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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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  • Hi John, a very good question, and the answer is fairly straight forward

     

    If the Darlington is getting overloaded due to exceeding the current limit of the Darlington then an additional one in parallel would help to fix the problem (or two, or three image ), you must limit the base current separately though with its own 3K resistor and the emitter current through its own 1R resistor, this will prevent one Darlington hogging all the glory.The LM317 has more than enough drive to handle it

     

    basically the Darlington's are acting as emitter followers and will be outputting about 2 diode (B-E junctions) drops less than the output of the LM317 (So about 1.5V ish), as long as you don't exceed the output current of the LM317 or the power supply, add as many as you like in the way I described above. The other possibility is the current capability of the Darlington is not being exceeded but the heat sink is inadequate if the additional devices have their own then this should fix it but if not you may still experience issues so check the heat sink before investing in the additional drivers.

     

    The other option if room is an issue and the heat sink is ok, is to replace the current Darlington with a bigger more powerful one

     

    Hope this helps

     

    Peter

  • in reply to Robert Peter Oakes

    wouldn't having a larger darlington rather than several smaller ones serve better as you wouldn't need to mach them up, probably fit the same mountings and probably cheaper than several smaller transistors.

     

    I think Mark may of alluded to that or similar?

  • in reply to Problemchild

    I did allude to that for sure way up there somewhere, and as it is working in common collector mode, the differences in parameters will have minimal if any impact to the performance

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