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Why do these transistors overheat?

Discussion in 'General Electronics Discussion' started by Eratosthenes, Nov 20, 2015.

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  1. Eratosthenes


    Nov 20, 2015

    I am trying to control a stepper motor (1.68 A , 1.65 Ohm) by using an arduino mega. Therefore I built an h-bridge which can be seen in the following circuit diagram:[​IMG]
    Here GPIO 1 and 2 are two digital output pins of the arduino and the 12V DC comes from an external power supply (12 V, max. 30 A). I connected the ground of the power supply with the ground of the arduino. To power the motor, I connected two of those h-bridges in parallel (one for each coil). The problem: When I switch the power supply on, the transistors (IRF9630 PNP and IRF630 NPN) of the h-bridge get extremely hot and if I do not switch off after 2-3 sec, they start to smoke. What did I do wrong? Is there anything wrong with the circuit itself or is it normal that the transistors get hot and I simply have to add large heat sinks to them?

    Some of the used parts:
    · stepper motor:
    · power supply:
    · arduino mega:
    · IRF9630 PNP:
    · IRF630 NPN:
    ·2N2222 (or rather a similiar model):
    · 5W 2Ohm high load resistors:

    Thanks for your support!
  2. duke37


    Jan 9, 2011
    1.Connecting two power supplies in parallel will only work if they are trying to do the same thing, however, I think that you are not connecting them in parallel.

    2. Each 2N2222 pulls it collector low when switched on and should allow its collector to rise to 12V when switched off. You have placed a resistance to ground to prevent this, thus the FETs are never turned off and short out the power supply.
    Tha fios agaibh likes this.
  3. dorke


    Jun 20, 2015
    Try this modified circuit: h-bridge.jpg
    Tha fios agaibh and duke37 like this.
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