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Topic: Transistor Switching (Read 1 time) previous topic - next topic


Hi everyone,

I need help choosing a transistor to switch some high power LED's on and off. I volunteered to figure out a cool flickering campfire prop for my churches kid's camp.

I have 6 Cree XRE-Q5 with an appropriate driver for them. The driver requires you to run the LED's in series, and then it provides a constant current to drive the LED's. The max current that the LED driver can push is 800 ma @ 17.5V (~14W output)

I would like to be able to control the brightness of the LED's using PWM output on pin5 by switching the LED's through a transistor. I plan on having the transistor making and breaking the hot leg of the driver output. I would like to switch the LED current ON when my output from pin 5 is high, thus I need an NPN transistor

I would like some guidance as to how to choose an appropriate transistor. Here is what I have so far.

1) a maximum collector current (IC) greater than my desired load current of 800mA (I would like a max 1A, as I plan on driving the LED's at 700ma. A 30% safety margin seems logical)
2) It needs to be able to switch with a relatively low IB I'd like to reserve as much current handling capability of the arduino as possible. Just in case I need it for something else.
3) If I need a resistor between pin 5 and the base of the transistor, how do I choose it? I know my source from the arduino will be 5V.
4) Do I need to place a diode between the transistor base and the arduino to protect it from potential overvoltage from the transistor?

Anyone know of cheap sources of a >15 Watt wall wart?

If you have an hints, tips, or guidance I would be grateful.

Thank you kindly in advance.



I would use a relay, but I need to fade the brightness smoothly. Mechanical relays are not able to switch rapidly enough, and I don't want to fork out $20 on an SSR than can pass DC through it's load side. Besides, i'm not sure SSR's can switch that rapidly.


then try do some work on your breadboard, play with different transistors, because each transistor has a beta (factor) so all you can do is use different resistors until you get what you want. also try using an amplifier the ua 741 is good and again use resistor to get the gain you want


To be honest, I was hoping for some guidance beyond "randomly try values". I appreciate your help though!

http://www.futurlec.com/Relays/SSR2A50D.shtml I found this SSR, it's only 9 bucks. it has a max turn on 0.5ms and off 1ms. Would a total on-off cycle time of 1.5ms be quick enough for smooth PWM effects?

How many times a second can transistors switch?



AH! Brain fart.

If I have a 1.5ms cycle time... that means 1000ms/1.5ms/cycle = 666 cycles per sec. or 666Hz. That should be plenty quick enough! Us humans can't really see flickering beyond 100 Hz.

I am thinking that this should work nicely. If anyone doesn't think this would work, please pipe up. 


For switching 800mA or more I would use a mosfet, not a bipolar transistor.

Your LED driver may not take kindly to having its output switched in this way.
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Doesn't have your driver a pin for switching it's load on and off? dc42 is right that switching the load side of a constant current driver isn't a good idea. You should switch the input side and not the output side. A description or picture of the circuit with details on the driver helps us to analyze your project.


May 07, 2012, 06:06 pm Last Edit: May 07, 2012, 06:10 pm by winner10920 Reason: 1
A bc337 (max 800ma)would probably work well with a resistor to get you a base current of like 12ma you may be able to get away with a 480 ohm resistor, and if you want even less current consumption on the base a tip122 (max 5A) would work with a 1k resistor for <.005amp
and with 700ma on the tip122 I wouldn't worry about a heatsink, maybe a small one if anything, the bc337 I've never drivin to near max so idk about the heat on that


I'm going to have to go with the MOSFET suggestion on this one.  Switching DC is what they do best.  Also, high input impedance, so you don't need to worry about current into the gate.

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