Maker Pro
Maker Pro

How can I boost the power of my junk box H-bridges?


Owen Lawrence

Jan 1, 1970
Hi. I want to drive a bipolar stepper motor marked 1.4V 3.8A. I
have built a PIC16F628 based circuit to drive two H-bridges made up
of 2N3904/06 transistors. Clearly they will not supply enough
power. The PCB is already made, so I won't be changing the
transistors on it.

What are my options for boosting the 5V low current output of the
H-bridges? (There are 1K resistors to the bases of all the
transistors. If I lower them, how low can I go before I start to
cause problems with the PIC?) I'm thinking of rummaging around for
four relays, and using one diode per coil to choose which coil gets
energized, based on the current direction. I'm worried about speed
(1.8 degrees per step!), and burning out the relay contacts, but I'd
rather get results at all.

Would I be able to use the same diode trick with power
transistors? If so, which ones would you recommend? Note: So far I
spent only $1, which was for the super glossy paper I used for the
toner transfer for etching the PCB. Everything else came from my
junkbox, so I'm loathe to spend any (significant) coin. It's a
matter of pride, you understand. :)

Assuming I zipped the right files, you can download the Eagle
project from

This circuit can drive small toy motors, but not this big stepper
motor (it just "clicks" but doesn't move).

Thanks for any advice.

- Owen -