Freewheel diodes

A project log for SPDT16: 16-bits arithmetic unit with relays

Let's imagine I could get about 200 SPDT relays. Of course I would know what to do with them :-D But will 200 be enough ?

Yann Guidon / YGDESYann Guidon / YGDES 04/12/2016 at 15:210 Comments

Electromechanical relays use an electromagnet. An electromagnet is a high value inductor. And inductors generate high voltages (and worse) when their power is removed.

The traditional method is to use a diode acrosse the relay coil's pins. But I don't like this method, which amounts to heal instead of preventing. Well, in the case of relays, where the coils are driven by other relays, there's little to be done.

I favor dampening the pulse with a capacitor. But then it creates a new series of questions, such as

I'll settle for 100nF/63V for now.: high voltage, medium value, low ESR.

For the rest of the dampening, I figured that diodes are necessary but the XOR configuration precludes the use of the traditional diode or electrolytic capacitor because the polarity of the current in the coil can change.

I'll clamp the coils to the power rails instead, using normal diodes (I got a reel of high-voltage ones, which should be enough) but when drawing the schematic, I realised something funny :-)

Yes, it's a full-bridge rectifier ! This means I have found a new use for the bridges I have in stock :-D

So even my XORs will not create arcs and jam the whole neighbourhood ;-)