NPN transistor dimmer/voltage controller

Thread Starter

rudyauction8

Joined Jan 27, 2012
250
I have a NPN power transistor and some assorted transistors and I want to use them to build a dimmer/voltage controller by quickly switching the power transistor on and off. It needs to run on 3-15 volts and control up to 20 watts (the rating on the power transistor). I have only the most basic electronic components: capacitors, resistors, a few diodes, a 555 and some other transistors.
 

PaulEE

Joined Dec 23, 2011
423
So use the 555 timer to the base of the transistor? and a variable resistor? but where do I put the variable resistor? Other than that problem, I'm good
Take a look at this:

http://www.dprg.org/tutorials/2005-11a/index.html

The output going to the MOSFET in this schematic will instead go to your NPN power transistor. I would use a worst-case gain of 20. At an output of V volts from the 555 circuit, you want 20 * V/R to be equal to the base current that'll saturate (turn the transistor fully ON) the transistor.

Example:
If you use a TIP31 power transistor and you want .5 amp pulses, you'd divide .5 amp by 20. This gives a required base current of 25 mA. If your output voltage is, say, +10v, the base resistor should be 10/.025 = 400 ohm; I'd try for 390 ohm.

If you have a power mosfet, say an IRF510 from Radioshack, I'd use that before I'd use an NPN transistor.

Also, in either case, if you plan to drive inductive loads (motors, fans, coils, transformers) be sure to use a snubber diode. Coils that are energized store energy...and when you abruptly take away the current through them, the electromagnetic field surrounding the coil collapapses. Faraday's Law says that the rate change of flux of that field is equal to the voltage across the coil...which, if you abruptly shut the coil off, you'll fry your transistor and possibly other things.

If you do have an NPN that you must use, I would use a 2n2222 or similar transistor in a darlington configuration at the output of the 555 timer circuit. This is because the 555 circuit probably won't be happy if you draw 25 mA from it.

Cool?
 

Thread Starter

rudyauction8

Joined Jan 27, 2012
250
OK, I've figured this out and finished the circuit, it's driving some flashlight bulbs. I'm using a PNP transistor to drive the base of the NPN power transistor because the 555 gives negative pulses. It seems to be working fine now. Thanks for all of your help.
 

PaulEE

Joined Dec 23, 2011
423
OK, I've figured this out and finished the circuit, it's driving some flashlight bulbs. I'm using a PNP transistor to drive the base of the NPN power transistor because the 555 gives negative pulses. It seems to be working fine now. Thanks for all of your help.
Great! Glad to hear.
 

PaulEE

Joined Dec 23, 2011
423
No.
An ordinary 555 has a max allowed output current of 200mA.
A Cmos LMC555, TLC555 or ICM7555 has a low output current.
http://www.dprg.org/tutorials/2005-11a/index.html

Take a look at the circuit a few figures down. The output is taken from "discharge", a 10K resistor, and ground.

The "output" terminal is referenced in this mini-article and is said to be able to sink 200 mA. It doesn't use the "output" terminal to drive the transistor.

Since the discharge pin is hooked up to an NPN within the chip, he used a PNP/NPN darlington buffer to drive his lamps, because the signal gets inverted once for the PNP and another time for the NPN...

Are we on the same page now? :)
 
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