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Topic: Transistor not giving expected voltage (Read 1 time) previous topic - next topic


I'm using a 12V power supply for powering my Arduino Uno board. I want to make a lamp blink that's apparently rated for up to 125 V (it lights enough under 12 V, but it's way too dim under the standard 5 V). Since the board's output pins' HIGH value is 5 V regardless of the power supply, I figured I'd use a 2N222 transistor as a switch to switch the 12 V to the lamp.

So I have the collector connected to the Vin pin, the emitter pin connected to the lamp, and the base pin attached to a 10K ohm resistor and then one of the output pins.

The problem is whenever the output pin is HIGH, the voltage read across the collector/emitter is still 5 V (a little less actually). Why is this? I figured the transistor would act as a simple switch and turn on the 12 V across it...?


Jul 10, 2011, 04:15 am Last Edit: Jul 10, 2011, 04:24 am by JoeO Reason: 1
Connect the emitter to ground.  The base through the resistor to the Arduino pin.  The bulb then connects between the collector and the +12V.
Then read the emitter to collector voltage.  It should be close to what you are looking for.

I assume you are using a 2N2222 transistor.....


Hmm, I don't understand fundamentally how that's different from my circuit, and the multimeter still read 4.5 V, but the light works now!


You had an emitter follower, what was needed was a common-emitter circuit.  Emitter follower cannot amplify voltage, only current.  Google these terms for more information.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


4.5 V is too much, decrease R from 10 k Ohm to 1 k Ohm or so.
Btw, transistor rated for 0.6 A max, what kind of lamp do you have, I'm afraid it take more current
than 0.6 A


The "lamp" I'm referring to is actually an illuminated pushbutton that says "125V MAX AC/DC, 3W MAX" on the full voltage unit. The light lights under the 12 V, albeit somewhat dimly. I know transistors can amplify so is it possible to somehow amplify the 12V to make the lamp brighter?


Yeah, I just looked up the bulb online and it states it a 28 V bulb. And I kinda figured transistors didn't have magical voltage-increasing powers, but thanks for confirming it. :)

Thanks for all the help everyone!

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