Relay and snubbers

Sometimes we think, "it is just an innocent relay", why add a freewheeling diode or some other snubber circuitry?

Well, take a look. I took those pics of a common 12V relay and a mechanical switch to toggle it.

I used a VC-6545, to measure. Storage mode and Norm. triggering.

This is the pure circuit, without any snubber:


Behold that you have a lot of spikes, the peak voltage is 284V, and the total "spiking" time, is about 150 µs.

Now just imagine a BC548 with those spikes across the collector (Vceo = 30V).
Or worst... A low voltage MOSFET would have its gate's oxide destroyed, if there was a series of relay switching events.

It is important to say that I was using a mechanical switch, so I'd have power loss on the contacts. With a transistor the spiking would have much more power. And more power means more voltage.


When I add a snubber capacitor (I used a 100nF ceramic), this is what it looks like:


Much more well behaved. The peak value is 160V.
The capacitor amortized the high frequency spikes, leaving a clean signal.
But still, we have a high voltage peak. That would not be a problem to a 200V rated device, but it is pretty high for a general purpose transistor/mosfet.

That's why in kettering ignition system there is a snubber capacitor with the contact breaker, to prevent contact erosion and capacitors can withstand high power spikes.


This is what happens when there is a freewheeling diode (used a 1N4148):


It is beaultiful, just a 12V step. No spikes. Of course if there was a high power application, a low power diode would fry. But for 12V relays that drains 100 mA, they are good.

There is lots of theory why that happens. But grossly speaking:
When you cut the voltage across a inductor, the stored EMF generates a flyback current with the same direction as the feeding current. (If the current flowed from A to B, the flyback still flows from A to B), therefore, a opposing voltage will build up.

That's why a freewheeling diode works well. In a normal operation, it is reverse biased, when there is flyback currents, the diode is direct biased, so that the power supply sinks the high voltage spikes.

Conclusions:
- No snubber: High voltage spikes.
- Capacitor snubbing: We still have a bit of high voltage, but the signal becomes well behaved. For high power application the capaciotor is one option.
- Freewheeling diode: The best results, but if the flyback current is too high, a high power diode would be required.
- Another snubber option would be MOVs (Metal Oxide Varistors).