Torn between SSR alternatives

A project log for J1772 Hydra

Charge two electric vehicles at once

nick-sayerNick Sayer 02/26/2019 at 19:030 Comments

I'm sort of torn between two SSR alternatives.

First, there's the CPC1972GSTR. This was the first one I tried, and it had a problem with reliably turning off. Adding a snubber (properly) solved this problem, though. On the one hand, it has a zero-cross circuit, which is strictly speaking unnecessary for this application and is theoretically problematic for an inductive load (though, again, in practice, with a proper snubber, it still works). On the plus side, it is physically smaller (6 pins instead of 7 - really 8), and has a blocking voltage spec of 800 volts, which is comfortably above the peak-to-peak voltage for 240 VAC and above the MOV's maximum clamp voltage. On the minus side, it's about 35¢ more expensive.

The alternative is the AQH0223A. It's absolute maximum repetitive peak off-state voltage is 600V, which concerns me. This is not only lower than a 240VAC peak-to-peak voltage, it's also lower than the maximum clamp voltage of the MOVs (though that's less concerning as that voltage would be non-repetitive). The only reason I could guess that this is working is that the load represents a voltage drop so that the switch isn't seeing the full peak voltage (but I can't really accept that fully as there would be very little current flowing with the switch off). It's also a physically larger package (but the creepage distance is not any larger - both relays have a .2" BIS separation between the switching pins, though the CPC1972GSTR does have an NC pin between that reduces the total creepage a little), but lacks the zero-cross circuit and is cheaper.

UPDATE: I think my concern about the p-p AC voltage is incorrect. The peak voltage the SSR will see is the peak, not peak-to-peak. So 339 V, which is well under 600V (and the MOV will prevent transient excursions higher than that).

So what’s left is that the CPC1972GSTR has the interloping pin that reduces the creepage distance and a higher cost.