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current sharing on psu pass transistors
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m3vuv:
hi all im building a psu that should be good for 0-30v 0-20a,i have 6x tip35 pass transistors on a common heatsink insulated,im using 5 watt 0.2 ohm emitter resistors ,it works ok but one of the pass transistors gets  way hot,the others are luke warm,its running 60 watts of bulbs at 12v,any ideas?
amspire:
0.2 ohms should share the current well enough, so something seems wrong.

It just seems that the hot transistor just does not have a 0.2 ohm emitter resistor. Have you measured the voltages across each 0.2 ohm resistor?
drussell:
Measure the current flowing through each by measuring the voltage drop across each of the emitter resistors. 

Are they all actually conducting the same current?  If so, then you must have an issue with the thermal connection of the one transistor to the heatsink.  (It could be an internally defective transistor, I suppose.)  ??

Edit:  Seems amspire beat me to the same suggestion by 30 seconds.  Kudos!  :)

If they're not conducting the same current, double check the actual value of your emitter resistors and also let us know how you are driving the bases.  Also double check that there is not some unexpected resistance in the collector connections to the source of the power.
Audioguru again:

--- Quote from: drussell on September 28, 2019, 02:48:28 pm ---Measure the current flowing through each by measuring the voltage drop across each of the emitter resistors. 

Are they all actually conducting the same current?  If so, then you must have an issue with the thermal connection of the one transistor to the heatsink.  (It could be an internally defective transistor, I suppose.)  ??
--- End quote ---
Many cheap fake Chinese transistors are like that.
drussell:

--- Quote from: Audioguru again on September 28, 2019, 03:39:35 pm ---
--- Quote from: drussell on September 28, 2019, 02:48:28 pm ---... (It could be an internally defective transistor, I suppose.)
--- End quote ---
Many cheap fake Chinese transistors are like that.
--- End quote ---

Indeed!  That was why I pointed that out in the edit. 

If everything else really does seem to be correct but one transistor behaves poorly, it certainly can be a horribly made defective transistor, especially these days, especially with commonly cloned parts like TIP- series or 3055s or whatever....  This was much less of a problem "back in the day" but sadly is very high on the suspect list of late.
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