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Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
I have this old linear PSU that needs some new series pass transistor emitter resistors installing and was just wondering if they need to have a low temperature coefficient (PPM) since they are used as balancing resistors for the series pass transistors (2 in parallel).
Both the transistors and resistors get pretty hot when the PSU is being used at its max output current of 3 amps, which for the transistors is to be expected when they have to drop the DC bus voltage down from 23.5v to 12v.
So I am just wondering if they need to be low tempco resistors or anything like that to maintain thermal stability.? They are .5 ohm 5w.
Thanks.
Schematic of the supply in question:
The series pass transistors and emitter balancing resistors are at the top of the image.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
The emitter resistors are normally wirewound with tempco. of wire, no compensation. The 'compensation' is that each base-emitter voltage will vary similarly with temperature, balancing current sharing. The sharing is rarely exactly 50:50, close is normally good enough.
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
As above, the ones I’ve seen are all ceramic wirewound or metal clad. But you may find 0.47 ohm easer to obtain, then I would definitely change all the emitter resistors so they are all the same type.
Edit, If it’s quite an old psu might be worth checking/replacing some of the electrolytic caps in it while your at it. I have a similar old Coutant one that the Auxiliary supply cap went open circuit in.
Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Must have forgotten to reply to this thread but thanks! I replaced the resistors and scoped the ripple on the capacitors which seem to be within 15% when at full load.
I don't have an ESR meter but non of them appear to be getting warmer than they should.
I have it powering an array of LED lights in my room drawing just under the maximum current capacity, the heatsinks for the series pass transistors make for a nice little desk heater that's for sure!
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Balancing resistor temperature coefficient
