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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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  • John

    a quick answer before heading to work.

     

    I'm not surprised the darlington fails.

    This is only working because there is a drop across the series 1 ohm resistor.

     

    The transistor is turning on, then its emitter is rising such that it turns off, and its always operating in never really ON or OFF mode so it will heat up.

    My suggestion is to look at putting the motor and diode in the collector, and then tweak the regulator to detect the voltage across the 1 ohm, which will be between the emitter and ground.

     

    Adding a second darlington might load share, but the basics are wrong.

     

    Mark

  • in reply to mcb1

    If you do that, you will change it from being a current amplifier (Keeping the voltage regulated based on the two adjustments on the LM linear regulator, im assuming a course and fine adjustment)

     

     

     

    After the change the transistor would be working as a voltage amplifier and very dependent on temp and other factors

     

     

     

    The circuit John posted is a classic circuit to boost the capacity of a regulator, I have been using variations of it for over 30 years with no issues

     

     

     

    Look at any linear regulator circuit and you will see this, think of the LM as a voltage reference with a higher then normal output ability, the pattern is also the same as almost every single linear bench power supply out there just without current limiting so I don’t think John needs to change the topology of the design

     

     

     

    The rising and turning off in this case is supposed to happen and is the negative feedback, the voltage gain is ideally 1. With a massive current gain, look at the WIKI on common collector BJT for more details and a fuller description

     

     

     

    Peter

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  • in reply to mcb1

    If you do that, you will change it from being a current amplifier (Keeping the voltage regulated based on the two adjustments on the LM linear regulator, im assuming a course and fine adjustment)

     

     

     

    After the change the transistor would be working as a voltage amplifier and very dependent on temp and other factors

     

     

     

    The circuit John posted is a classic circuit to boost the capacity of a regulator, I have been using variations of it for over 30 years with no issues

     

     

     

    Look at any linear regulator circuit and you will see this, think of the LM as a voltage reference with a higher then normal output ability, the pattern is also the same as almost every single linear bench power supply out there just without current limiting so I don’t think John needs to change the topology of the design

     

     

     

    The rising and turning off in this case is supposed to happen and is the negative feedback, the voltage gain is ideally 1. With a massive current gain, look at the WIKI on common collector BJT for more details and a fuller description

     

     

     

    Peter

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