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Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
I have observed a curious phenomenon while looking at a relay circuit. The assembly is very simple: A voltage source, a mechanical switch, and a relay coil in series. A 12 V supply, a micro-switch, and a 12 V relay, for example. Observe in an oscilloscope the voltage over the relay coil. It's well known that when the switch opens a large voltage pulse appears over the relay coil, caused by the sudden interruption of the relay current, and a spark occurs at the switch. The oscilloscope shows this pulse clearly, or several pulses in series, negative if a grounded positive supply is used. The curious observation is that the pulses never exceed ~300 V. The reason appears to be "Paschen's law", that says that in air there is a minimum voltage that can cause a spark, at about 300 V. So, when the switch opens interrupting the current the voltage over the open switch, that is still opening, and with the contacts at a small distance, must get close to 300 V before a spark limits the voltage. In most cases (I tried several relays) what is observed is a series of ~300 V pulses, but in others, when the current is larger, a long pulse limited at ~300 V appears. Smaller pulses sometimes appear too, but they are probably due to contact bounce. A capacitor in parallel with the switch reduces the voltage rise rate, but up to the point where there is no more spark, the voltage limit appears to be the same ~300 V in the tested relays.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
assuming that your 'scope probe is not significantly loading the circuit, this is the reason for R.C snubbers across contacts to limit the voltage gradient to allow time for contact separation distance vs. time capacitor sets dV/dt (=Icoil/C), resistor limits cap pulse current when contacts close. (and absorbs some of the back-emf, and during contact bounce at closing) (I've been considering reed switches in a long life motor) any switch has a maximum dV/dt on turn-off.
I see what you mean about the 300v, a good guide, the opening contacts get an equivalent 300V 'head start' in the race of distance vs. voltage rise.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
there is also a 'minimum distance' at which the gap can artc. Too close and the voltage required is higher. The points must open to this minimum distance before the arc occurs'
There are cases, due to Paschen's law, where the discharge path is not the shortest distance, due to this phenomenon.
Registered Member #2390
Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381
This may not be the case here but, some scopes can only measure 300v before the signal is internally clipped. If your scope does do that, try a scope with a 600v input and see if your voltage goes over 300. I watched an engineer try to take measurements on a large dc bus with a portable scope meter for 3 days before he realized it was not rated to read what he was looking for.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
Really, I have to verify if the 10x probe used was rated for more than 300 V. But I looked at the current coming from the power supply using a current transformer, and the spark current could always be seen, indicating that the sparking was at the switch.
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Curious experiment with a relay
