Driving a LED from an IC

Jul 24, 2006 2 Replies
N

Hello.



I have successfully used a transitor as a switch to drive a LED from an Intel PXA255 CPU, pretty much exactly like this link describes:

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I used a BC109 transistor for the job.



The LED's maximum forward current was 20mA, but I now have the need to drive a brighter LED rated at 350mA. Can I use a transistor-based circuit similar to the one above to do this? My electronics knowledge is minimal and the time-frame is short - so I may just be happy with what I've got which is the new LED running at 90mA or thereabouts.



TIA, Nick



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H

I'm not familiar with the Intel PXA255 CPU. (Is it a micro-processor CPU or a microcontroller?) How much drive current can it provide from a pin? For a

350mA load, you could use a 'Darlington' pair, (a high-current transistor driven by a BC548/BC549 or similar), but a small logic-level power MOSFET might be the best answer. I use a 2SK2231 for most things up to a couple of amps, but someting smaller would suffice in this case.

... Humphrey B. Bear

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D

How bright does the LED have to be?

I know some LEDs that get plenty bright for indicator lamp duty at half a milliamp - one example is Nichia's NSPG520S. Or get a Nichia NSPG500S (10,000 or so mcd green 15 degree 5 mm diameter) (or similar) and sand down the tip about a millimater and re-round the tip with sandpaper if you need a wider viewing angle and need to not buy 100 from a Nichia sales office.

I know some 350 mA LEDs have brightness in the ballpark of 7-watt nightlight bulbs that have reflectors behind them. Like any white, blue-green, green, orange or red part by Lumileds.

Meanwhile, my favorite way to have an IC control an LED is to use a power MOSFET. My favorite MOSFET for "general purpose work" is Zetex ZXMN3A01FCT when input voltage is guaranteed to be less than 10 volts, although this is a small surface mount part. It will switch well over an amp quite comfortably. For higher input voltages (20 volt limit), I like IRFZ14 or the like. If you can still get an IRF511 at Rat Shack, it will work.

- Don Klipstein ( snipped-for-privacy@misty.com)

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