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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

     

    Is this a custom design you created or a commercial unit your repairing ?, just curious

     

    Thanks

     

    Peter

  • in reply to Robert Peter Oakes

    This is a commercial design that I just pulled off the circuit board onto paper. It is an inexpensive Chinese knock off of an inexpensive Korean unit, which is probably why I am having problems. As I mentioned I followed you last advice and put in an equivalent Darlington with about 40% more tolerance for voltage and current. We will have to wait to see if it comes back to me or not. I have been really impressed with the support I have gotten on this question. Everyone has such good ideas and helpful suggestions. Lucky to have smart friends.

    John

  • in reply to jw0752

    John, guys

     

    While you are quite right that it's really just a voltage source with transistor to boost the current, the fact the design destroys transistors means there is a design issue.

     

    The schematic shows a 36vac input and with no smoothing cap gives around 36v output (give or take). This non smooth supply is a bit brutal on the regulator ...

    Full Wave Rectifier and Bridge Rectifier Theory

     

    My checks show that V2 can adjust the regulator from (approx) 2.55v to 38.75v, and the only current limiting is the R-6 1ohm 5w resistor.

    The TIP122 is rated at 5A max, and without knowing the current rating of the motor, seems very light to me.

     

    A TIP140 at 10A might be a simple substitution and it might require a slight adjustment to the R-3 3K base resistor.

     

    One thing I would suggest next repair is to try the motor with just the 36v and a variac to measure what currents you're seeing at the loads.

     

     

    Mark

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  • in reply to jw0752

    John, guys

     

    While you are quite right that it's really just a voltage source with transistor to boost the current, the fact the design destroys transistors means there is a design issue.

     

    The schematic shows a 36vac input and with no smoothing cap gives around 36v output (give or take). This non smooth supply is a bit brutal on the regulator ...

    Full Wave Rectifier and Bridge Rectifier Theory

     

    My checks show that V2 can adjust the regulator from (approx) 2.55v to 38.75v, and the only current limiting is the R-6 1ohm 5w resistor.

    The TIP122 is rated at 5A max, and without knowing the current rating of the motor, seems very light to me.

     

    A TIP140 at 10A might be a simple substitution and it might require a slight adjustment to the R-3 3K base resistor.

     

    One thing I would suggest next repair is to try the motor with just the 36v and a variac to measure what currents you're seeing at the loads.

     

     

    Mark

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  • in reply to mcb1

    Hi Mark, You are right I should have taken the time to do a little more research on the characteristics of the motor. Victor wanted more info on the motor too. I will be more thorough next time. I too wondered at the unfiltered supply of the LM317. I decided to leave it alone as this was the design and it seemed to be working. I have learned that even my best intentions to make improvements always have side effects. I have to admit that I have created problems for my self enough times that I now always spend extra time considering side effects before I try something. Peter suggested I try a heavier Darlington so I found I had a TIP102 which is the same TO-220 case and has higher specs on the data sheet. I found that it worked well and so I sent the unit back to the dentist. Fortunately this unit is used in the Lab to work on dentures and prosthetics out of the mouth so the operation isn't critical.

    John

  • in reply to jw0752

    I must be slipping, I did not even pick up on the fact there was no smoothing there

     

     

     

    Good catch, and yes that is not a good design

     

     

     

    Peter