Control one relay via multiple 12 V DC signals

Question

I'm seeking help designing a circuit wherein we would have multiple 12 V DC signal inputs, with the goal of:

If one or more 12 V signals is ON, the main relay is energised and closed.

Basically, we have a vent fan that serves multiple appliances. When any appliance reaches a specified temperature, it sends out a continuous 12 VDC signal. This is intended to activate a relay to run a ventilation fan.

Because of the number of these appliances in use, we use one large fan, ducted to all appliances. We are manually switching the fan on and off at this time but I am aware of the 12 V heat-triggered output on each appliance and would like to take advantage of it (especially so the fan doesn't run all weekend when not needed).

What exceeds my knowledge is how to design the circuit with an "any or all" function, where either, any one 12 V signal by itself, or any number of 12 V signals at once, would turn on a relay.

(total # of collectively vented appliances, each with its own 12 V sender, is 7)

Each signal is sent via a two-wire DC lead (pos & neg).

Thanks in advance for any comments/thoughts.

Edit: the 12v signals need to be isolated from one another, so the most basic way I've thought of doing this is to have 7 "pilot" relays, one for each incoming 12v signal, and those relays would be switching a single "bus" 115v current that subsequently activates the main relay. But I am wondering if there's a better/more efficient design that would work.

Answer 1

Since you need isolation, you will need a relay for each appliance.

Even if you didn't strictly need the isolation, it would be recommended since it might be hard to safely share the same voltage (ground) reference among all 7 appliances.

I would locate the individual relays at each appliance and only bring the parallel combination of the relay contact outputs to the fan control circuit. That way you won't have to worry about a voltage drop in the 12V circuits due to wire length.

Here's a schematic for two appliances. Expand for more.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

When any appliance reaches a specified temperature, it sends out a continuous 12 VDC signal. This is intended to activate a relay to run a ventilation fan.

Then those 12 V signals are powerful enough to power the relay's coil directly so then we can use a simple circuit like this:

^{simulate this circuit – Schematic created using CircuitLab}

The diodes form a simple "OR" function meaning that if on one (or more) inputs a 12 V is present, the fan switch will close.

The diodes connect to the 12 V + of each wire.

All the - (negative) wires are connected together and connected to the relay as shown. The ground symbol is just a virtual thing, there's no need to actually connect to ground.

The diodes also prevent that input2 and input3 are pulled up to 12 V when only input1 is receiving 12 V.

Note how my circuit doesn't need any transistors.

Update:

Edit: the 12v signals need to be isolated from one another,

Thanks for mentioning that after me writing this answer.

Then what I propose cannot be done, you need multiple relays in parallel, see Theodore's answer.

Answer 3

The critical question: do the three 12 V signals share a common Ground (12 V return, -12 V, etc.)?

And if so, do they share that ground with whatever supplies power to the relay coil?

If you are lucky, the various 12 V signals and power sources are either already common-grounded, or floating so all of their negatives can be connected together. If so, then Bimpel's answer with nothing but 3 diodes should work fine. Do you have a make/part number, datasheet link, or other information about the relay?

Update:

It sounds like the three control signals are floating, so their commons might be joinable. If that makes you nervous, then your are looking at four ground-isolated circuits, the three control signals and the relay power supply. This is the (relatively) worst case.

One way to solve it is with three opto-isolators. Depending on the opto and fan relay choices, they might be able to drive the relay coil directly. Otherwise, it's three opto output transistors in parallel, turning on a single driver transistor.

Answer 4

Something like this circuit may work, it uses mosfets on the low side of the relay to switch on the relay, if any of the mosfets are 'on' the relay will be on. If both of the mosfets are on, the relay will also be on/closed (if you select a normally closed relay, most relays come in normally open and normally closed versions.).

^{simulate this circuit – Schematic created using CircuitLab}

The mosfets must be sized so that whatever switching voltage you use for their gate is high enough to turn the mosfets on, but not higher then the gates rated voltage. R1 might be necessary to get the right amout of current for the relay (but any 12V relay should work)

Answer 5

Edit: This answer was given before the isolation requirement was given.

If you need to use a transistor to energize a relay, you can do this:

^{simulate this circuit – Schematic created using CircuitLab}