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Feasability of Adding a Second Parallel Darlington?

jw0752

        I am currently working on a small circuit that controls the voltage to an electric dental lab handpiece. Please see the schematic below. This circuit has had a chronic problem with failure of the TIP-122 Darlington. The problem arises when the handpiece operator applies pressure which adds load to the motor and the motor then demands more current from the circuit. This circuit is very simple with basically no overload protection. The original design did not even have a flyback diode across the motor. My question is if it is practical to add another TIP-122 Darlington in parallel with the first one in order to improve current handling capacity. Does anyone have any experience with this type of modification? I also have a curiosity codicil to my question. If it is practical to add another Darlington, what would be the limiting factor to how many parallel Darlingtons can be added in this way? Any insights would be appreciated.

John

 

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  • The pwr on led needs a reversed signal diode in shunt.  LEDs have a reverse breakdown potential of about 6V.  When we parallel bipolars, as an earlier poster stated, we need ballast resistors to minimize current hogging due to the negative tempco of the Qs.

     

    Hanging an emitter follower  on the VR produces an output a diode drop below the regulator output.  As a prior poster noted your Q is operating in the linear mode, probably you should embiggen your heatsink.  When one does that (hangs a Q on a VR) he does increase the ampacity of the regulator.  But he loses the thermal shutdown feature.

     

    The http://www.ti.com.cn/cn/lit/ds/symlink/lm395.pdf LM195 is the transistor for you.  Even though it is bipolar, they can be paralleled freely as they have thermal feedback control on-board.

     

    You can do better than this.  I suggest you read a book or two on operational amplifiers and maybe one on SMPS design.

  • in reply to D_Hersey

    Hi Don, Thanks for the information on the LM195. This looks like a very nice component to know about. As for the design of the circuit we are all in agreement that it is not a particularly good one. Not saying that I personally could have done better but at least in this case it can not be hung around my neck as it is a commercial product that I have been tasked with fixing. I took the circuit board and drew the schematic from it so that my friends here could better advise me. Button up your coat as some really cold weather is blowing by headed in your direction.

    John

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  • in reply to D_Hersey

    Hi Don, Thanks for the information on the LM195. This looks like a very nice component to know about. As for the design of the circuit we are all in agreement that it is not a particularly good one. Not saying that I personally could have done better but at least in this case it can not be hung around my neck as it is a commercial product that I have been tasked with fixing. I took the circuit board and drew the schematic from it so that my friends here could better advise me. Button up your coat as some really cold weather is blowing by headed in your direction.

    John

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