High amperage motor control

Thread Starter


Joined May 30, 2011
I'm building a project using an old 19.2 craftsman cordless drill as the driver.

I want to be able to switch the directions of the motor using a PIC controller. In the past I have used a DPDT relay to flip the + and - going into the motor. In this case I would use a 24v 20a DPDT relay.

Is this the best way to go without over-engineering it?

I know enough to protect the circuit from EMF feedback and to use a transistor to actually switch the relay.

Thanks in advance,


Joined Jun 22, 2012
A simple NPN transistor and DPDT relay will do for the output on the pic.

So Output High = relay on = Motor forward
Output Low = relay off = Motor reverse


Joined Oct 2, 2009
I would choose to interrupt the power and wait for the motor to come to a halt before reversing the voltage. Hence I would add a second ON/OFF relay to control the supply voltage.

Thread Starter


Joined May 30, 2011
Okay, great, I will pursue the relays.
I am going a slightly different direction, no PIC involved. The user will flip a DPDT switch to have the motor move the ACME nut up and down the drive screw, so timing should not be an issue.
I'll have switches at each end to break the circuit so that the motor stops.

HELP: I just blew up the "6A4-MIC DIODE GUARD JUNCTION STANDARD RECTIFIER" that is part of the cordless drill trigger assembly. (I was pulling it without the motor attached and apparently it didn't like that.) So is there a part from Radio Shack that I can replace it with?



Joined Oct 3, 2010