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PWM application question

KX36

Aug 26, 2011
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I know you can use PWM to adjust average power level in a device such as a motor or light bulb, but can you use it to power something from a supply beyond its specifications?

E.g. I have a 6Vdc light bulb and a 30Vdc power supply. Can I use a 20% duty cycle PWM to power the bulb, or will the bursts of 30Vdc damage it?

(Let's just say for the sake of argument I can't use a different power supply, but the high currents involved make resistive voltage dropping underirable, wasting lots of power in heat and needing a very large, expensive, heatsunk resistor)
 

rootboy

Aug 26, 2011
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I know you can use PWM to adjust average power level in a device such as a motor or light bulb, but can you use it to power something from a supply beyond its specifications?

E.g. I have a 6Vdc light bulb and a 30Vdc power supply. Can I use a 20% duty cycle PWM to power the bulb, or will the bursts of 30Vdc damage it?

(Let's just say for the sake of argument I can't use a different power supply, but the high currents involved make resistive voltage dropping underirable, wasting lots of power in heat and needing a very large, expensive, heatsunk resistor)

I think that you would be just fine. And keep in mind that you could always put a reactor in series (it's what we do for VFDs, and it really helps)
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Just make sure it's not a nuclear reactor...

Do you mean an inductor? If so you may need to provide additional protection for the PWM drive so that it doesn't suffer from the effect of switching a highly inductive load.
 

KX36

Aug 26, 2011
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Thanks for the replies. How would I work out the right inductance for the inductor? (if that's what u mean by reactor)? With these currents, the dc resistance will have to be low, so that means the part would get big and expensive quickly with increasing inductance, so I would aim for the minimum inductance that does the job.

And would the protection just be the usual parallel diode to avoid flyback voltages?

Cheers,
Matt
 

rootboy

Aug 26, 2011
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Just make sure it's not a nuclear reactor...

Do you mean an inductor? If so you may need to provide additional protection for the PWM drive so that it doesn't suffer from the effect of switching a highly inductive load.


Well, in this application they are known as reactors, but yeah, it's an inductor. But when you go shopping for one, search for "reactor".


(KX36); said:
How would I work out the right inductance for the inductor?

As for sizing the inductor, that's a good question. Usually we let the companies that sell them size them for us. But some sites have an online calculator that you can use.


Here's a short writeup on load reactors:

http://www.mtecorp.com/load.html

I don't know if the math scales, but this pdf might help with the calculations:

http://www.pge.com/includes/docs/pd...e/energystatus/powerquality/line_reactors.pdf


Enjoy! :)
 
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