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jocanon said:you get smoke coming out of the turned off one
Here is one solution, although it got a little complicated. But it has the bells and whistles of fault indicator etc.
Here's how it works and a couple of questions?
This would replace your on off switch box for the 2 supplies. It has a momentary type rocker switch that in one direction turns on the ac to the supplies. So you press it and release it and the supplies turn on. If the combined voltage is over about 19 volts the relay turns on and keeps the ac going to the supplies when you release the switch and the green power on light is on. To turn it off in normal operation you push the switch in the opposite direction and it shuts off. If there is a fault and only one supply comes on when you push the switch to start they will power on but not stay on when the switch is released. While the switch is being held the red fault light will be on. If the supply fails in operation the red light will blink briefly and the supplies power down. It would be a pain to build 100's of these on a breadboard so if you go this way a little pcb might be in order though it will probably cost more than the rest of the parts. There is probably $6 in parts.
The 2 relay idea is simple. You could turn it on the same way, but I haven't found a switch type to turn it off. The diode is probably a good idea too, but it needs a pretty big heat sink or it will smoke.
Just to make sure. You still have 12 volts with one supply bad?