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using a darling transistor as a switch? Answered

Hi, i want to use my arduino to switch a set of high power LED's on and off (maybe PWM dimming later) but there are two things wrong.
my set up is an arduino uno and a NPN TIP122 darlington transistor, the LED's should draw about 1.7A  max (on measurement its more like 0.2A average)

  1. even when the base of the transistor isn't connected to the arduino, the LEDs are still on. 
  2. when I set the base High using 5 volts from the arduino though a 1K resistor, the LED's are really dim 
Ive looked though just about all of the tutorials online and I dont understand two things that are wrong with my really basic circuit.
Im a serious beginner when it comes to electronics so any suggestions in layman terms would be appreciated 
Thanks

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steveastrouk (author)2014-06-08

your circuit for powering the leds isn't right. Forget the regulator. Put your leds in chains of 5 leds, with an 18 ohms, 0.5watt resistor in each chain. Tie, the chains in parallel, connect them to the transistor

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jim_2000 (author)steveastrouk2014-06-12

yep, lose the regulator and connect your grounds together

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framistan (author)2014-06-08

Is your ground from the arduino connected to ground of your darlington?

Is your arduino turning the lights on and off at a fast rate like 5khz? That would make the lights look half-brite because they are on and off 50 percent of the time.

If you overdrive the base of a darlington, it may burn out the transistor. Also, too many LED's will overheat the darlington and burn it out. Possibly your darlington is bad or is overheating due to no heatsinking?

Must you power it with 3.3 volts? That seems kind of low. If the LED's have a voltage drop of 2.7 volts... doesn't leave much to power them with. Maybe you could power them with 5 or 12 volts and re-calculate what resistor you need.

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bowlen199 (author)2014-06-07

update: I've managed to make them turn off completely now, but they are still much dimmer than they should be

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bowlen199 (author)2014-06-07

I've got 17 of these LEDs http://docs-europe.electrocomponents.com/we docs/0ff5/0900766b80ff5816.pdf

And here is a schematic, I would show the actual circuit but it's a really messy jumble of wires on a breadboard at the moment

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steveastrouk (author)2014-06-07

show your really basic circuits, and say what 'high power'leds you have- voltage and current. I'm guessing right now your two power supplies don't have a common ground.

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