The principle is ok, but that FET has a specified 'on' resistance of 160mΩ, so will dissipate about 10W when the bulb is on. It will get hot. Better (lower resistance ) FETs are available.
PWM a 12v bulb
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30th March 2012 08:30 PM #1Conrad_Turbo
PWM a 12v bulb
I want to build something that can dim an automotive bulb from 100w from full brightness to just glowing. I will be using a micro controller to do the PWM, but I am not sure where I should look in terms of a simple 12v (14.4v when the engine is running) PWM controller for a resistive load. Any suggestions to get me started. If it's just a few components to source I can easily build on that.
Doing some searching, an IRF530 looks like it could work?
http://www.datasheetcatalog.org/data...ild/IRF530.pdf
Run a microcontroller PWM to control the mosfet to control the 12v light?Last edited by Conrad_Turbo; 30th March 2012 at 09:07 PM.
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30th March 2012 09:47 PM #2alec_t
0My circuit designs should be regarded as experimental. Although they work in simulation, their component values may need altering or additional components may occasionally be necessary when the circuit is built. Due safety precautions should be taken with any circuit involving mains voltage or electrostatic-sensitive components.
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Every problem has a solution (given the right information and resources).
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30th March 2012 11:35 PM #3duffy
Like Alec said, there are better mosfets. If you can stand dealing with a surface-mount part, you might try a IRFH6200TR2PBF. This has an RDSon of 1.2mΩ with a 4.5V gate voltage. This will dissipate a mere .083W driving that 100W bulb - won't even get warm. Costs less than $3.
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31st March 2012 03:15 AM #4KeepItSimpleStupid
BTW, this will work nicely. http://www.jameco.com/webapp/wcs/sto...0001_120539_-1
There will be an oddity at the low brightness level though. if you were to plot intensity vs %ON, there will be a difference for up/down, especially at the low duty cycles. In fact, the lamp may not even turn on.0
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31st March 2012 03:20 AM Thread Starter #5Conrad_Turbo
I could possibly make that SMD chip work...wow it is light years better than the one I suggested. 45A!?!? I wouldn't mind spending a few extra bucks for a non-SMD unit though, just to make assembly easier and to know that it is heatsinked properly. However if there isn't something like that I could make it work. I'm going to be driving two lights total.
KISS, something like that would work, but I will be using the microcontroller to do the PWM. I just need the muscle to control the current of the lights.0Last edited by Conrad_Turbo; 31st March 2012 at 03:35 AM.
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31st March 2012 03:54 AM Thread Starter #6Conrad_Turbo
What about this? http://search.digikey.com/us/en/prod...1PBF-ND/811792
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31st March 2012 04:07 AM #7duffy
That will work fine.
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31st March 2012 05:04 AM #8bountyhunter
You can even use a simple 555 timer circuit and a pot to adjust the pulse width.
Originally Posted by Conrad_Turbo
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31st March 2012 10:36 AM #9MrAl
Hi,
As already pointed out, the bulb will not dim in accordance with the pulse width but will dim more than the pulse width would have you believe. The reason for this is because the bulb voltage vs brightness varies as a power law, not a linear one. So at 75 percent of max pulse width you wont get 75 percent of max light output but actually less than that. At 50 percent pulse width you'd get much less than 50 percent of max light output.
Since you are using a micro controller however you could program it to sense the potentiometer position and adjust the pulse width based on that and the control law for the bulb. You could discover the law for the bulb by setting the PWM to several levels and then measuring the light output, then using that data as a lookup table and interpolate between table entries, or do a curve fit to fit an approximate curve for the control law of the bulb. This would work pretty nice, so with the pot set at 75 percent of max rotation you'd actually be able to get 75 percent of full brightness, and likewise at 50 percent.
The control law for the lumen output of a vacuum tungsten filament bulb is approximately:
PercentOutput=(Volts/NominalVolts)^3.5
so you could fit the curve:
y=(Volts/NominalVolts)^a
or something like that. This would mean you could program the formula into the microcontroller and use that to calculate the PWM requirement for a given pot setting.
'NominalVolts' above is the voltage the bulb normally operates at, so for an automobile bulb this would be 12v or nearly that.
'Volts' is the average voltage of the bulb as it is actually being operated, so 75 percent would mean this would be 9v.0Last edited by MrAl; 31st March 2012 at 12:19 PM.
One test is worth a thousand expert opinions, but one expert specification is worth a thousand tests.
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31st March 2012 03:28 PM #10chandan28
dude you need not have to do all these circus just buy a regulator used for fan that wil do this job
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