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Topic: TIP31 or TIP122 as a substitute for a TIP120 (Read 24041 times) previous topic - next topic


I need to control a little bit of nichrome wire (heating element) with an arduino. The wire draws 3.8 amps. I found this circit for a light which i think would work:

But it calls for a tip120. I can't get tip120's at my local electronics store although they do stock TIP31's and TIP122's. Would it be possibal to use one of these as a substitute?

If not, here is the full list of TIP transistors they stock, which would be the best substitute?


TIP120 is a 60V 5A NPN Darlington with an Emitter-Base Voltage of 5V and Base Current (DC) of 120 mA.
TIP121 and TIP122 are the 80V and 100V versions.

The TIP122 will work as a replacement for the TIP120.

TIP31 is a 40V 5A NPN. It has a Base Current (DC) of 1 Amp making it unsuitable to drive with an Arduino pin.
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Use a TIP122 and instead of the 2k2 resistor use a 150ohm resistor to get good saturation at that high current.  You'll almost certainly need a large heatsink (given the Vsat information in the datasheet).

You might be better off with a logic-level MOSFET with Rds(on) < 0.04 ohms, should run cooler.
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Yes, a darlington transistor is a poor choice for this application (and most applications) because of the heat it will dissipate. Use a logic level mosfet instead.
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