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I'm a mech engineer trying to use 2 vandal resistant, SPDT, latching switches to control a power distribution module. The module has an internal bi-stable relay to turn the outputs on and off. The trigger wire for the module is fed 12v to turn on and then turns off when grounded. I need to use 2 switches for activation (1 inside the vehicle, 1 outside) and don't want to use rocker switches (although i wouldn't be asking this question if i did).

My current setup is using the 2 latching vandal switches in a conventional 3-way setup which works fine to activate the module but i can't figure out how to get the circuit to pull down the trigger to ground when I want to shut it off. I was thinking a motor reversing circuit with relays but was hoping for a simpler, and more space saving solution. Any suggestions would be great.

Here's the datasheet for the module that shows the recommended control wiring: https://www.littelfuse.com/media?resourcetype=datasheets&itemid=99c7c610-ef0f-487c-a3ff-aa85e05d47be&filename=sl-series-pdm-datasheet

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  • \$\begingroup\$ Does the power distribution module have any outputs that are always on when the module is? If so, use that to control a SPDT relay which feeds the switches with either 12V or ground. \$\endgroup\$
    – Klas-Kenny
    Commented May 4, 2022 at 11:55
  • \$\begingroup\$ The module is fed main battery power and has 3 high amp, switched outputs, 4 30A max un-switched outputs and a plug for the bi-stable relay control. Contacts in the plug are a constant power (from main battery input), ground, led pull down when relay is closed and the trigger wire. The hot lead in the connector is there to use for relay activation, so i'm using that to power the trigger wire through the switch assembly. That trigger wire needs to be grounded though in order to move the relay to the off position. \$\endgroup\$
    – Nuttcase21
    Commented May 4, 2022 at 12:43
  • \$\begingroup\$ Here is the data sheet for the module showing the recommended pinout for the control: littelfuse.com/… \$\endgroup\$
    – Nuttcase21
    Commented May 4, 2022 at 12:56
  • \$\begingroup\$ Are your switches latching or momentary? They're exactly the opposite so they can't be both. And do you need to be able to switch the load on and off from both switches? \$\endgroup\$
    – Finbarr
    Commented May 4, 2022 at 19:08
  • \$\begingroup\$ Now that i think about it, you're right, they're latching, apologies. I need to be able to switch the load from both, yes. \$\endgroup\$
    – Nuttcase21
    Commented May 4, 2022 at 19:24

1 Answer 1

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The simplest way is probably to use an SPDT relay controlled by the switches in the same way you'd control a two-way light:

schematic

simulate this circuit – Schematic created using CircuitLab

The downside is that the relay will be powered whenever you want the output to be powered, so you need to use one with low enough current consumption not to add too much extra drain on the battery.

To design an electronic version that could use less power than a relay, you'd need to know how much current the trigger input needed to work. I can't see that information in the datasheet so it would be a question for the manufacturer.

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  • \$\begingroup\$ Thank you much! I did measure the trigger current and it's fairly low, 0.004A. Any reason I couldn't use a reed relay for this? \$\endgroup\$
    – Nuttcase21
    Commented May 5, 2022 at 10:52
  • \$\begingroup\$ 4mA is practically nothing, but as ever check the datasheet for the relay you intend to use. I'm curious why the PDM says to use 1.5mm cable for the trigger connection as even 0.5mm would be plenty if it's just 4mA. \$\endgroup\$
    – Finbarr
    Commented May 5, 2022 at 12:24

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