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Old 28th April, 2002, 03:53 PM
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Question NPN Transistor Fan speed controler questions

just got done building one of these:
http://www.hardforum.com/showthread...=npn+transistor

I used a 50kohm Liner Pot and NPN transistor MPS2222A(RadioShack Part)
(http://filebox.vt.edu/eng/ee/faculty/lai/mps2222.pdf)

My problem is.........

When ever I turn the pot to 7.00v to 5.00v I get an odd buzzing from my power supply, I opened my power supply to see if it was arching or something visable and found no clues as to what was making the noise. Has anyone ever heard of this before and if so what was making the noise?

Here's some more info.

Ok I went and hooked it all up and put a delta screemer(.48A) on it and.....(With 50kohm pot)

1. Using a Multimeter I checked voltages- I could change it from 0.00 to 11.43v
2.The NPN got hot with only 1 fan on it I would say about 120-130F(I'll check in the morning with my temp probe.)

I also used the 100kohm liner pot and got the exact results only when turning the nob it was more touchy(voltages increased/decreased more with the same amount of turn on the pot than the 50kohm).

O and here's how I hooked it up....

12V in to Pin 3(NPN) and middle leg of pot
Left leg of pot to Pin 2(NPN)(Nothing on right leg of pot)
Pin 1(NPN) to to fan

Should something be hooked to the other leg of the pot?

Thanks
Kook
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Old 28th April, 2002, 08:40 PM
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No idea about your PSU, but problems with the circuit...

It's ugly, poorly designed, and likely to kill the transistor eventually.

What's wrong with it?
  • No base current limiting resistor. Any variable resistor has a 0 ohm rating when turned fully up (down). Not good for transistor
  • No design work with reference to the linear curve of the transistor.
  • Transistor current output varies with temperature(!) for a given setting.
  • Not being familiar with your fan, you might be close to the maximum dissipation of the transistor.

At a guess, you'd be aiming for a base current of 4mA for full power, and 1mA for minimum power. This assumes your transistor is at 25C. Running it above 25C you'll find that you need less base current for the same output current.

Still no idea why your PSU would be growling, unless it's unhappy about the level of load presented to it. What else is on the 12V line?

AidanII
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Old 29th April, 2002, 05:59 AM
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I prefer linear voltage regulators myself, easy to implement, few components, and quite reliable . . .

I havn't given PWM a try yet, but it definately has advantages, so I may give it a shot . . .
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Old 5th May, 2002, 02:08 PM
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For variable control this low voltage drop out reg is the way to go. Can get up to 11.7 volts and up to 3 amps with heatsink.

http://www.bit-tech.net/article/52/5

RS Components have this reg
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