Audio Amp Motor Driver - motor polarity oddities + parallelling experiments TODO

A project log for Random Ridiculosities and Experiments

Sometimes yah's just gots tah try somethin', regardless of whether it'll become a full-fledged "project"...

Eric HertzEric Hertz 03/22/2016 at 10:142 Comments

Crazy Update some five+ years later, at the end....


The Class-AB (linear) audio-amp chips I'm currently using for my motor-driver (see past logs) have *4* channels... So, for low-enough-current motors this single chip could handle *4* separate DC-motors. (cool).

Update-Wise: it seems to be working quite well for several weeks now. It definitely gets warm, and maybe on a few occasions has been dropping-out (plausibly due to thermal protection-circuitry combined with the fact the heat-sink it's attached-to has a *channel* (for screws) right under the thermal-pad...). But actual overheating-protection seems few and far-between.

There is a more pressing problem that oddly seems to be related to the motor itself... when spinning in one direction it works fine, but in the other direction it acts a little strange, kinda stutters. This only seems to happen when driven at full-speed (when the amp's outputs are close to ground and V+).

Now there's a few oddities, here. Watching the negative output from the audio-amp, it's very clear that that output stutters *with* the motor's stuttering... When it's *just above* ground, it runs smoothly, but when it stutters, the output actually reaches ground. OK, maybe thermal protection...

But then, there's oddity 2. When the motor's *loaded*, it doesn't do this. The output doesn't go to ground. When it's *loaded*, the motor draws *more* current (right?) so then thermal-protection seems illogical.

OK, now there's oddity 3. As I recall, reversing the polarity of the motor's connection to the audio-amp/motor-driver doesn't do what I expected. I expected the motor to continue to "stutter" when the motor-driver is driven the same way (now, the reverse-direction), thinking the "motor-driver" might be a bit 'biased' toward one polarity over the other (maybe one of its channels has a slightly more-powerful low-side transistor than the other channel?).

In fact, what seems to be happening is that the motor itself is responsible, when powered with one polarity over the other. Is that plausible? I should measure its current-draw... But, again, loading it causes the stuttering to *stop*, so a differing amount of current-draw doesn't seem like the problem, right...?

The motor is from the 1980's, so it's likely had years of wear... maybe it was used in one particular direction more than the other, so its brushes aren't worn evenly, or something...? Is it possible that the sparking caused by the brushes' breaking-contact with the windings could be larger in one direction than the other, (again due to wearing) causing, maybe, over-voltage protection?

Again, the weirdest part about it is that when I actually apply a load to the motor's shaft, this problem isn't a problem. When I drive the audio-amp's input such that the outputs aren't *quite* at their limits, it's also not a problem. BUT it only seems to happen when the motor's powered/spinning in the same direction (but not the other), regardless of the polarity it's been wired to the driver-chip.

Oh, and, yes, I did (early-on) throw in freewheeling/flyback diodes, despite my experimentation-goals of seeing whether the Class-AB amp really needs them with its push-pull abilities.


Now, onto the experimentation-TODO:

It was suggested, if I understand correctly, at that Class-AB amps might be paralleled?! OK, I've always thought this to be a bit risky, since it'd be hard to have two identical transistors/gains, etc... So then the loading on one output would be higher than the other... (and maybe worse in this particular case, since the gains are fixed, there's no [external] feedback-path, they might differ slightly). But, maybe it's actually possible... and something worthy of my looking into. Here's a good start: I found some other resources, as well, but one was about Tube-Amps, and the other was nothing more than SPICE simulations (which seem to me like they wouldn't take into account differences in silicon).

Also, per more discussion at #Powerful Portable Speaker, with @HP (who apparently can't be '@'ed without insider knowledge) and @Moritz Walter:
Are these for Class-AB...? More importantly, maybe... Are my chips Class-AB? I need to refresh myself on the classes!
And... Again, the commentary at: wherein @Moritz Walter's log-entry--largely regarding charging batteries in series, which could be quite helpful down the road--has been hijacked. Apologies-yo!

Why...? Would there be any benefit to using the four channels of a single chip to drive a single motor...? If overheating's the concern, then wouldn't it just redistribute the heat around the same piece of silicon, and ultimately generate the same amount of heat on that same piece of silicon? I don't know. Another plausibility is using separate chips to redistribute that heat. Or maybe there'd be some benefit besides heat... I don't know. I guess that's where experimentation is worthy. I'd throw out there that the effective "on-resistance" would be lowered, but that doesn't make sense in a Push-Pull Linear-output... right?


Anyways, this has been my go-to circuit for motor-driving since I started this crazy endeavor months ago, and it seems to be working surprisingly well. Maybe I should do a full writeup/schematic. Chip-wise, we're talking the TA8251AH.


August 2022: 


These dang chips are on sale for *70* chips for $20. 280 motors. 

You gotta know I'm having a hard time justifying *not* making this purchase. Even if all I used them for was driving every "toy motor" and cassette motor and every itty-bitty bipolar stepper from floppy and CD drives in my collection, attaching nothing to their shafts but a piece of tape. Heh!

Or, do I really need all that fancy circuitry from before? What was wrong with simply using AC-coupling and an RC-filter? Or even dropping the RC filter and using PWM at low frequency?


Actually, having vaguely-remembered the parallelling findings, here, I kinda thought about making a "pure-sine" inverter for my oscilloscope... Though, I kinda doubt I've a suitable transformer. And, a brief look through a doc linked here from TI suggest what I initially thought: 'servoing' (with much additional analog circuitry, which I, tend to have bad luck with) is necesary unless the tolerances are *finely* tuned (0.1% resistors, and what about the chips themselves?). And, again, these guys don't have typical feedback pins available. And, well, it'd be an interesting experiment if it wasn't 120V... And, well, I've still 14(?) of these chips unused anyhow...

but... 280 motors!


Oh no, more ideas:

These are 4x Bridge-Tied-Load speaker-amps. each BTL output-pair has a single input. The difficulty of all my circuitry in the past logs was due to the fact that input is centered at 0V; the device is, and my circuit was, single-supply.

OK. But using *two* speaker-drivers, one could, maybe, use the non-inverted outputs... Then drive the two inputs separately. Essentially: PWM on one (0V-3.3V) and "Direction" on the other. Single-supply. And could still drive two small motors, or a small bipolar stepper, from each chip.

 .... Sheesh, brain!


HP (@banjohat) wrote 03/22/2016 at 12:35 point

I get the idea of using AB chips for this.. I wanted to say something about using a screwdriver as a hammer but... it's hackaday ;) this is interesting!

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Eric Hertz wrote 03/22/2016 at 14:10 point

Haha, Thanks! 

Actual screwdrivers get used as hammers 'round here *way* more often than they deserve to, especially what with two hammers being within arm's reach of my keyboard... But as far as metaphors (similes?) go... Yeah, these "screwdrivers" are maybe being a bit abused, but technically they're rated for nearly the wattage of the H-Bridge chips I'd been using before... I think it's a bit more like using a rubber-mallet... with a metal end-cap ;)

Thanks for the paralleling-idea, definitely going to keep it in the ol' experiment-banks.

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