W
Watson A.Name \"Watt Sun
Guest
Tweetldee wrote:
http://www.ampslab.com/c200cfp.htm
"Albert" <pass@pass.com> wrote in message
news:8daq809a40983ebqc6abs9te4hja7vn2p6@4ax.com...
17 volts at the secondary for your transformer sounds quite normal for a
12
volt supply. The flag that I saw raised was the fact that it drops to
15.5
volts under moderate load. Sounds like there may be a problem with the
transformer or a connection to it, either on the primary or secondary
side.
Eliminate defective connections as the source of your problem, especially
around the high-current secondary.
If the pass transistors are mounted in sockets, make sure the connections
are good there as well.
All the connections are good, I checked them mechanically and
confirmed by measuring the voltage drop across the high current
connections.
But there's a bigger problem with your transistors... they probably
aren't
in parallel, since the 2N3055 is an NPN transistor, and the MJE2955 is a
PNP
unit. The MJE2955 is in a totally different case than the 2N3055, so
it's
very unlikely that these are supposed to be in parallel. Recheck your
wiring and see if maybe the MJE2955 is used to drive the 2N3055. Even if
the wiring indicates that it's the driver for the pass element, it's
strange
that it is a PNP transistor. Maybe you meant to say MJ2955 instead of
MJE2955 ????
Ah, yes! You caught me! Yes, it is an MJ2955, my error. Sorry.
Another problem lies in the fact that the 2N3055 has a spec'ed max
collector
current of 15A. If this is the only pass transistor in the supply, it
can't
possibly handle 25A. It's possible that if the MJ(E)2955 that you found
in
the supply should be another 2N3055. That would be much more believable
than the MJ(E)2955.
Not sure which transistor should be there, but they are definately in
parallel! The 2N3055 blew, the MJ2955 still operates and I used it to
test the supply with.
Sounds like the someone who attempted repair on this supply didn't really
know what he was doing, and may have put some wrong parts in there.
I am sure someone with a low level of competancy repaired it the last
time it blew. Many of the soldered connections left a lot to be
desired although I did not find any connection that actually failed.
It says 25A on the front, but looking at the innards, it is clear that
it probably never produced 25A. I suggested to the owner that we
rebuild it as a 15 A constant duty with 20A intermittent output
because it probably can't produce 25A no matter what we do to it. So,
we are going to rebuild it from scratch.
My hope was to gain some extra head room with a quick and dirty pass
transistor and rectifier swap and build a simple regulator from the
ground up.
Should I be looking at a switching supply or try to squeek out a few
extra volts of headroom by maintaining the origianl design with some
improved parts?
Thanks for all comments.
Maybe they're a Sziklai pair?Well, that only adds to the confusion!!! As I said in my earlier post, the
MJ2955 is a PNP unit, whereas the 2N3055 is NPN. They can't operate as
series pass elements in parallel!!!!!!
Is the output of the supply regulated now? Is it holding the output
constant with changing load?
If the supply is operating (albeit limping) with only the MJ2955 in the
circuit, then I have to ask for a schematic, or at least a sketch of the
circuit. I'm curious as to how this supply is built. What's the make/model
of this supply?
If it's simple enough to sketch, that may allow us to offer more intelligent
answers to your dilemma.
Cheers!!!!!
http://www.ampslab.com/c200cfp.htm