How do I decide what current rating these Zeners need to be?

[Chris Wilson]

OK, the easy way is to ask the guy who designed the circuit. That is quite possible, I am in contact with him, but I won't learn anything much with that approach. Instead, would someone mind giving a quick run through of how I myself should assess this given just the schematic please? I am able to measure relay current draw, and get semiconductor data sheets for the devices used. Thanks.

[Zero999]

Use Ohm's law to calculate the power, P = VI

[Andy Watson]

I can only see D8 as taking any appreciable current, and it only does so when the relays turn off. Worst case would be 2 x 35mA. The remaining zeners supply negligible bias currents to control the MOSFETs, except for Q4 which appears to float!

Edit: What exactly are RL2 and RL3? It's not clear from the web whether these are solid state, or reed relays. If it's the former, you don't need D8. In either case, are the relays actually needed? They appear to be driving another relay.

[Chris Wilson]

I too thought there seemed an abundance of relays, but the designer claims it all works. I wasn't going to argue, as he is far more qualified than I with this RL2 and RL3 are mercury wetted reed relays, the spec sheet is attached.  Thanks!