Driving Multiple LEDs from a transistor

Zoot

I want to drive 4 or 5 LEDs (@ ~17ma each) from a transistor.

I wired up one LED to a transistor and a micro pin following the attached schematic (which has been reproduced many places in these forums).

So... if I wanted to hook up several LEDs, can I tie the cathode of all of them to the transistor and the anode of each to Vdd through it's own limiting resistor? Or would I need a transistor for each (I would think not, since the 2N3904s I have on hand should be able to handle up to 100ma).

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MikeK

Depending on Vdd, the forward voltage drops of the LEDs and the Vce of the transistor, you can put more than one diode in series with each resistor. You'd use a smaller resistor than you would with only one LED. The advantage is that you're using the power that would have been dissipated as heat by the resistor to light an LED instead. You still only need one transistor. Don't put LEDs directly in parallel. It isn't a good idea, no matter what some web sites say.

Mike

Mr. Richard

Just my inexperience, but why do you want to avoid parallel LEDs?

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Magic Smoke Theory of Electronics –
Inside every electronic part there is magic smoke.
The magic smoke is what makes everything work.
If you release the magic smoke, the part stops working!

Bean

Mr. Richard said...
Just my inexperience, but why do you want to avoid parallel LEDs?

Because each LED will have a slightly different forward voltage drop. So what happens is much more current flows through the LED with the lowest voltage drop, and very little flow through the LED with the greatest voltage drop. Of course this results in very uneven LED lighting, and can damage the LEDs (because one LED is carrying a much larger portion of the total current).

It okay to parallel them if each LED has it's own dropping resistor.


Bean.


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Zoot

Thanks all. I wired up my LED banks -- each with it's own limiting resistor. Nice and bright! Dang, transistors are cool.

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When the going gets weird, the weird turn pro. -- HST