I just installed a solid state relay as the main power switch for a robot I'm working on. The robot is controlled by a Windows 8 tablet and I thought it would be cool to use the tablet to turn the bot on and off. Not long after I started using the relay I realized that the battery wasn't charging (the charger was installed after the relay). Once I connected the charger directly to the battery it started charging – this means that the relay was acting as a large diode. Is this an effect of all Solid State Relays? If so, this could be a very large problem for some applications.
Do all Solid State Relays function as diodes
solid-state-relay
Related Solutions
You need to monitor the current using a low resistance in series with the load (bulb). Or possibly, the drop across the relay can be used to calculate the amount of current taken by the load.
Once the drop exceeds a threshold, the relay should be turned off. Make sure that the detection mechanism and relay is fast enough to allow for abrupt termination.
Otherwise, go for a fast blowing fuse in a holder; solid state stuff has low thermal mass. Replace the fuse while replacing the bulb.
You could combine multiple SSRs to do this. Two form B and two form A will give you a DPDT SSR. Cost would be fairly high (about $20 in singles) but this is really a weird set of requirements. Switching will be slow.
For example, two LCB710 and two CLA230 (IXYS) would handle 700mA.
You could also buy PV optoisolators and use them to drive back-to-back MOSFETs for each switch. Since the former come in pairs you'd end up with 8 parts (plus maybe 4 resistors). Again, slow switching.
Edit: Since you imply in the comment below that there is no requirement for power-off normally closed, the simplest method is probably to use two dual form A SSRs such as TLP222A-2.
Again you could use dual MOSFETs and PV drivers, but I don't see much point in that since the current requirements (and presumably your voltage requirements) are met by common parts.
Best Answer
Some SSRs do work like that, some don't. If the output leads are marked with a polarity, then it matters. There are "AC" SSRs that are two back to back light-activated SCRs. In that case the polarity doesn't matter.
One issue with SSRs is that they can have significant voltage drops, from a few 100 mV to over a volt. You don't want that between the charger and the battery anyway. Even if this were a mechanical relay, you still wouldn't want it between the charger and the battery. You'd then only be able to charge when the unit is on, which makes little sense.