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So I got some HSR412s and connected them to my Christmas lights in a serial circuit. It worked great and as expected except for one caveat.... It was getting really hot. I realized I was putting double the rated current in and decided to go with a parallel circuit which could take more current.

I hooked up the wires like the picture said and when I plugged in the lights, they turned on, which is not what they should do when connected to these relays. I also tested to see if a parallel connection would turn the relay into the nominally open type, but that also was incorrect. I had my 6 and 4 pins connected to positive power in a Y split and had my 5 connected to the lights. I may have a misunderstanding of how a parallel circuit works but have no idea where I went wrong, and why the relays aren't working under my wiring. The datasheet is here.

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The parallel configuration can take more current, but look closely and see that it is only applicable to DC. By "christmas lights", I expect you mean something intended to be plugged into normal house power, which would be AC.

A MOSFET can be a switch when used in forward mode (drain to source positive for N channel). However, these devices have a diode from source to drain as a byproduct of their construction. The series case puts the two diodes back to back, so one FET is always blocking when off. The parallel configuration is only for one power polarity since the diodes will conduct in the other direction. AC power will be on for at least every other half cycle with the parallel configuration.

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  • \$\begingroup\$ Ah. Didn't know that this was an DC only configuration. Well, time to check out some better relays then! :) \$\endgroup\$ – Kyle Sep 30 '11 at 1:57
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    \$\begingroup\$ @Kyle: The diagrams show a AC or DC source used in the series case, but only a DC source in the parallel case. I expect this was elaborated on in the datasheet too. \$\endgroup\$ – Olin Lathrop Sep 30 '11 at 11:51
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You shouldn't be using solid state relays in parallel. They work great in a series configuration to bring up the working voltage, but they do not work in solid state for added amperage like a conventional relay would. One will trigger before the other and take the current, at which point the other will not trigger. The first one will eventually fail due to the high current and the next one will trigger, etc.

The following is a snippet from the manufacturer Crydom:

Placing AC relays in parallel to increase the load current of a circuit is unfortunately not possible. The relays will not turn on simultaneously, and thus one relay would carry the entire current at turn-on. As the AC relays require a voltage across the output terminals to function correctly the second relay would then not switch on as the voltage is now dropped across the load, all it now “sees” is the on state voltage drop of the first relay. The first relay will carry on conducting the full load until it fails, at which point the second will turn on, also to then fail at some point due to overcurrent.

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