Stepper Motors, Part V: Taming the Wild Voltage

With a desired motor voltage of ~2V and the bridge feeding the coils ~11V, the motor draws way more current than it needs, and the FETs get really hot. In fact, the PC power supply I’m using, which is rated for 7.5A of 12V, can’t keep up with the load. It makes sense:

11.4V / .85Ω ≅ 13.4A

And that’s per coil, so almost 27A draw for the two-coil motor. When I measured the voltage drop across one coil, it was actually only a little over 3V. I had known I was going to want to drop the bridge voltage so the motor coils didn’t get the full 11.4V, and I had thought about doing it with diodes or resistors. Before going any further (laying out a PCB), it was obvously time to deal with the overvoltage; so last night, I sat down to do some calculations.

Dropping Voltage with Diodes

I don’t know why, but my first inclination was to try to drop the voltage with a string of diodes. Given

bridge voltage: 11.4V
desired motor voltage: ~2V


required drop: ~9.4V

With a .6V drop for small-signal diodes:

drop per diode: .6V
diodes required: 9.4V / (.6V / diode) ≅ 15.7 diodes

And assuming I was willing to string together 15 diodes in each direction, for each coil of each motor (uh, 15 * 2 * 2 * 3 = 180 diodes in the milling machine just for dropping voltage), how close would that get?

drop for 15 diodes: 15 diodes (.6V / diode) = 9V
motor voltage with 15 diodes: 11.4V – 9V = 2.4V
current with 15 diodes: 2.4V / .85Ω ≅ 2.8A

Granted, the rectifiers I’d actually use have a voltage drop more like 1V to 1.25V per diode; but that’s still 9 * 2 * 2 * 3 = 108 diodes in the project. Way too wasteful, and way too many.

Dropping Voltage with Resistors

It seems that Tom McGuire’s suggestion to use power resistors is the right way to go. Big sand resistors can dissipate a lot of power, but how much would be required? From above,

required drop: ~9.4V

And given

coil current: 2.5A

then each resistor would need to be capable of

9.4V * 2.5A ≅ 23.5W

That’s a pretty hefty resistor!

I looked through my parts bin, but most of my power resistors have a much higher resistance. I also browsed power resistors at All Electronics and found a few ceramics rated for 25W–but I didn’t want to have to come up with enough stuff to place a minimum (reasonable) order, wait for it to arrive, and then take a chance of it not meeting my needs. As it turns out, Radio Shack carries an 8Ω 20W resistor, so today I trotted on down and picked up a couple.

I hooked one in series with each motor coil and ran through my barrage of tests. After reconnecting a loose signal wire from the controller (oopsie!), I found that the motor was much easier to stop by gripping it with my fingertips, it had a much lower top speed without losing steps, and the coil voltage was only a little over 1V.

Dropping Voltage with the Right Resistors

At no point in the resistor discussion do I describe figuring out exactly what resistance is appropriate, because I was too impatient to get on with prototyping to be bothered with calculating. Sigh.

Okay, so I want ~9.3V dropped across the resistor and ~2.1V dropped across the motor; and the motor coil is ~.85Ω.

9.3V / 2.1V = R / .85Ω
R = .85Ω (9.3V / 2.1V) ≅ 3.8Ω

So I was embarassingly far off with my 8Ω resistors, which explains the low voltage and poor performance when I tested them.

Hmmm . . . 3.8Ω is really close to half of 8Ω. Now I’m tempted to go back and buy two more of the 8Ω resistors, to test in parallel.

On the other hand, it’d be nice to try a range of values centered around 3.8Ω, to graph the motor performance and find whether the curve has a knee in that vicinity. So now I’m back to looking at All Electronics, thinking of ordering myself an assortment. They don’t have anything in the 4Ω range, but they have 1.2Ω, 1.3Ω, 1.5Ω, and 2Ω in power ratings I could work with.

Leave a Reply