An earler post asked about using snubbers to save the contacts of switches & relays.
Many of the same issues apply to both relay & switch contacts.
I looked into factors reducing relay contact life a while ago & there are a couple of Application Notes that I have saved.
An Application Note "Maximizing the Life Span of Your Relays" discusses relay selection, contact life & snubber systems.
This App Note has been arround for some time & a recent Keysight version is at:
An earlier Agilent version can be found at:
One factor that can dramatically reduce the life of relay contacts is the common Back EMF "Flyback" protection diode fitted across the coil.
The diode slows the relay operation making arcing far worse & that destroys the contacts.
The App Note "Coil Suppression Can Reduce Relay Life" recommends the simple addition of a Zener Diode in series with the usual diode to maximise contact life.
An article "Back EMF and snubber" has a nice short discussion.
He has a section that I find very interesting: "I have seen the diode replaced with MOV (Metal Oxide Varistor) or a bipolar TVS (Transient Voltage Suppressor) but I am not fond of that since both types of devices seem to age over multiple breakdowns and finally stop working. In the case of the MOV it shorts across its leads when the back EMF exceeds its breakdown voltage, so it is no better than the diode. The TVS keeps a fixed voltage on breakdown and the time the current flows for is reduced."
** SAFETY WARNING **
This last comment leads to a safety warning.
I once worked on a piece of equipment damaged by a close lightning strike.
I turned the item on & off a few times & then opened it up to do some voltage probing.
It had been on for a short time & I hadn't touched probes to the circuit when the Surge Suppressor / Varistor / VDR exploded & embedded shrapnel in my face.
Some of it had to be dug out.
I had not not touched the circuit with my probes so the Surge Suppressor must have been just about ready to fail - a direct short circuit I guess.
I had noticed that SOME manufacturers cover the Surge Suppressor with heatshrink or even fibreglass sleeving but didn't think anything about it.
After that incident I always fitted heatshrink (a thicker type) to every Surge Suppressor that wasn't already covered before I powered up with covers off.
I did see one piece of equipment where part of an exploded Surge Suppressor penetrated a large electrolytic capacitor beside it but the heatshrink appeared to have stopped any major damage to a couple of other items I saw with exploded Surge Suppressors.
It is known that Surge Suppressors eventually fail & some of them have internal fuses but it appears that some do not have internal fuses or that protection is not always effective.
Take care & protect yourself against a possible shrapnel risk.
Regards,
Brian.
