[R-390] OT: Pass Transistor Question

[Dave Maples]

Bob: I may be behind here, but the equalizing resistors in the emitter leads
of the pass transistors are really there to keep one transistor from hogging
a lot of current, failing, and then allowing the failure to cascade through
the other transistors.  I just scrapped a modern import power supply because
it omitted the resistors, and half the pass transistors were nuked.  It
wasn't worth it to me to redesign it with the proper stuff.

As far as bypass resistors and caps across diode strings, that's a different
story, I believe.

All replies welcome!

Dave WB4FUR

-----Original Message-----
From: r-390-bounces at mailman.qth.net
[mailto:r-390-bounces at mailman.qth.net]On Behalf Of rbethman
Sent: Wednesday, October 15, 2008 12:07 PM
To: R-390 at mailman.qth.net
Subject: Re: [R-390] OT: Pass Transistor Question


Barry,

The equalizing resistor use is the result of broader mfr tolerance in
days of yore.  You pointed out a reference to a 1978 handbook.  Remember
in that time when we built ridiculous bridge rectifiers for HB
Amplifiers?  Same philosophy!

If you are using more modern components, don't need the equalizing!  If
using "OLD" saved parts, equalize!

I built a power supply about 1980 - 1981.  used 5 2N3055s on one BIG
heatsink.   Go for it!

Bob - N0DGN

Barry wrote:
>> A few weeks ago, I mentioned I wanted to build a regulated,
>> low-voltage power supply that uses an LM317 with some pass
>> transistors to enable it to handle a larger current than the LM317
>> can handle.  The circuit uses a 2N2905 to drive a 2N3055 in a "pass
>> transistor" configuration around the LM317 and the circuit
>> description states it's capable of delivering up to 3 amps in this
>> configuration.
>>
>> I have a transformer with a 6-amp capacity and I would like the power
>> supply to deliver up to 5 (maybe all 6 amps) if I want.  Is it
>> possible to put another 2N3055 in parallel with the one in the
>> circuit allowing thus allowing the pass transistors to deliver the
>> extra amperage.  Will this work?
>>
>> I ran across something like it here:
>>
>> http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm
>>
>> (Scroll down to the middle of the page).  The two 2N3055s are
>> paralled and have 0.1-ohm resistors in the emitter lines.  Not sure
>> what these equalizing resistors do, but perhaps that's all I need to
>> add?
>>
>> By the way, the original circuit is on page 127 of the 1978 ARRL
>> handbook.
>>
>> Thanks guys for the OT help.
>>
>> Barry
>
> Hmmm.  I backed up a page or two (p. 124) in the ARRL Handbook and it
> discusses parallel pass transistors, specifically 2N3055's.  I'm still
> wondering what the equalizing resistors do.  Anyone care to explain?
>
> Thanks again,
> Barry - N4BUQ
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--
Bob - NØDGN

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