How might I test this pass transistor pack?

[Chris Wilson]

I had big problems with an elderly Farnell 60V @ 50 Amp linear adjustable bench supply a long time ago and thanks to help here I fixed it. Sadly it went bang the other night again, blowing a 50A internal fuse between the full wave bridge rectifier / smoothing caps and the pass transistor block. I replaced the fuse with a temporary 10 A one and on turn on see over 70V DC at the output, with no voltage control. I believe if a pass transistor was shorted the fuse between the bridge and itself would blow? I attach the schematic of the bridge, I changed all the pass transistors from 2N2055 to MJ15003 about 2 years ago, which have a much higher 120V DC rating, as last time this thing had an uncontrolled output from the bridge it blew some 2N2055's.

Note, the input to the bridge is supposed to be controlled via a circuit that uses mark space from the main transformer, the idea being to not have a high drop voltage needed on the pass transistor block. This may have gone berserk but i am not sure if a pass block issue could be feeding the control circuit bum info. So I thought I would try and test it in chunks first seeing if I can use an external supply on the base and wherever on the pass block to see if it will control the output. Then work backwards towards the input of the supply.


So how can i externally control the pass block from a separate bench supply? Where to connect what to DISconnect, and what voltage to apply?

Many thanks!

[David Hess]

I changed all the pass transistors from 2N2055 to MJ15003 about 2 years ago, which have a much higher 120V DC rating, as last time this thing had an uncontrolled output from the bridge it blew some 2N2055's.

Failures are usually related to excessive junction temperature from excessive junction-to-ambient thermal resistance (lack of cooling) or secondary breakdown related which is quite possible in a 60 volt power supply.  Just increasing the Vce breakdown voltage of the transistors does not necessarily improve either.

With that said, the MJ15003 is probably the best transistor in this application for the price.