Lamp Current vs Voltage

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Dean Huster
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Re: Lamp Current vs Voltage

Post by Dean Huster » Wed Sep 29, 2004 9:31 am

"I bet its resistance is relatively stable with temperature changes since it has been the most commonly used material in manufacturing resistors."<p>Joseph, carbon resistors, especially carbon composition resistors, have TERRIBLE tempcos! Carbon film resistors don't seem to be quite as bad although their lack of bulk tends to make them pretty fragile to transient overloads that carbon comp resistors would normally take without batting an eye. Carbon comp resistors also aren't very stable WRT time, usually shifting high in value. It's pretty tough, even with NOS carbon comp resistors, to find components that are still within tolerance after all these years.<p>Those 6S6 and 3S6 lamps, although rare in most households, were extremely popular in electronic shops. As mentioned, they were often used for pilot lamps and panel illumination as well as being there as a trouble light for those tiny little engineers inside an hp 200CD audio oscillator. The infamous night light or 7-1/2 watt Noma Christmas tree lamp were popular even back in the 1950s. There must have been something about them that keep engineers from using them as electronic components. Maybe they didn't like the idea of red, yellow or green lamps inside their boxes.<p>Dean
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Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

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Joseph
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Re: Lamp Current vs Voltage

Post by Joseph » Wed Sep 29, 2004 9:52 am

Dean, thanks for that information about carbon resistors. It goes without saying, I am quite surprised.<p> <blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr> Carbon film resistors don't seem to be quite as bad although their lack of bulk tends to make them pretty fragile to transient overloads that carbon comp resistors would normally take without batting an eye. <hr></blockquote><p>Recently, I was adding resistors to the output of a varmit zapper for peak current limiting, and the carbon film ones burned through while the metal film ones have been holding up. BTW, the raccoons around here have the nasty habit of trying to tear their way into attics. I feel sympathy for at least two of my neighbors at whose houses they were successful. <p>I was recently in the local Dollar Tree store and saw a pack of 4 night light bulbs for a buck. One reason they may not be used in electronics is that they are physically larger than standard pilot lamps.<p>[ September 29, 2004: Message edited by: Joseph ]</p>

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Re: Lamp Current vs Voltage

Post by Enzo » Wed Sep 29, 2004 4:01 pm

I would be hesitant to use those huge bulbs and sockets in something when the smaller bulb would work as well.<p>Those resistors not only shift value with age, in our guitar amps they also get noisy. I have to replace noisy resistors in something several times a year in my shop.

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Re: Lamp Current vs Voltage

Post by toejam » Thu Sep 30, 2004 1:09 am

Joseph
What i was thinking about was the mechinacal stress due to the constant polorazation reversal caused by ac. I once had a couple of carbon filament edison bulbs and although the glass was darkened they still worked. Now bulbs seem to last at the very most as long as thier stated life.

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Re: Lamp Current vs Voltage

Post by rshayes » Thu Sep 30, 2004 1:30 pm

Carbon filament lamps would have a totally different curve. The resistance of a carbon filament decreases with temperature, so I would expect something closer to a square law for a carbon filaments current vs voltage characteristic.<p>I don't know when the last carbon filament bulbs were made, but I would guess at some time in the 1930's. Carbon filaments were less efficient and had a shorter life, but they may have cost less for a while, due to the problems that had to be solved in working with tungsten.

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Re: Lamp Current vs Voltage

Post by Will » Thu Sep 30, 2004 4:26 pm

Yes Joseph - I see that the equation for the 6 volt lmap looks a little odd - that's probab;y because I only put three sample points into the curve fit program - The program also produced a result for the 6 volt in terms of the first equation but it didnt correlate quite as well as the one I used. In fact it's easy to see from both sets of data that the resistance has a positive temperature co-efficient i.e. that the resistnce increases with current/power. If it did not change then your graphs would be straight lines , if the temperature co-efficient was negative then the current graphs would curve upward i.e. less resistance - as it is they both curve downward i.e. for a positive temp co-efficient, resistance increasing with power/current/temperature.
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Re: Lamp Current vs Voltage

Post by Enzo » Thu Sep 30, 2004 10:12 pm

Joseph, your critter zapper resistors were burning up because you exceded their voltage rating. If you chew into a film resistor you will find it is a ceraminc form or tube with a spiral stripe of resistive material around it like the lines on a barber pole. There is a very tiny gap between adjacent turns of the spiral. when you put too high a voltage across the resistor, the turn to turn voltage can get high enough to jump the gap - it arcs - and then the resistor is history. Your metal films were doing better because they had a higher operating voltage limit, or you were just lucky.<p>Resistor voltage ratings rarely come up in normal stuff, but we have to worry about them in such things as tube amps that run on high voltage. Tube amps and critter zappers.<p>I have a coon in my attic as a matter of fact.

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Joseph
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Re: Lamp Current vs Voltage

Post by Joseph » Fri Oct 01, 2004 9:38 am

toejam, I heard of that idea. But nowadays I wonder if florescent may be the way to go for longevity and efficiency. Before long, LEDs may even replace even those.<p>Steven, the negative tempco of carbon makes some sense to me.<p>Will, thanks for that explanation of how you came up with the formulas. The upward or downward curving that you mentioned for a set of graph points holds up in my thinking.<p>Enzo, I saw the 75 ohm resistor burn through as you described. A flash occurred between two of the adjacent carbon spirals. It left a gap in one of the spirals as it also blew off a section of outer coating. I suspect the metal film ones hold up because the film is solid instead of spiraled. An arc would tend to jump around the entire resistor. It helps that the resistors are rated 2 watts each. A general guideline is that higher wattage resisters handle higher voltage. I saw on the Yageo carbon film resistor datasheet some working voltage ratings for resistors:
  • 1/6w 150v
  • 1/4w 200v
  • 1/2w 300v
  • 1w 400v
  • 2w 500v
The dielectric strength ratings are twice the working voltage ones.<p>I saw on my house how the bottom of the siding was bent out where coon tried to rip it off. I surely need to prevent one from getting in since I am highly allergic to them.<p>[ October 01, 2004: Message edited by: Joseph ]</p>

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Re: Lamp Current vs Voltage

Post by Dean Huster » Sat Oct 02, 2004 10:43 am

I'm amazed at the number of folks that don't realize that resistors have voltage ratings. It's really not their fault because that fact isn't taught in any schools that I'm aware of and I've never read it in any books or electronics courseware, ever. But more than once, I've seen folks try to make a voltage divider for their DMM so that it can measure around 10,000 volts and try to use a single 1/2-watt resistor for the series element and the DMM's 10M ohms for the other end. Zxxxxxzzzzzxxxzzzttttt!<p>Stephen, I've seen carbon filament repros and in fact have one. It LOOKS like a carbon filament, but it might be something else. Pretty cool looking, though.<p>Dean
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Chris Smith
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Re: Lamp Current vs Voltage

Post by Chris Smith » Sat Oct 02, 2004 12:46 pm

When using high voltage always string a long line of many resistors.

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Re: Lamp Current vs Voltage

Post by Grampadan » Sat Oct 30, 2004 3:23 pm

If light bulbs burn out "too fast/often", measure the voltage (several times thru the day/night) and if much higher than 'normal' (which has been 110, 115, 118, 120 volts in the past) the power company can adjust the voltage taps on line-loss compensating transformers in the distribution system. (Worked for my daughter in rural Vermont) Some are self-adjusting, have a round indicator (about 6" diameter?) on them to show the tap selected. <p>Sylvania Lamp, Pennacook?, NH published spec books for miniature lamps, and for automotive bulbs ( + aircraft, railroad, mil, agriculture) - circa 1965/1985 - with a great graph (just inside the cover ?) showing relation of voltage, current, life, probably brightness, power. That "Life = V to the 12th power" was really impressive. hard to wrap your head around, until you try running a bulb at twice rated voltage. Poof!<p>Molded plastic nightlights, with two sockets in series ( the center contacts of the 2 bulbs touched each other IIRC ) were sold in the 60's or 70's. I had one, lasted a long time, BUT bulbs were not identical; the higher resistance one dropped more voltage, thus evaporating filament faster, going to even higher resistance. Once started down that slippery slope, the brighter bulb would burn out in "just a few years." Still a good device.<p>A series diode Should give same effect, and be stable. Using a lot of them, on only one branch of the 220 service - 1 phase - might dirty up your power line waveform some.<p>In these days of switching power supplies, perhaps only of concern to those who buy MonsterCable (r) power cords for over a hundred dollars, to keep their crappy commercial power 'pure.'

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Re: Lamp Current vs Voltage

Post by REBCO » Sat Oct 30, 2004 5:26 pm

Very interesting, the different points of view on a seemingly simple question. May I add two more - when you reduce the voltage the life expectancy is tremendously increased (I dont like to say infinite), however the efficiency of light emision is correspodently decreased, you will be paying for a lot of electricity to get very little light. (2)The light bulb manufacturers are supposed make the cost of electical power about equal to the cost of the bulb to get the best overall light for the dollar. Use long life bulbs only where they are difficult to replace

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Re: Lamp Current vs Voltage

Post by Will » Mon Nov 01, 2004 3:48 pm

joseph,
I had another thought about your lamp test data. Trying to fit a curve to either the volts or current data is not easy since the temp co-eff of the lamp filaments are not constants. More to the point is the relationship between applied power and the resistance - so I noted all of your points from the graphs, calculated the resultant power and curve fitted resoistance against that - It came out to be R = 4.9191*P^0.2385 for the 3.0 volt lamp - the correlation factor was about 0.9 and that was possibly due to inaccurate data taking (By me) The 6 volt lamp was R = 9.8058*p^0.2113 and the correaltion R^2 was 0.9724 - both results are saying that the resistance is a function of somewhere between the 4th and fifth roots of the power applied. Still having fun !
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Joseph
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Re: Lamp Current vs Voltage

Post by Joseph » Mon Nov 08, 2004 9:30 am

Will, if you get a chance sometime, can you add the variable for current or voltage into your equations for calculating R?

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Re: Lamp Current vs Voltage

Post by n7hqk » Tue Nov 09, 2004 5:52 pm

Carbon is a Negitive Coefficent Conductor. I used to run carbon arc projectors for movies. When the arc is established, the carbon of course heats to white hot and the current in the circuit DROPS to a relitivly stable point. Some units used this to automaticly adjust the arc gap with a spring and solenoid arrangement...
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