Driving A Motor With An Audio Amp Chip

[InazumaDenki] wanted to answer the question: can you drive a motor with an audio amplifier chip? The answer, of course, is yes. The TDA7052 has a single input, and a bridge output meant to drive a speaker differentially. It should work if the motor doesn’t present more of a load than a speaker.

The plan was to use a resistive divider to provide several discrete voltages to the input. At precisely the half-way mark, there should be no voltage across the load. Altering the input to go higher than halfway should make the motor turn one way, and making it go lower should turn the motor the other way. As you can see in the video below, it does work, although it may not be ideal for this application.

By varying the voltage at the input, the motor changes speeds, although this won’t be as efficient since you probably would prefer to control the speed using PWM to preserve the motor’s torque output. Since the motor is a very inductive load, the circuit has didoes to trim back kickback voltages to protect the device.

Still, it is an interesting idea, and if you are only interested in direction control, it probably isn’t a bad thing. Feeding PWM through the device might be difficult, though, since it would have to be referenced to the midpoint, not ground. Otherwise, the motor would just spin in one direction for a bit and then back in the other direction — not really what you want.

This works, of course, because it isn’t a single-ended output. We’ll stick with a classic H-bridge circuit.

25 thoughts on “Driving A Motor With An Audio Amp Chip

  1. Even better: Use a stereo amp to drive the two phases of a stepper.
    With a pair of DACs driving sine waves into the amp it’s ‘infinite microstepping’, and near dead silent.
    Burns a lot of power though, but you can reduce the drive current when torque isn’t needed..

    1. not that many audio amp are happy supplying DC, which you’ll need when the stepper isn’t moving. And with a bipolar supply rather than a bridge, you risk pumping energy from from one rail to the other eventually killing the amp

      1. All true statements. You obviously can’t use a stock audio system.
        But a stereo amp module driven by an appropriate power supply works a treat, and is a tidy solution for an electrically and acoustically quiet drive.

      2. Reminds me of an old saying from the era of the tube radio – “DC doesn’t belong in the headphones!”

        This was from a time when loudspeakers were still uncommon in broad use and detector headphones with high-impedance were in use (chassis made of bakelite; the coil was pre-magnetized so reverse polarity was bad).

      3. I think every audio amp chip will happily supply DC to it’s load, assuming it’s not an integrated amplifier circuit. Only if it is DC blocked (usually through capacitors) will it not pass DC, but keep in mind that any AC signal IS DC at any single point in time.

        Like every component, it won’t kill the amp unless you run it outside of it’s limits.

    2. We actually have an in-house product that does exactly that.

      We use a pair of OPA548 drivers to drive linear current into a small stepper motor in an application where heat, noise, and vibration are extremely critical.

      Even the smoothest chopper drive creates noise and inductive heating every time the current is switched on and off, even when the motor is sitting still.

      Driving a step motor with with a linear current source eliminates the switching noise and reduces motor heating to just the resistive term, which is as low as you can go while (and like many stepper applications, we can use reduced holding toque).

      Yes, it’s very inefficient, but all the heat is dissipated at the *driver*, not the *motor*, and that was important in our application.

      If you try this, you may have to fiddle with the feedback loop, make sure the gain is bypassed well at frequencies above the working response of the motor, or you can have high-frequency oscillation because of all the inductance in the system

  2. Of course you can. An amplifier is an amplifier; if it amplifies linear signals it can also amplify digital ones, therefore it can be used for both linear and PWM load driving. Using parts for other uses instead of the ones they were made for is a very instructional practice I would suggest anyone to experiment with.

  3. Feeding PWM into an inductive load is not only not difficult, it’s preferred. The inductance will act as the desired low-pass filter. 50% duty cycle output would be zero DC voltage, and if the low-pass filter provided by the circuit inductance is sufficient, there’s negligible AC current too.

  4. Maybe I’m wrong but I think this similar (not equal!) to those chips like BD8263EFV-M system motor drivers for cd/dvd’s actually I wonder why we don’t see those chips being used by maker community maybe they are too complicated?

  5. Back in the early seventies I saw an audio amp used for vertical deflection of a color tv, not a big deal as the signal is 50Hz but it also served as vertical shift adjustment by a potmeter to modify the DC offset.

    1. Cool! The people back then had a lot of phantasy and didn’t hesitate to try out unconventional things, if they were useful. That’s why I think that era was the pinnacle of ham radio, too. Along with the upcoming DIY it was a time of experimentation. Everything seemed possible with a bit of creativity. Vy73s

    1. Possibly yes, but overkill. Class D amplifiers are almost identical to switching power supplies, that is, they drive the speaker with high frequency PWM, whose duty cycle is modulated by the audio source. There’s an unneeded modulation step that can be eliminated.

    1. I think I saw a 70s RC servo design, that used audio amp chip. At that time there was maybe only one special purpose chip for it. DIYers couldn’t get hold of them easily, but the amp chip was in a lot of catalogs.

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