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Author Topic: Humming 12v halogen bulb - PWM dimmer, Solved  (Read 4690 times)
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Manchester (England England)
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So can a mechanical (lamp) produce energy at a frequency lower than its being driven at.?
Yes it could. It could be vibrating at it's natural mechanical resonant frequency. The fact that the excitation frequency is higher would not stop it. This is known as sub harmonic resonance. and it happens with planetary / moon orbits in some cases.
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Sydney
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Aha, thats interesting. Alright I will find out the frequency the lamp is making. My feeling tells me it's actually the frequency it is driven at. As I mentioned I can't hear it at 31khz, but that might also be because the mosfet is not switching at all atm. Will report back.
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Sydney
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Alright, for those interested, I made three recordings with an Edirol R-09.

The results:

Arduino running at standard pwm frequency:

15/255pwm
35/255pwm
47/255pwm

As for the frequency, well it's not that simple, analyzing it it with the Audition Frequency Analyzer visualizes much better what I hear:

15/255pwm:

35/255pwm:

47/255pwm:


I ultimately perceive a pitch of about 3200Hz which seems to increase with increasing the pwm speed as well become more quiet which can also be seen on the screenshots.

Running the pwm frequency at 3900Hz gives me a perceived pitch of about 8000Hz. This is the reason why I'm quite certain that If I manage to switch the mosfet at 31khz it will become inaudible. I ordered the driver and it will hopefully be here in a week.

The cheap dimmer I bought results in the same pitched noise the arduino makes when running it with a pwm frequency of 3900. Looking at the design it also does not incorporate a driver but a resistor just like mine. I guess it's the limitations of the design that do not allow for faster switching. Trying to minimize costs they also decided to leave out the driver and stuck with the maximum pwm frequency they where able to drive the mosfet using this circuit.

Just using a resistor such as some hookup wire of course does not make any noise. The noise mainly comes from the and is certainly because of the lamp. It can also be heard from wires when held together by hand.

Cheers,
Adrian
« Last Edit: June 01, 2011, 03:27:11 am by alcs2000 » Logged

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Mike
Thanks for the lesson.
I realised afterwards that a true square wave contains all the odd frequencies, and hence it could easily be related to that.

Adrian
Nice piece of recording gear. In this case proving very useful.
Interesting that there is nothing below about 3kHz.

You test with the resistor shows it is definately from the lamp.
I can only imagine that will either get louder or quieter when its mounted....

Mark
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Sydney
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Just thought I should check how it looks at 3000Hz PWM frequency.

Looks less noisy, but louder and higher pitched:

64/255pwm at 3900Hz (recorded to wav instead of mp3 thus the increased range on the high end)


Interestingly enough the lamp would no longer light up at 16/255pwm which it did at the lower pwm frequency.

Looking at the Spectrum analyzer shows the annoyance quite beautifully:


64/255pwm at 3900Hz


The audible noises are clearly 7800Hz and 11700Hz! They are all multiples of the pwm frequency! Beautiful.


Any theories why exactly this is the case?

I have great hopes for 31khz now!


35/255pwm at 488Hz for comparison


And now of course I realize that what I see here is that its all multiples of 488Hz...

It seems like the pitch did not change at all with the increased pwm frequency but it got louder, so my hope to shift the noise into the ultrasound spectrum is diminishing. It kind of concentrated the noise frequencies resulting in a higher amplitude. However, because the noises are all multiples of the pwm frequency I am hoping that the lowest will be 31khz with the mosfet driver. We shall see when what happens when I get it. Will also play around with some ceramic caps when I get some time.

Cheers,
Adrian
« Last Edit: June 01, 2011, 09:30:06 am by alcs2000 » Logged

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Sydney
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Hey Guys

The mosfet driver arrived and I had finally time to try it out. It works exactly as predicted. The Mosfet driver can charge and ground the mosfet much quicker, 31khz is not a problem. As predicted, the first frequency emitted by the bulb is 31khz, the less loud 62khz and I guess further multiples of 31khz that I can not detect anymore. I can no longer hear the humming anymore of course, so that's fantastic!

Cheers,
Adrian
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Switching 2A on and off will make two wires move, certainly will move filament in Earth magnetic field. When you PWM it's down, spices of current become higher, as resistance of bulb goes down, with cold state (PWM < 5%) I'd expect current up to 12-14 A.
 I wonder how much RF interference this circuitry generate, is your radio still working?  smiley-confuse
BTW, where 12V coming from, can you just use "dimmable" driver for your halogen and regulated voltage produce by driver?

edited: There is more to use "dimmable" driver, the wiring to bulb how long is it?
It would explain, why brightness jump between 50% and 100%. With PWM 100% you have a DC, and no voltage losses on wire inductivity. And it's also good indicator for EMI, whatever break you have in linearity (50% or so) all this energy is radiating in RF or dissipating in a heat.
« Last Edit: August 09, 2011, 08:11:43 am by Magician » Logged

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Actually, the dimmer works perfectly now, it doesn't jump anymore from on to a semi on to off. No heat is generated anywhere and the 31khz ultrasound emitted is also of very low amplitude, and of course not audible.

The reason why it didn't work was that at a higher frequency the circuit could not charge / discharge the gate of the mosfet fast enough which left it in a semi on state (and of course getting really hot). PWM less than 5% doesnt really work for halogens, I think they only start to give a shimmer of light at about 20% if i remember correctly.

12V is comming from a standard 12v laptop power supply, rated 3A.

Cheers,
Adrian
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Interesting analysis all, thanks for sharing.
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I'm still worry how your radio works, or your neighbors.
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PWM less than 5% doesnt really work for halogens, I think they only start to give a shimmer of light at about 20% if i remember correctly.
I understand this, I put 5% just as an example. It could be any value, when bulb start to glow, may be 10 or 30%. It's not easy to explain for me, but you also should take into account, that the faster you turn on/off mosfet, the more EMI your wiring radiate. The solution only to place mosfet as close to load(bulb) as possible, and pass to transistor control, low signal wire.
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that the faster you turn on/off mosfet, the more EMI your wiring radiate.
True but PWM of any ratio will turn the FET on at the same speed.

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The solution only to place mosfet as close to load(bulb) as possible
No there are lots of solutions to slow the rise time of the current or reduce the radiated emissions with things like ferrites. 
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that the faster you turn on/off mosfet, the more EMI your wiring radiate.
True but PWM of any ratio will turn the FET on at the same speed.
Yes, PWM duty cycle is not changing speed of the mosfet, but current spikes are higher/bigger for lower duty cycles, as resistance of the filament goes down.  Variation between cold - hot resistance could be in order 10x, consequently EMI would be stronger for low PWM.
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consequently EMI would be stronger for low PWM.
No not really, the difference in resistance between a just glowing filament and one burning at full brightness is small compared to that of a glowing on and an off.
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