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I have a microbit setup with a relay controlled by pin 0. The relay is being used to turn on a simple motor. Everything works fine when the motor is connected to a separate 3v power source, but if I try use the gnd and 3v outputs of the microbit to power the motor the microbit immediately resets once the relay switches on.

These are my connections to the relay: Microbit 3v > relay DC+ Microbit gnd > relay DC- Microbit p0 > relay IN

This fails: Microbit 3v > relay NC > relay com > motor > microbit gnd

This works: 3v battery + > relay NC > relay com > motor > 3v battery -

I have tried adding a resistor in the path, but the smallest I have is 170 ohm which is too much to power the motor.

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Sounds like you might be trying to draw too much current through the Micro:bit itself (through its 3V Pin). You could try using the same powersource wired to both Micro:Bit AND the motor (in parallel) - with the relay on the positive motor wire.

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    \$\begingroup\$ Indeed, routing either power or ground to a motor through an MCU board is fundamentally the wrong idea. \$\endgroup\$ – Chris Stratton Jul 27 '18 at 4:33

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