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Real Quick LED Driver Question

I am trying to drive an LED (2.1Vf, 20mA) from a digital signal (3.3V). From what I have been able to understand, the logic level output I am using does not provide enough current to drive an LED brightly. I have another power souse (7.4V) on board to drive the LEDs, but I do not know what transistor to use. There are so many different ones. What would be my best option? Can someone point me out to a model number?

Thanks, Tom

Reply to
Tom
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Pretty much anything will do, if it's just for blinkenlights (if it's a fiberoptic laser LED, that's a different issue). You need either an NPN bipolar or N-channel MOSFET.

On my furnace board, I used ON Semi's DTC144EET1G, mostly because I wanted a SOT-523 (i.e. really small) package. This is an NPN bipolar with a built-in base resistor, since you need some sort of resistor when driving bipolar transistors. Then you have to do the math - how much Ice current do you want, divide by worst-case Hfe (gain) to get Ibe, determine voltage difference between worst case Voh of your driver and highest Vbe, select base resistor.

For N-MOSFETs it's a little easier, as no base resistor is needed and the turn-on voltage drop across the MOSFET is very little; you can almost pick the resistor as if the resistor/led were connected right to the voltage rails. You just have to pick any N-MOSFET with a Vgs turn-on voltage sufficiently below your minimum Voh, and a Ids current capability high enough for your LED.

To recap:

  1. Select a physical size, like TO-92 or SOT-23.

  1. Choose between bipolar or MOSFET.

  2. Current capability: For bipolar, select for Ice. For MOSFET, select for Ids. Choose at least 2x your expected LED current.

  1. Pick the cheapest one that's still listed ;-)

Reply to
DJ Delorie 

--
For 20mA, almost anything will work. 2N2222 (PN2222) and 2N3904 are
popular and cheap.
Reply to
John Fields

My personal favorite NPN is MPSA06 (TO92), and for higher power PNP I like MJE170 (TO225). Mostly because I have several hundred of the one and a couple thousand of the other, among my surplus stock. I design many of my projects around these and other components I have in bulk, unless I have to compromise too much on specs.

For driving 8 LEDs (or even relays) it's hard to beat the 18 pin ULN2803 octal darlington driver.

Paul

Reply to
Paul E. Schoen

Wasn't quick enough.

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Reply to
Meat Plow

any GP NPN signal transistor will do really - 2N4124, BC107, BC184, ZTX300 anything really - provided you drive it in common emitter mode (i.e the LED & resistor to the collector, emitter to ground and drive the base with your 3.3v signal via a resistor). Base resistor needs to be about 1K5 and your LED resistor will be about 270R. Should light up nicely.

Reply to
feebo

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