Control two circuits (invert) from single switch

Question

I'm not an electrical engineer, so please excuse the incorrect terminology in this post. I have 2 circuits/devices that are powered and controlled independently. Some info on each circuit which may or may not help:

• Circuit A: contains an appliance switch (rocker switch) which controls the AC supply to a device
• Circuit B: part of a device powered by a DC adapter.

I would like to disable Circuit B when Circuit A is switched on. So the way I envisage things working is that there is something that goes from the Switch of Circuit A to Circuit B, along with something to change the AC power to DC. Can someone please help me work out what I need?

From my basic research, I think I need a resistor (based on How to create a circuit to turn off LED when switch is "ON" and turn on when "OFF").

Thanks in advance!

Answer 1

If I understand your question then you have two easy options.

^{simulate this circuit – Schematic created using CircuitLab}

• (a) If you have a 2-way switch you can arrange the wiring so that one appliance or the other is on at any one time.
• (b) If the switch is not 2-way then you can solve the problem with the addition of a relay.
  • When SW2 is open Applicance 1 is off and RLY1 is off. Appliance B is powered through the normally closed (NC) contact of RLY1. (This would feed your AC/DC adaptor for Appliance B.)
  • When SW2 is closed Appliance 1 turns on and RLY1 is energised. The NC contact opens and power is cut to Appliance B.

You need to choose a relay with an AC coil rated for your mains voltage. The contacts need to be rated for the amperes drawn by Appliance B's power supply.

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The technique used in the linked question works OK for an LED which has a resistor in its path in normal operation. It is not a suitable technique for this problem.

Answer 2

A resistor is not the correct tool for this purpose. I can't even think of any way you could use one for it.

There are two ways of doing this that come to mind immediately. Option 1 is to use a relay, like so:

^{simulate this circuit – Schematic created using CircuitLab}

Using a relay with an AC mains-voltage coil will mean that circuit B is turned off whenever circuit A is turned on. The relay doesn't have to be SPDT; it could also be SPST-NC. This circuit has a slight delay of a few milliseconds between turning circuit A on and turning circuit B off. You could also use an SSR, which may give you faster response time to circuit A being turned on or off.

Option 2 is to replace the power switch in circuit A with an SPDT switch, like so:

^{simulate this circuit}

Here, the same switch that turns circuit A on also cuts power to circuit B's power supply. You may want to use a three-position (ON-OFF-ON or OFF-ON-ON) switch, so that it's possible to have both devices turned off at the same time; a two-position switch will mean that one of the two will always be powered. This circuit will cut power to one device before applying power to the other, but a potential downside is that it necessarily cuts power to the entire power supply. If you need other things to stay active that share a power supply with circuit B (which I suspect may be the case as you said that circuit B was only "part" of a device), this won't work for you.

Answer 3

Normally a regulator with DC Ok detection is best to enable its output until Ok threshold is with 10% or less is used.

Otherwise a Vac detect with a time delay to enable Vdc out can be made with a known time delay can be defined.