Electrical sizing question (Solved)


OK, finishing up my little project and have a question about how to determine what kind of diode I need, and whether I need two or four of them.

Basically I have a load at 100W (max 125W) / 12 V that I need to run either forward or backward. I figured I can do this with a set of relays and wiring as follows:

What I need to know is:

  1. What kind of diodes do I need at A?
  2. Do I need a second set of the same diodes at B?



Were it me, I would use an H-Bridge:

The only thing to mention is make sure you don’t have the vertical pairs on at the same time (S1 and S2 or S3 and S4) or you are shorting your supply.


After looking at this for a bit I realized this is exactly how a car window motor is controlled. I hooked up either end of the load to the common terminals of the relays, and the NC relay contacts to ground, and the NO contacts to +12… this way when I switch one relay on, it provides the +12 via the common to the load, which exits out the common / NC to ground on the other relay… and vice versa.

Just had to write it out and let it mill about in a different part of my brain one I looked at the picture :stuck_out_tongue_winking_eye::exploding_head:

No diodes necessary!!!


FWIW, that’s the same circuit that a “Useless machine” uses with a small DC motor as the load. A DPDT switch is wired such that it move a motor clockwise in one position and counterclockwise in the other. Human pushes switch to cause a “hand” to move out of box. When it reaches limit, the hand flips switch to move it back into the box.