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Transistor/ current question

TheGrovesy

Dec 19, 2011
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Dec 19, 2011
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Hi all, I am a software engineer by trade but have recently started playing with some basic electronics.

I am creating a fan controller for a 12v fan which is driven by a microcontroller (arduino) and a transistor.

I successfully got this working with a 5v supply passing through the transistor (I tested this with a multimeter and saw that it was reading 5v for half a second and 0v for half a second, as programmed).

Great i thought, now lets try it with 12v (without the fan) and with a puff of smoke the transistor was dead!

Now i assume this has happened because there was no resistor before the transistor causing all 1.7A to pass through the transistor, is this correct? Below is an illustration on my setup.
Flickr Link

Also, will there be an issue having two different power sources? This wont be true for the final pcb, but as a mock up the arduino is powered by the USB on the PC and the 12v is coming from a variable power supply.

Many thanks,
Chris
 
Last edited:

jackorocko

Apr 4, 2010
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Apr 4, 2010
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The transistor released some magic smoke and saved your battery from exploding. Without a load, when the transistor conducts it creates a short between the + and - side of the battery.

FYI, most people use an LED/resistor to help visualize what is happening.
 
Last edited:

Laplace

Apr 4, 2010
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From your description I don't believe that the 1.7A current was the problem. In both cases the base current from the microcontroller driving the transistor would have been the same, and the collector current would have been Hfe times the base current. The difference was the collector-emitter voltage. In the first case the transistor power dissipation (IxV) was 1.7x5 watts whereas in the second case it was 1.7x12 watts. 'Twas power killed the transistor, not current.
 

BobK

Jan 5, 2010
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You also need a resistor from the micontroller to the base of the transistor. BE is a diode, so this also looks like a short when the output is high. This is likely pulling more current than the miccontroller can safely ouptut.

Bob
 
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