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Is it possible to trigger a 3V relay with a 3V signal voltage carrying no current?

I want to be able to operate a 12V ebike lighting circuit via the brake lever which also operates the motor cut off for the ebike.

The relay operates fine when connected to a 3V battery.

enter image description here

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  • \$\begingroup\$ As already answered, you can't use absolutely no current, but you can certainly minimise the current. My only query is why you need no current to be drawn on the signal line? \$\endgroup\$
    – Joe Mills
    Commented Jan 31 at 16:51

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No, but kind of yes.

Standard electromechanical relays physically require current to function. But that current doesn't have to come from your 3 V signal--you can use it to switch a MOSFET to connect some other power supply to the relay.

schematic

simulate this circuit – Schematic created using CircuitLab

With a configuration like this, and a suitably chosen MOSFET, you can use the current from the 3 V supply to actuate the relay, while requiring only enough current from the signal to drive a 10 kΩ resistor (which you can even omit, if you can be sure the signal will never be left floating).

You can also use a 12 V relay, since you definitely have a 12 V supply available, as long as the MOSFET is chosen accordingly.

The DMN2058 shown here will definitely work for any 3 V or 12 V relay, but depending on the relay's current requirements you may also be able to use a cheaper one like a DMN63D8 or NX138/BSS138; you can find information on choosing a MOSFET elsewhere on this site so I won't go into any more detail than that. Even the DMN2058 is still only $0.38 right now, so "cheaper" literally only means saving a handful of pennies.

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  • \$\begingroup\$ Thanks so much for the quick reply. If I was to use a 12v relay like the one pictured do you know what MOSFET would be required as I am a newbie to this. \$\endgroup\$
    – Jasperhound UK
    Commented Jan 31 at 17:19
  • \$\begingroup\$ @JasperhoundUK Any of the same MOSFETs I mentioned would work at 12 V, just check the current ratings. \$\endgroup\$
    – Hearth
    Commented Jan 31 at 20:40

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