Question: I haven't followed this thread from the beginning but if you're using a switch to turn on a MOSFET to turn on a motor, why can't you just use the switch to turn on the motor?
Good question.
This is for a sailboat. The only source of power is 200 watts of solar. On long passages, every watt is critical.
Right now, automotive relays are being used. But they have drawbacks, including drawing a couple watts each while in use (about 2-3 watts each). There are 5 onboard right now. 15W is a significant portion of the available power which could be better used for lighting, safety equipment, communications, navigation, etc.
The control panels for many devices (pumps, refrigeration, winches, etc) are located sometimes 40ft return trip away from the device.
That means dealing with voltage loss. Big conductors aren't really an option, because of space and cost. Tinned marine wire is spendy, but also it's not good practice to have "large" currents running back and forth (chaffe, corrosion, space).
That's why automotive-type relays were traditionally used. However we're now in 2021 and there's surely a better way.
I believe with the right components it can be made much more efficient, solid state, reliable, affordable, and allow for drop-in replacement for automotive relays (which unfortunately means high-side switching).
A commercially available DC SSR would be another option, but the
efficient ones are very expensive. A plain-Jane DC SSR is cheap, but lacks in efficiency.
If I can figure out this circuit, it should be able to accomplish the same goal at a fraction of a Watt and very inexpensively. It'll replace the current relays, but it will also be used on every new electrical appliance I install. It'll mean the ability to use small conductors for the signal, which is super useful when routing wires.
As an aside... SSRs and Automotive relays also decouple/isolate. Every conductor in this implementation is fused, including the signal/control wire. So, it's not a critical feature. Nonetheless, I may consider adding an optocoupler once I get this figured out.
There are a lot of DIY SSRs out there, but none of the ones I've found would work as a drop-in replacement for this application.