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using class D amplifier as motor speed controller?
using class D amplifier as motor speed controller?
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Old 5th April 2017, 06:54 AM   #1
realflow100 is offline realflow100  United States
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Default using class D amplifier as motor speed controller?

Can I set a 1khz tone or higher with squarewave signal input from computer.
and use bridge rectifier and smoothing capacitor on speaker output.
to power a small DC motor (like pc fan) off 12v power supply to run it at variable and precise speeds?
I can use a combination of adjustment knob on the amplifier. as well as on my computer software.

Would sinewave be a more efficient input waveform than squarewave?

it seems like it would work (the audio amplifier is rated 50W per channel max and 25W per channel at 12v 4A
fan is only going to be using a few hundred milliamps maximum at full output.
just a small 80mm PC fan

or is this a horrible idea and should not be attempted for some reason thats oblivious to me?

Last edited by realflow100; 5th April 2017 at 06:57 AM.
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Old 5th April 2017, 07:10 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Good idea unfortunately the PC fan requires whatever the rated voltage as they have a regulator internally making them very unpredictable. But, if an AC fan, the class D would be ideal. Feed the amplifier from a sine wave oscillator and as it is synchronous, keep the AC voltage at the same level, to keep the power and vary the frequency for a speed controller.
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Old 5th April 2017, 07:13 AM   #3
realflow100 is offline realflow100  United States
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Why would a PC fan have an internal regulator?
Don't they run just fine even from a couple AA batteries? Wouldn't that be almost the same thing?
the bridge rectifier rectifies the AC from the amplifier. the capacitor smooths the output high frequency noise and you can vary the voltage from very near 12v to anywhere in between 0v and 12v with a 12v input power supply
(Maybe even slightly above 12v since its class D and it might be very close to the efficiency of an actual motor speed controller at that point.
Just need the right frequency range thats optimal for the load (i think 1 to 10khz is fine)
PC fans don't always run at full speed. of course they can run at any speed below their maximum.. thats what a computer does when it controls the fans speed whether it be 2 pin. 3 or 4 pin fan. it amounts to the same.
and class D would be nearly like PWM control or even buck/boost.
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Old 5th April 2017, 07:20 AM   #4
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Two things here, use your idea if you want but do you know what DC is? It has no frequency as it is direct current, so what is the point of a frequency to drive it? If your amp has a frequency response from DC upwards, that is fine, use a DC input, that will be amplified and produce a DC output to run your fan.
Why not use a variable resistor instead, (5k 3W POT), that will adjust your DC voltage and therefore your motor speed. I realise that takes away the fun of what you are doing, so make is and see for yourself.
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Old 5th April 2017, 07:23 AM   #5
Mooly is offline Mooly  United Kingdom
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using class D amplifier as motor speed controller?
Have a read at this:
Pulse Width Modulation Used for Motor Control
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Old 5th April 2017, 07:27 AM   #6
realflow100 is offline realflow100  United States
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Whats wrong with applying a sinewave from my pc and then rectifying the output of the amplifier with a bridge rectifier and a smoothing capacitor?
it would make the same result.
And no i cant because the amplifier is AC coupled with a decoupling capacitor on the input and outputs. so theres no way I could just apply DC input. it would just make the speaker pop outwards a little bit then it would shift back to the center.

its still more efficient than using a potentiometer.
lots of power would be wasted in the potentiometer.
if I were to use a 1W motor I would need at least a 5W potentiometer and it would get HOT
Plus what if the motor is stalled. it would draw over 5W stalled for sure and burn up the potentiometer. No good.
the amplifier wouldn't even break a sweat and be much more efficient.

it would be like using a buck converter and varying the output voltage continuously to the motor. so the efficiency would be as good if not better than PWM.

and I can generate any kind of sinewave input from my computer that I would want.

the amplifier is has capacitors decoupling the input. so I cant use a PWM input signal.
it wouldn't work that way.
the only way to make it work is with a bridge rectifier on the output and a smoothing capacitor. then applying a high enough frequency AC input signal.
It would work just as well either way.

Last edited by realflow100; 5th April 2017 at 07:29 AM.
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Old 5th April 2017, 09:17 AM   #7
DF96 is offline DF96  England
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Why use a Class D amplifier (high efficiency) to drive a rectifier (lowish efficiency)? Why not just use a programmable DC-DC converter if efficiency is your aim. If efficiency is not needed then simply use a programmable regulated DC supply.

You need to tell us what you are trying to achieve, and why.
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Old 5th April 2017, 09:20 AM   #8
realflow100 is offline realflow100  United States
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I just want something with a knob to control a motor/fan and dont want to go purchasing something if I don't need to.
and I already have these things so was thinking it'd be the fastest and easiest way to set it up.

and I dont need quite that much precision. just want to adjust the fan/motor speed variably.
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Old 5th April 2017, 09:47 PM   #9
PRR is offline PRR  United States
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using class D amplifier as motor speed controller?
PWM is the classic way to control a DC fan. The pulses do not matter because the fan has inertia.

Using a full Class D audio amp is the same idea with many many more parts.

A potentiometer is inefficient, true. A buffered pot (TIP120) gets to linear inefficiency which is *often* tolerable.

A rheostat will work quite well if you do not need to go to super-slow speeds. (Many fans just won't do super-slow; not how they are made.)

Most sub-Watt (even some larger) fan motors may be stalled indefinitely. In fact a common PC fan failure is bearing oil so gummed the fan quits. When re-oiled they rum fine again, even secondary fans which may have been "run dead" for months.

AC induction fan-loaded motors may generally be slowed well below 3/4RPM 1/2CFM with series resistance. I have a 69VA motor in my heating duct, and relays to switch in 25r and 100r to give my preferred balance of roar and airflow in different furnace modes.
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Old 6th April 2017, 07:07 AM   #10
realflow100 is offline realflow100  United States
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I'd never use a fan that large
I'm just running a PC fan. it'll happily run on two AA's which is only 3v.
so surely it can run on a class D audio amplifier's output rectified and smoothed to average 3v to 12v when running the fan?

I use a drop of hairclippers oil in my pc fans and it makes them last for years and years. they run silently and smoothly
It's still at least 75% or more efficient compared to using a linear potentiometer!
The amp has protection in it so it would cut off if there was a fault or short anywhere in the output.

Last edited by realflow100; 6th April 2017 at 07:11 AM.
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