Using the electronics from a servo to drive a gearhed motor

As we all know, a hobby servo consists mainly from 3 section.

  1. The motor
  2. The servo control board
  3. The feed back pot.

Is there anyway to rip the electronics and the pot from a hobby servo and connect a regular DC gearhead motor to these. Can an external H-bridge be connected directly to the motor contact or does it have to be connected before the on-board H-bridge that the servo controller has.


The details of what you can do will depend on your particular servo and H-bridge. For instance, if your H-bridge has a dedicated PWM input pin, you can’t just get that from one of the two motor outputs. Also, depending on your mechanical setup, the servo might not be tuned correctly for good operation.

If by H-bridge you mean just the four discrete transistors, you might have issues there, too, since a single line per half-bridge doesn’t give you control to limit shoot-through, and the signal might be too small (voltage-wise) to fully turn on or turn off your transistors.

- Jan

I’ve actually had decent luck position-controlling gearhead motors with harvested servo electronics and potentiometers. They’re not a great solution, and can be a bit jittery around the home position, but if your output shaft speed is going to be really slow (like slower than the servo the board came out of) it should work alright.

I did run into some problems delivering enough current to larger motors, but I discovered that the big Hitec servos (HS-705 and HS-805 in particular) have four big TO-220 package discrete transistors powering the motor. They actually use two pairs of Fairchild Semiconductor KSA1010’s (PNP) and KSC2334’s (NPN). I’m not sure why this is, but both types can handle 7 amps (Yowza!). They’re also good up to 100V, so you could theoretically desolder the collector pins of the four and hook them up to a higher voltage to drive your big honking gearmotors.

I’m not sure what you’re thinking of doing with that second h-bridge, but maybe this way you won’t need it after all. What’s your project?

The project I working at is this

Since I could not find a servo with a worm gear I decided to scrap the servo and go with a gearhead motor with a 33rpm speed. Do you think that this can be linked to a hobby servos in-built control electronics with no problem ?
I will be trying to use the servo electronics from a Hitec HS-645MG servo

motor specs:
Operation voltage 12VDC
No Load Current 140mA
Max Current 440mA

Oh, hi. I totally didn’t link your three threads together. Sorry this project hasn’t worked out for you yet.

The 645MG is a great little servo, but its published specs only go up to 6V, and a max current of 450mA. I’ve run some off 7.2v for a while and they wore out mechanically before their electronics gave out. The problem is that the H-bridges in Hitec servos have their own internal brand markings on them, and I have never been able to find published specs for them, but I imagine something would fail at 12V (you could always try and see). I would try running the gearmotor off the servo electronics at 6V first to see if it’s good enough.

Now, for less than the price of a single HS-645MG, you could get one of the afore-mentioned HS-805BB, with the discrete driving transistors (with the original commercial markings that you can look up data sheets for). Again, I wouldn’t try running one of these off of the straight 12V from your car, but you could supply just the big transistors with 12V, and the rest of the electronics with a 5V from a small regulator, and you’d be set.