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Overheating components in PWM

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riccardo

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Hi,

I made a 555 based PWM circuit but am having problems with it. It consists of a 555 timer and comparator which drives the gate pin of a MOSFET. My test load is 5x 5R wire wound resistors in parallel.

It operates ok but has some nasty ringing. I have also noticed that when I turn up the frequency (about 80kHz) the electrolyitic capacitor gets really hot.
This capacitor is just placed between power and ground. Why would it get so hot?

Cheers
 
Effective Series Resistance in the capacitor. Lots of resistance between your power supply and the electolytic cap. Look up ESR for your electrolytic.
 
The capacitor is supplying most of the switching current which goes from zero to the load current and back to zero for each cycle of the switching frequency, generating a large AC current in the cap. The higher the frequency, the more power is dissipated in the capacitor's ESR.

You may need to go to low ESR caps designed for switching regulators. Also smaller caps in parallel will tend to have a smaller equivalent ESR.
 
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Why use 80KHz to control a motor?

The circuit will probably do nicer at 400 to 800Hz.

If there is still ringing you might eliminate it with a 100 to 470nF cap parallel with the motor.

Boncuk
 
Why use 80KHz to control a motor?

Because higher frequency means smaller ripple current (with inductive loads). Smaller ripple current means higher efficiency (less heat) and longer life (especially for the electrolytic capacitors).

Riccardo didn't say what he is going to control with his circuit, but if its an inductive load (like a speaker or a motor), then the test load should be inductive also.
 
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misterT makes a good point. If your actual load is inductive, then the ripple currents will be much smaller and your cap won't get as hot.
 
Hi both, misterT and Carl,

the OP mentioned that he is using 80KHz. That frequency should suffice for any inductive load.

(Electrolytic caps are designed for mains frequencies of 50 and 60Hz for most applications)

May be his circuit design has a flaw which causes the cap to overheat.

I suggest he posts his circuit here for further assistance.

Regards

Boncuk
 
He stated his test load was resistive, not inductive, hence the increased ripple current and capacitor power dissipation.
 
Hi both, misterT and Carl,
the OP mentioned that he is using 80KHz. That frequency should suffice for any inductive load.

I know this forum is for hobbyists and that 80kHz is sufficient for many applications, but I wouldn't say that it is sufficient for "any inductive load". If somebody is using a switching frequency of 80kHz there is a reason to it. If the application is an audio amplifier, I would use even a higher switching frequency than that.
 
I know this forum is for hobbyists and that 80kHz is sufficient for many applications, but I wouldn't say that it is sufficient for "any inductive load". If somebody is using a switching frequency of 80kHz there is a reason to it. If the application is an audio amplifier, I would use even a higher switching frequency than that.

Just let's wait and see what the OP is about to control. :)
 
Thanks for all your comments.

The circuit is intended as a multi function test circuit. Sometimes I will use it to test resistive loads, sometimes inductive ones. The frequency and duty are both adjustable.

The capacitor is RS 228-6773

The capacitor is C2 in the attached diagram.
 

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The capacitor is C2 in the attached diagram.
I cannot locate it in your chematic.

Anyway, have you tried to replace the cap so that you can rule out defect cap?
 
In the middle, near the top, there is a voltage regulator, with some diodes and capacitors, it is there.

Yes, I tried different caps (of the same type)
 
Is this circuit built on a already finished print board or you have it on a bread board? If you have a bread board, try to connect two 100µF caps in paralell instead. They might share the amount of active effect so each of them won't get so hot.

But frankly, capasitors isn't the first component I would expect pick up heat. You sure that there isn't other components located nearby that may heat the caps as well?
 
Wikipedia states that a 100µF electrolytic cap has a typical ESR of 0.05Ω to 0.5Ω. For a 12V PWM signal driving a 1Ω resistive load, the ripple current from the cap could be up to 12A. It's easy to see that this could cause a significant power dissipation in the cap, even for the minimum ESR of 0.05Ω.
 
In his first post he says his load is wirewound resistors (which are inductive at 80kHz).
 
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