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Topic: PWM on LED too slow (Read 3371 times) previous topic - next topic

DrAzzy

Well, that wouldn't work. Sorry for little OT, but I'm starting to get confused. Everytime I build something using arduino I order all the parts, wait a month for them, and repeat only because I thought that "in theory" it should work.
Yup. Do you know about digikey? (in the US, at least) They don't have the modules and cheap chinese boards, but they've got oodles of components, and you can ship via usps for like $3, and it's really fast.

The chinese electronics vendors on ebay and elsewhere are usually cheaper, but they only have common parts, and the shipping is miserable. My strategy is to buy anything that looks neat and is absurdly cheap, that way if I decide to do something that uses it, I don't have to wait for shipping (now if only I could find my desk...)

When I tested the circuit I used a salvaged PC power supply (with 12V 10W LED instead of those 30V I'm using with drivers). And it just worked! Are there any risks of doing that pernament? I'm speaking of connecting the 12V LEDs straight to the DC12V of the power supply (and switching it with TIP 120...). (I'm having a feeling that I should connect a resistor in series with the led, but what if the voltages are equal?)

Thanks M.
Yeah, you have to be a bit concerned about the PC power supply's power quality. I'm not sure how severe of a problem this is, though. You can also buy surprisingly cheap 12v power supplies... on amazon, not just ebay-from-china.

You probably do want a resistor in there - IIRC, those LEDs are happier at 10-11v than 12. I'd measure the current going through the LED (with multimeter), and compare that to the expected current, and then pick a resistor value based on that. I'd expect something in the 1 ohm or less range.
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Paul__B

My strategy is to buy anything that looks neat and is absurdly cheap, that way if I decide to do something that uses it, I don't have to wait for shipping (now if only I could find my desk...)
Oh, how I feel for you (on both counts)!   8)

MtHzR

What you have not done so far, is explained (with a link) what the LED actually is, so that we can understand what is properly necessary to control it.
It is just one of these There are no datasheets so all I know is that it runs off 9-12VDC and it should draw 400-450mA.
Yup. Do you know about digikey? (in the US, at least) They don't have the modules and cheap chinese boards, but they've got oodles of components, and you can ship via usps for like $3, and it's really fast.
Unfortunately I'm a high school student from the Czech Republic (central Europe) so I haven't heard of that. I usually order most things off the eBay just because it is cheaper and I usually don't have to hurry.

Thanks for trying to help me out.

Paul__B

It is just one of these There are no datasheets so all I know is that it runs off 9-12VDC and it should draw 400-450mA.
Right, now we know exactly what it is - it is a 3 by 3 series-parallel array of LEDs with no other components.  As such its voltage drop is three times that of a single high-brightness white LED.

You have your description backwards.  It does not "run off 9-12VDC" and "should draw 400-450mA" at all.  It runs on 425 mA which you have to control with a current controller and you may expect its voltage drop whilst doing so to be in the order of 9V.  You were powering it more-or-less appropriately with the possibly deadly power supply you first cited except that it was delivering less-than-optimal current and could not of course, use PWM.

What you need to do to operate from the 12V supply is to use your TIP120 transistor - with heatsink - as the second transistor in the circuit Mike has given you to control the current while you apply the PWM signal as the circuit indicates, to the input resistor.

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