I have two sensors (represented in the schematic below by a switch) connected to a light. If one of the sensors trips, I would like the light to switch on.

The sensors and light are directly on AC, and there is where my problem is. In the DC alternative (on the right), I would put diodes behind my switches to prevent a short circuit when two switches are on at the same time. But on AC, diodes won't work. So how do I prevent a short circuit when two switches are on at the same time in an AC setup like on the left? Cheers,



  • 2
    \$\begingroup\$ Don't see how the diodes are helping you in any way in the DC circuit. When both the switches are on the current would simply divide itself equally. You will not the shorting the source. \$\endgroup\$ – Sada93 May 15 '15 at 1:29

If the switches are directly in parallel as shown in the left drawing, there is no problem of a short circuit, as both switches are connecting the same two points together.

If the switches were supplied by separate power sources, there would be a problem. That could be solved by having one switch operate a relay, with the relay's contacts in parallel with the second switch.

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  • \$\begingroup\$ I don't know why my simulation software gave an error when both switches were closed on the left side, but indeed it doesn't seem like there is a problem, as there is indeed only one single power source for both switches. \$\endgroup\$ – Dolf Andringa May 15 '15 at 3:07
  • \$\begingroup\$ @DolfAndringa probably gave an error because its solver can't cope with having wire loops with no resistance, you can usually solve that by adding a 1mOhm resistor or smaller. \$\endgroup\$ – Tom Carpenter May 15 '15 at 4:28

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