulse would be similar to one discharge pulse on your CRT. Also, years ago in the B&W TV area, it was common practice to discharge CRT hi- voltage (about 15Kv at that time)
by a manner in which MrAl described with screwdriver and ground lead.
Standard transmission line cable ( RG-58U) has a breakdown voltage of about 15 Kv and I have seen these cables used many times in H- voltage probes. For grounding application only, they were constructed by tieing the center conductor and sheild together at one end to a ground clip to chassis. The other end had a string of resistors tied to the center conductor. This assy. was enclosed in one of the larger plastic ball point pen cartridges. A point protruded from the far end in which to make circuit contact. These were used as continuous duty test probes and all the ones I had seen (and used) worked quite well. The cable sheild was run well into the cartridge and gave the operator a pretty good measure of protection.
Hi again,
Interesting Robert, perhaps you can elaborate a little on what you were
using those 20meg resistors for, as to what purpose they served.
I am wondering what effect they had on the total voltage.
I guess i have been lucky in the past because i have been shocked
by several different kinds of sources of high voltage, including lawn
mower spark plugs and a 400vdc power supply used in a vac tube
amplifier. Out of all of them, the 400vdc was the worst because it
went through my whole body from hand to foot, and it was really
what i would call a very nasty shock. The lawn mower spark, although
higher in voltage, didnt feel as bad for some reason, i think because
the total energy available was less.
I was lucky i didnt get killed with that 400v supply, or get some
kind of other damage. After that people said i repeat myself sometimes,
but i havent noticed anything at all myself. myself. myself. myself.
(ha ha). Seriouly though, to those using any voltage over 20v there
is always the chance of shock and with high voltage like 100v or
over and especially 1000v and over there is real risk of loss of life
so the most important thing to remember is safety first. You also
might want to make sure any pets can not get near either.
Interesting Robert, perhaps you can elaborate a little on what you were
using those 20meg resistors for, as to what purpose they served.
I am wondering what effect they had on the total voltage.
I guess i have been lucky in the past because i have been shocked
by several different kinds of sources of high voltage, including lawn
mower spark plugs and a 400vdc power supply used in a vac tube
amplifier. Out of all of them, the 400vdc was the worst because it
went through my whole body from hand to foot, and it was really
what i would call a very nasty shock. The lawn mower spark, although
higher in voltage, didnt feel as bad for some reason, i think because
the total energy available was less.
I was lucky i didnt get killed with that 400v supply, or get some
kind of other damage. After that people said i repeat myself sometimes,
but i havent noticed anything at all myself. myself. myself. myself.
(ha ha). Seriouly though, to those using any voltage over 20v there
is always the chance of shock and with high voltage like 100v or
over and especially 1000v and over there is real risk of loss of life
so the most important thing to remember is safety first. You also
might want to make sure any pets can not get near either.
LEDs vs Bulbs, LEDs are winning.
-
Robert Reed
- Posts: 2277
- Joined: Wed Nov 24, 2004 1:01 am
- Location: ASHTABULA,OHIO
- Contact:
Hello MrAl
To be a little more descriptive in that setup, there were four 20 meg resistors in series with a much smaller tapoff resistor to ground at which junction a scope's 10:1 probe was attached. This was setup to give a 1000:1 reduction of the hi-volt. as presented to the scope probe.The calibration accuracy was very loose as we were more interested in wave shape than actual voltage, but never the less gave a fair accurracy of actual level at the plug. You would think that with this high of attenuator values one would need some compensating capacitance around the string for accurate AC presentation, but subsequent test setups showed this was not neccessary and also various resistive loading (within reason) had no effect on voltage levels, as apparently the coils output was a fairly stiff source.
We had assumed that the firing level of the newer cars at that time would be on the order of 8 KV, given the 0.060" gap they were using. Add to that a less than ideal situation of normal gap wear and some fouling, we calculated a 10 KV max. for the test and in reality the tests did bear this out.
My point was that even with 2500 volts across each resistor, there was no arcing or degradation that had occurred.This was a cheap and dirty setup in which the resistors were soldered end to end with full lead length totaling a string of about 8 inches and then supported in such a manner as to be well away from any conductive surfaces. Like I said - cheap and dirty! But it performed OK for our tests.
As to shocks, the most painful one I ever experienced was from a 2400 regulated supply - fortuneately no damage done except for a very small burn on my finger. The thing that has always worried me is that after years of working with low voltage circuits (5-20V) I have become very complacent about where my hands are in regards to the circuit and never giving any thoght what so ever about shock potential. So if I do get involved with HV in the future, I will really have to change my habits and concentrate on that danger.
As to 'lethal voltages" - there are so many scenarios present above and beyond actual stated voltage, I think it is impossible to predict. I look at contact area and body current path as being most important here. For example, I had a co-worker that would amaze the lab by actually grasping live 120VAC leads in his hands without even feeling a tingle.Extremely dry skin? Haven't seen him for years. Wonder if he's still around
. On the other end of the spectrum, I have felt a pretty good tingle from 12 volt automotive batterys when a sweaty forearm was pressed against the top of a fender for support and the other was grabbing a 12 volt source - CONTACT AREA!
To be a little more descriptive in that setup, there were four 20 meg resistors in series with a much smaller tapoff resistor to ground at which junction a scope's 10:1 probe was attached. This was setup to give a 1000:1 reduction of the hi-volt. as presented to the scope probe.The calibration accuracy was very loose as we were more interested in wave shape than actual voltage, but never the less gave a fair accurracy of actual level at the plug. You would think that with this high of attenuator values one would need some compensating capacitance around the string for accurate AC presentation, but subsequent test setups showed this was not neccessary and also various resistive loading (within reason) had no effect on voltage levels, as apparently the coils output was a fairly stiff source.
We had assumed that the firing level of the newer cars at that time would be on the order of 8 KV, given the 0.060" gap they were using. Add to that a less than ideal situation of normal gap wear and some fouling, we calculated a 10 KV max. for the test and in reality the tests did bear this out.
My point was that even with 2500 volts across each resistor, there was no arcing or degradation that had occurred.This was a cheap and dirty setup in which the resistors were soldered end to end with full lead length totaling a string of about 8 inches and then supported in such a manner as to be well away from any conductive surfaces. Like I said - cheap and dirty! But it performed OK for our tests.
As to shocks, the most painful one I ever experienced was from a 2400 regulated supply - fortuneately no damage done except for a very small burn on my finger. The thing that has always worried me is that after years of working with low voltage circuits (5-20V) I have become very complacent about where my hands are in regards to the circuit and never giving any thoght what so ever about shock potential. So if I do get involved with HV in the future, I will really have to change my habits and concentrate on that danger.
As to 'lethal voltages" - there are so many scenarios present above and beyond actual stated voltage, I think it is impossible to predict. I look at contact area and body current path as being most important here. For example, I had a co-worker that would amaze the lab by actually grasping live 120VAC leads in his hands without even feeling a tingle.Extremely dry skin? Haven't seen him for years. Wonder if he's still around
. On the other end of the spectrum, I have felt a pretty good tingle from 12 volt automotive batterys when a sweaty forearm was pressed against the top of a fender for support and the other was grabbing a 12 volt source - CONTACT AREA!Hi again,
Robert, wow im surprised that even worked at all, especially since
the distance across one resistor isnt that great either so i would have
expected problems at elevated humidity at the very least even if the
part itself did survive the extra high out-of-spec voltage.
I guess from past experience i have learned to observe tbe
manufacturers specs because they go through a lot of trouble to
test their products so the end users dont have any trouble,
and it's very hard to get that kind of multiple unit data without
testing a whole pile of their product.
Robert, wow im surprised that even worked at all, especially since
the distance across one resistor isnt that great either so i would have
expected problems at elevated humidity at the very least even if the
part itself did survive the extra high out-of-spec voltage.
I guess from past experience i have learned to observe tbe
manufacturers specs because they go through a lot of trouble to
test their products so the end users dont have any trouble,
and it's very hard to get that kind of multiple unit data without
testing a whole pile of their product.
LEDs vs Bulbs, LEDs are winning.
-
Robert Reed
- Posts: 2277
- Joined: Wed Nov 24, 2004 1:01 am
- Location: ASHTABULA,OHIO
- Contact:
Yeah, when you think about it, it should be prone to problems. BTW, these were carbon comp 1/2 watt resistors which would be somewhat higher than the 1/4 watt films that have been discussed from time to time here. There is one thing in the back of my mind that i picked up somewhere along the way and that is in relation to HV breakdown vs. time. In other words, time enters into the sustaining voltage rating ( be it usec, sec, or minutes ?). Do you recall any info on that particular subject?
I wonder if that might apply here due to the very short HV duration of discharge circuits or the tests that were run in my case.
I wonder if that might apply here due to the very short HV duration of discharge circuits or the tests that were run in my case.
Hi Robert,
i have heard that carbon comps are better for high voltage discharge,
but i think what happens is the resistance decreases with the number
of discharge cycles and so at some point the resistance is much lower
than it was when the resistor was new.
Perhaps you have some of those resistors you used still laying
around and can measure one? Of course it would have to be one
that was used over and over again to discharge a capacitor,
not a new or slightly used resistor.
I wouldnt doubt it if the resistance went down with very high voltage
and causes the cap to discharge even faster.
The 1/2 watt resistors i have seen are rated for 350v continuous
(power allowing of course) and 700v surge, but they dont say how long
the pulse is for that surge, so it makes it very hard to predict just
how long a string of resistors will last for when they are seriously
overvoltaged.
i have heard that carbon comps are better for high voltage discharge,
but i think what happens is the resistance decreases with the number
of discharge cycles and so at some point the resistance is much lower
than it was when the resistor was new.
Perhaps you have some of those resistors you used still laying
around and can measure one? Of course it would have to be one
that was used over and over again to discharge a capacitor,
not a new or slightly used resistor.
I wouldnt doubt it if the resistance went down with very high voltage
and causes the cap to discharge even faster.
The 1/2 watt resistors i have seen are rated for 350v continuous
(power allowing of course) and 700v surge, but they dont say how long
the pulse is for that surge, so it makes it very hard to predict just
how long a string of resistors will last for when they are seriously
overvoltaged.
LEDs vs Bulbs, LEDs are winning.
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