# bi-directional motor using SPDT relay and diodes

I have a bi-directional motor that I am trying to control using an SPDT relay. It is only working in one direction when I remove the circuit for the opposite direction, and it is either shorting or just not starting no matter what I try. Below is my circuit. Any ideas on how to improve this so it works?

EDIT: I realize the diodes are drawn backwards here, but I did put them in the correct direction

• This can't possibly work because (with the diodes the other way around) the battery is basically shorted out on the bottom side of the motor Apr 28, 2022 at 12:26
• The only way something like it could possibly work is if you get the other side of the motor to stay at half the battery voltage, which requires you to build a circuit that's harder than using a DPDT relay Apr 28, 2022 at 12:27
• wouldn't the diodes prevent the current from flowing backwards to the battery? Apr 28, 2022 at 12:31
• The problem is that I don't have a DPDT relay; I do have access to resistors, transistors, resistors, diodes etc; is this something that can be done with a SPDT relay and other components? Apr 28, 2022 at 12:41
• If you have transistors, build an "H bridge" from 4 of them. Apr 28, 2022 at 13:52

Unfortunately this can't possibly work. The battery is basically shorted out on the bottom side of the motor.

The only way something like it could possibly work is if you get the other side of the motor to stay at half the battery voltage, without shorting it out. That's possible, but you'll only get half the motor's speed, and you'd have to either waste tons of power (wasting the battery really quickly) with a voltage divider, or make a more complicated circuit that's more complicated than getting a DPDT relay.

(noting that the diodes are actually the opposite way to how they're drawn in the picture) The diodes do not prevent current from flowing backwards to the battery. I don't understand what you mean by that. The diodes allow the current to flow in the same direction that it wants to flow anyway - i.e. from one side of the battery to the other - so they don't make any meaningful difference. If you turn either diode around it will block all the current and nothing at all will happen.

• Ok.. how come it only blocks the current in one direction but not the other? Apr 28, 2022 at 13:42
• @SamanthaCruz That's literally the point of a diode Apr 28, 2022 at 13:43
• ok, but how come when I turn the diode it doesn't prevent the current in the other direction, to clarify Apr 28, 2022 at 13:46
• @SamanthaCruz It does. But that's no use because now it blocks all the current coming from the battery. Apr 28, 2022 at 13:47
• @SamanthaCruz You are aware that current flows in loops, right? it doesn't come from the battery and go to the motor, rather it comes from the + side, goes through the circuit and goes to the - side. And if it can't go the whole way around, it won't go anywhere at all Apr 28, 2022 at 13:53

This is how I would do it: -

It requires two change-over relay contacts operating in tandem. You might also wish to apply back-emf quenching diodes too. Here's another version of the same circuit (from here)showing how the 12 volts get routed to one side and then the other: -

The relay contacts form, what is known as, a H-bridge and, a H bridge can also be made from transistors if you ever decided that might work better for you.