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Topic: Humming 12v halogen bulb - PWM dimmer, Solved (Read 6 times) previous topic - next topic


I am not sure that a 20W Halogen will respond to such a high frequency because of the thermal time constant involved. You do need a scope to find out what is happening.

Why does my circuit hum?
All together now:-
Because it doesn't know the words.

I think you might have gathered that it is the filament in the Halogen



Yeh I was thinking it could be the filament, however even the connections on the breadboard are humming, if however barely audible.

I will order a TC4429 and see how that goes. The oscillator still has to wait for a while, waiting until the dso quad becomes readily available...

www.alcs.ch - will put some arduino projects online soon!


Glad to see you have progress.

The fact the mosfet is getting warm is a sign that its not switching cleanly (either on or off).
You could check by seeing if it gets warm with just an ON only.

I did a similar thing way back with a single 2n3055 and a 555 IC.
I found that putting a capacitor across the series resistor to the base, squared up the drive very nicely.
It dimmed a 12v 25w normal sized lamp with no heatsink or heat at all. (Can't remember all the cct)

(mine didn't hum so it must have known the words)...very good mike

The fact that 0-50 is still off, just shows how non linear lamps are. It only requires a small drop in voltage to produce a large drop in light output.
If your supply is variable, you may be able to plot/check this, and then apply it to the pwm average.



May 30, 2011, 11:03 am Last Edit: May 30, 2011, 12:05 pm by alcs2000 Reason: 1
Yep, the mosfet is definately not switching properly. Having looked at it again I actually only get 3 states at this frequency: off, on and 30% which is when the mosfet heats up and i guess its neither on or off.

The cap didn't make a difference. It just seems that the mosfet's gate is not charged/discharged quickly enough. The mosfet driver will tell. At least the humming is gone =).

www.alcs.ch - will put some arduino projects online soon!


Correct if I'm wrong here...but
If the cct is operating at 31khz (the Mhz is not right), then we are talking about 32nS cycle.

The specs on the IRF1010z say its for Automotive use, (not switch mode power supply), and the rise time is 150nS.

Could Adrian's problem be its being driven too fast.??
(The opto on time is only 7.5-10uS)

By comparison looking at an IRF520 (10A) the turn on is only 4nS, which significantly less capacitance.

You could try finding a fet that is designed for switch mode supplies, and if you only want to dim a 20w lamp, something closer to 3 Amps.


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