Question about relay voltage

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
zmwworm
Posts: 41
Joined: Wed Dec 20, 2006 4:26 pm
Location: Bonners Ferry, ID
Contact:

Post by zmwworm » Fri Mar 21, 2008 5:52 pm

MrAl wrote:Faster opening contacts
sometimes reduces wear caused by pitting or even partial welding
of the contacts simply because the contacts can move away from
each other at a faster rate which reduces arc time.

The way to acheive this faster turn off time is to allow the coil
voltage to go higher, but not so high that it would cause a problem
with other parts of the circuit as to their voltage handling ability.
Is this considering the 68 ohm resistors before the relays? They drop both the current and voltage, meaning less arc, right? Or will the reduced voltage cause worse wear than normal?

dyarker
Posts: 1752
Joined: Fri Aug 22, 2003 1:01 am
Location: Izmir, Turkiye; from Rochester, NY
Contact:

Post by dyarker » Fri Mar 21, 2008 7:57 pm

Is this considering the 68 ohm resistors before the relays? They drop both the current and voltage, meaning less arc, right? Or will the reduced voltage cause worse wear than normal?
No. That resistor is in series with the switch and relay coil. It reduces voltage across, and current through, the coil when the switch is ON; so the coil stays cooler. It's only effect when switch changes to OFF, would be after the arc has formed. Not much help.

A resistor, or zener, in series with the diode that is parallel with the coil causes current to decrease faster when switch is changed to OFF.

With a 3A rated switch (I think you said), and 133mA coil current, your switch is safe with just the diode.

For lights it doesn't matter if the relay contacts open 2mSec or 10mSec after the switch opens. There are cases where it is important that the relay contacts open as quickly as possible. So it is worthwhile to remember this discussion for future reference.

Cheers,
Dale Y

zmwworm
Posts: 41
Joined: Wed Dec 20, 2006 4:26 pm
Location: Bonners Ferry, ID
Contact:

Post by zmwworm » Fri Mar 21, 2008 10:48 pm

dyarker wrote:With a 3A rated switch (I think you said), and 133mA coil current, your switch is safe with just the diode.
It's rated 3A at 125VAC, but as Bigglez told me in a different discussion AC rated switches are not as hefty as DC switches for the same voltages and amperage.

User avatar
MrAl
Posts: 3862
Joined: Fri Jan 11, 2002 1:01 am
Location: NewJersey
Contact:

Post by MrAl » Sat Mar 22, 2008 6:20 am

Hi again,

Actually, the faster switch time i was referring to was not in the switch
itself, but the relay contacts. When the voltage is allowed to go
higher across the coil (resistor,zener, whatever) the relay contacts
can open faster which means less wear.
Not sure how much wear is saved though with a very light current.
If the lights draw some current then it would be worth it to install
a zener too as this should make the relay last longer.

Again, the reason for the extra wear in the relay contacts is because
the two contacts move away from each other slower with only a
single diode than with a single diode and zener, thus giving rise
to more arc across the contacts as they open. With the diode and
zener, the change in coil current is more abrupt (decreases much
faster) so the magnetic force holding the arm in decreases much
faster and this allows the spring to pull the contacts apart faster.

Another way of thinking about this is to imagine the arm of the relay
starting to move as the current in the coil starts to decrease below
the holding current. As the arm moves, it eventually allows the contact
to start opening. When the contact first opens there is some inductance
in the line which could create an arc. As the contact reaches 0.0005 inches
away from the other contact the arc might still be supported, but as
the contact moves farther and farther away it eventually breaks the arc.
The wear is related to how long this arc lasts, and the arc damage is worse
with a long time period where the contacts are slightly open but not
touching. Reducing the time where they are say 0.001 inches apart
would reduce wear, so doing something that makes the current
decrease faster causes the contacts to move apart faster too, which
reduces wear. Since the inductance can release its energy faster with
a higher voltage across the coil, allowing the voltage to go higher
would definitely reduce the time of the arc.

Note that if a switch is being used to turn the relay coil on and off
the switch has to be rated for this extra voltage. If a 24 volt zener is
used and a 12v power source, that brings the total voltage up to
36 volts, so the switch would have to be rated for at least 36 volts.
If the switch is only rated for 12v then it might be better just to use
a diode and put up with the extra contact wear.

You might also try viewing the relay opening and closing in the dark
to see if you can spot an arc inside, especially when the contacts
open up.

BTW, the 68 ohm resistor will play a part in the opening of the coil
if the clamp device (diode or diode+zener) is across both the coil
and the resistor because the resistor will allow more voltage to
appear across the coil during turnoff. It will however, also slow down
the turn on time which might not be desirable either.
LEDs vs Bulbs, LEDs are winning.

User avatar
MrAl
Posts: 3862
Joined: Fri Jan 11, 2002 1:01 am
Location: NewJersey
Contact:

Post by MrAl » Sat Mar 22, 2008 8:15 am

Hi again,

Here are some drawings showing the difference between using a
single diode and using a zener in series with a diode.

Note that the pictures of the contacts are when using a single diode.
Also note below that the graph shows an improvement with the
addition of a zener diode as the time of the arc is reduced to half.
In real life it could actually be even better than this. To test, try
operating the relay in the dark and watching for the arc.

Image
LEDs vs Bulbs, LEDs are winning.

zmwworm
Posts: 41
Joined: Wed Dec 20, 2006 4:26 pm
Location: Bonners Ferry, ID
Contact:

Post by zmwworm » Sat Mar 22, 2008 11:00 pm

MrAl wrote:Actually, the faster switch time i was referring to was not in the switch
itself, but the relay contacts.
I understood that.
When the voltage is allowed to go
higher across the coil (resistor,zener, whatever) the relay contacts
can open faster which means less wear.
Not sure how much wear is saved though with a very light current.
If the lights draw some current then it would be worth it to install
a zener too as this should make the relay last longer.
I'm thinking that the extra money and effort to install zeners would not be worth it, if a relay wears out I'll just replace it. These relays shouldn't be turning on and off all the time anyway, they will only see occasional use. I don't know if this might have any impact, but I'm powering the lights through the normally open contact, and I connected the normally closed contact to ground (because that's the way the other relays in the truck were, no other reason).

The one thing I would do differently next time I do something like this is put everything in an enclosure. So far I've had 5 fuse holders, 3 relays, 15 connectors in 3 sockets for the relays, 3 resistors, and 3 diodes all soldered together between many, many wires (that also go to 3 switches and 3 indicator lights). If any one connection comes loose I have to take apart the dash and pull the whole mess out, cutting and resoldering wires for any modifications. Hopefully anyone considering doing something similar will take my advice, it will be much easier to add things like zener diodes and resistors, and it will probably last longer too.

User avatar
MrAl
Posts: 3862
Joined: Fri Jan 11, 2002 1:01 am
Location: NewJersey
Contact:

Post by MrAl » Sun Mar 23, 2008 1:29 pm

Hi again,

Well, zeners are cheap these days anyway.
Of course if you have to rip everything apart just to do that i
wouldnt bother.

Good luck with it.
LEDs vs Bulbs, LEDs are winning.

Post Reply

Who is online

Users browsing this forum: No registered users and 25 guests