low voltage drop transistors for power supply

[Albert]

My neighbor brought his 25A (12 volts) power supply over for repair
last evening. I found the bridge rectifier shorted and one of the 2
pass transistors shorted. The 20A fuse in series with the output was
not blown, the 4A line fuse was blown.

I patched it enough to test it, putting in a 10A bridge rectifier and
using only one of the pass transistors (they were in parallel).

I found the transformer which looked hugh, was only putting out 17
volts no load (dc, measured at the bridge output) and 15.5 at 10A
load. While the transformer probably has the current capacity, it
probably loads down under full load (at 20A). I couldn't test at 20A,
but he confirmed that the ham radio transmitter had some quirky
behavior, probably due to the voltage drop off at the higher load
currents.

It looks like someone (previous owner) disconnected the metering in
order to get around the voltage drop associated with the meter.

I'd like to put the thing back together for him, and have it operate
properly.

It had a 2N3055 and an MJE2955 pass transistors-not sure why they were
different types, they were in parallel.

I need suggestions for low collector to emitter voltage drop pass
transistors that will handle 15A each. I also wonder if a bridge
rectifier made from schottky diodes. Suggestions for the schottky
diodes??

Thanks,

A



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

 

[Tam/WB2TT]

"Albert" <pass@pass.com> wrote in message
news:t66q80p0b2jndp3b9413glj3lecodgcdfi@4ax.com...

My neighbor brought his 25A (12 volts) power supply over for repair
last evening. I found the bridge rectifier shorted and one of the 2
pass transistors shorted. The 20A fuse in series with the output was
not blown, the 4A line fuse was blown.

I patched it enough to test it, putting in a 10A bridge rectifier and
using only one of the pass transistors (they were in parallel).

I found the transformer which looked hugh, was only putting out 17
volts no load (dc, measured at the bridge output) and 15.5 at 10A
load. While the transformer probably has the current capacity, it
probably loads down under full load (at 20A). I couldn't test at 20A,
but he confirmed that the ham radio transmitter had some quirky
behavior, probably due to the voltage drop off at the higher load
currents.

It looks like someone (previous owner) disconnected the metering in
order to get around the voltage drop associated with the meter.

I'd like to put the thing back together for him, and have it operate
properly.

It had a 2N3055 and an MJE2955 pass transistors-not sure why they were
different types, they were in parallel.

I need suggestions for low collector to emitter voltage drop pass
transistors that will handle 15A each. I also wonder if a bridge
rectifier made from schottky diodes. Suggestions for the schottky
diodes??

Thanks,

A



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----
Albert,

The 2N3055 is way too wimpy for this use. They run out of gain at around 5
amps. They should have used 4 of them. Probably what happened is that one
transistor was carrying all the load, due to uneven load sharing, and
shorted. I would use a minimum of a pair of 2N3071 transistors.

Schottky rectifiers would be good. If the transformer is open frame, I would
see if you can thread a few more turns of wire on there for the secondary. I
would start with 4 -5 turns. If this reduces the voltage, reverse the
connections. I have done this with #14 stranded wire, it worked OK.

I suspect this supply might have started life as a 12.0 V supply, not 13.8
which is what your friend needs.

Tam/WB2TT

 

[budgie]

On Mon, 26 Apr 2004 10:14:18 -0400, Albert <pass@pass.com> wrote:

My neighbor brought his 25A (12 volts) power supply over for repair
last evening. I found the bridge rectifier shorted and one of the 2
pass transistors shorted. The 20A fuse in series with the output was
not blown, the 4A line fuse was blown.

I patched it enough to test it, putting in a 10A bridge rectifier and
using only one of the pass transistors (they were in parallel).

I found the transformer which looked hugh, was only putting out 17
volts no load (dc, measured at the bridge output) and 15.5 at 10A
load. While the transformer probably has the current capacity, it
probably loads down under full load (at 20A). I couldn't test at 20A,
but he confirmed that the ham radio transmitter had some quirky
behavior, probably due to the voltage drop off at the higher load
currents.

It looks like someone (previous owner) disconnected the metering in
order to get around the voltage drop associated with the meter.

I'd like to put the thing back together for him, and have it operate
properly.

It had a 2N3055 and an MJE2955 pass transistors-not sure why they were
different types, they were in parallel.

I need suggestions for low collector to emitter voltage drop pass
transistors that will handle 15A each. I also wonder if a bridge
rectifier made from schottky diodes. Suggestions for the schottky
diodes??

Sounds like a can of worms. One issue is that the venerable 2N3055 isn't all it
is cracked up to be, and was often mis0used in serious power supply roles. If
you still can, get hold of a REAL data sheet with curves, and you'll find that
hFE drops off to single digit gains at 7A or so. Thius often leaves them
under-driven and hence the series volt drop becomes a problem.

Also, if this is an NPN-emitter-follower-in-the-positive lead then the
configuration is worthy of review. When the filtered bridge output is
struggling for headroom, high-side drive is going to be hard to achieve when the
going gets tough, compounding the 2N3055 gain problem. Although you probably
want a drop-in replacement rather than a major design project, a decent PNP or
two would make a better regulator in the face of low headroom. At a pinch, look
at the whole 2955/3055 family with particular attention to the MJE types rather
than the 2N3055.

Schottky diodes will "buy" you something less than a volt IF you get a suitable
high-current type, but a 40A device is unlikely to gain you much at 25A - you'll
be looking at 80A or bigger devices to get the lean volt drop @25A.

 

[Tweetldee]

"Albert" <pass@pass.com> wrote in message
news:t66q80p0b2jndp3b9413glj3lecodgcdfi@4ax.com...

My neighbor brought his 25A (12 volts) power supply over for repair
last evening. I found the bridge rectifier shorted and one of the 2
pass transistors shorted. The 20A fuse in series with the output was
not blown, the 4A line fuse was blown.

I patched it enough to test it, putting in a 10A bridge rectifier and
using only one of the pass transistors (they were in parallel).

I found the transformer which looked hugh, was only putting out 17
volts no load (dc, measured at the bridge output) and 15.5 at 10A
load. While the transformer probably has the current capacity, it
probably loads down under full load (at 20A). I couldn't test at 20A,
but he confirmed that the ham radio transmitter had some quirky
behavior, probably due to the voltage drop off at the higher load
currents.

It looks like someone (previous owner) disconnected the metering in
order to get around the voltage drop associated with the meter.

I'd like to put the thing back together for him, and have it operate
properly.

It had a 2N3055 and an MJE2955 pass transistors-not sure why they were
different types, they were in parallel.

I need suggestions for low collector to emitter voltage drop pass
transistors that will handle 15A each. I also wonder if a bridge
rectifier made from schottky diodes. Suggestions for the schottky
diodes??

Thanks,

A

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.

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 ????
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.

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.

--
Dave M

Good judgement comes from experience, and a lot of that comes from bad
judgement.

 

[Albert]

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.


-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

 

[John Popelish]

Albert wrote:

My neighbor brought his 25A (12 volts) power supply over for repair
last evening. I found the bridge rectifier shorted and one of the 2
pass transistors shorted. The 20A fuse in series with the output was
not blown, the 4A line fuse was blown.

I patched it enough to test it, putting in a 10A bridge rectifier and
using only one of the pass transistors (they were in parallel).

I found the transformer which looked hugh, was only putting out 17
volts no load (dc, measured at the bridge output) and 15.5 at 10A
load. While the transformer probably has the current capacity, it
probably loads down under full load (at 20A). I couldn't test at 20A,
but he confirmed that the ham radio transmitter had some quirky
behavior, probably due to the voltage drop off at the higher load
currents.

It looks like someone (previous owner) disconnected the metering in
order to get around the voltage drop associated with the meter.

I'd like to put the thing back together for him, and have it operate
properly.

It had a 2N3055 and an MJE2955 pass transistors-not sure why they were
different types, they were in parallel.

I need suggestions for low collector to emitter voltage drop pass
transistors that will handle 15A each. I also wonder if a bridge
rectifier made from schottky diodes. Suggestions for the schottky
diodes??

There are very few choices in Schottky bridges in this size, but there
are lots of single diodes big enough for this task. The best choice
probably has more to do with how you can mount them on the heat sink
than anything else. This is a good idea just to reduce the heat
inside the box.

For the pass transistors you might look into something much beefier,
like the 2N5684 2N5686 size. These are rated for 50 amps but have
good low voltage gain to about 10 or 20 amps. They have enough
capacitance that they might destabilize the regulator.

--
John Popelish

 

[Tim Wescott]

Albert wrote:

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.


-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

What is it using for a regulator? You may want to consider boosting the
main supply by putting in a little 6.3-volt transformer + bridge to make
a base supply for your pass transistor. This will let you drive the
pass transistor into saturation, which will give you 1/2 to 1 volt more
headroom.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Tweetldee]

"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.

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!!!!!
--
Dave M

Good judgement comes from experience, and a lot of that comes from bad
judgement.

 

[Albert]

On Mon, 26 Apr 2004 12:01:15 -0400, John Popelish <jpopelish@rica.net>
wrote:

There are very few choices in Schottky bridges in this size, but there
are lots of single diodes big enough for this task. The best choice
probably has more to do with how you can mount them on the heat sink
than anything else. This is a good idea just to reduce the heat
inside the box.

I was just at the Fairchild semi site looking for big diodes. The only
ones they have are in TO-220 packages and although they have 30 and
40A ratings, the power dissipation is only 3 watts. How can the thing
drop .7 volts at 40A and only dissipate 3 watts of heat?? Now I'm
confused:>:

For instance:

http://www.fairchildsemi.com/ds/MB/MBR3050PT.pdf

Who else makes high current schottky diodes??

I just received an email from a user suggesting syncronous
rectification using MOSFETS. Although it's a little more complicated,
it should be easier to implement at 60 hertz and I hadn't thought of
this. It sure would give me the head room I need as the drop across
fully turned on mosfets should be in the millivolt range.

I'm really tempted to suggest to my friend that we just assemble it as
a 10A supply with junkbox componentws that I already have on hand!

For the pass transistors you might look into something much beefier,
like the 2N5684 2N5686 size. These are rated for 50 amps but have
good low voltage gain to about 10 or 20 amps. They have enough
capacitance that they might destabilize the regulator.

Thanks for the suggestion of pass transistors.





-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

 

[Tam/WB2TT]

TYPO - TYPO

I meant to say 2N3771, not 3071.

How are the pass transistors driven? If it is a Darlington configuration,
you can get back about a Volt by using a PNP driver, but you now need a
current sink to drive that. Also, the guy with the 6V transformer had a good
idea.

Tam

 

[John Popelish]

Albert wrote:

I was just at the Fairchild semi site looking for big diodes. The only
ones they have are in TO-220 packages and although they have 30 and
40A ratings, the power dissipation is only 3 watts. How can the thing
drop .7 volts at 40A and only dissipate 3 watts of heat?? Now I'm
confused:>:

For instance:

http://www.fairchildsemi.com/ds/MB/MBR3050PT.pdf

That rating applies if you just have the diode sticking up into air,
with no heat sink. The thermal resistance spec applies when you mount
it in a heat sink and need to figure out how hot the junction gets
compared to the heat sink. You need heat sink date to figure out how
hot the heat sink gets.

Who else makes high current schottky diodes??

International rectifier makes lots of big ones. So does Diodes
Incorporated. Digikey sells both these brands.

(snip)
--
John Popelish

 

[Albert]

On Mon, 26 Apr 2004 13:36:37 -0400, John Popelish <jpopelish@rica.net>
wrote:

That rating applies if you just have the diode sticking up into air,
with no heat sink. The thermal resistance spec applies when you mount
it in a heat sink and need to figure out how hot the junction gets
compared to the heat sink. You need heat sink date to figure out how
hot the heat sink gets.

Makes perfect sense now. Thanks.

Who else makes high current schottky diodes??

International rectifier makes lots of big ones. So does Diodes
Incorporated. Digikey sells both these brands.

I did have a quick look at IR, but probably the high power diodes
listings.




-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

 

[Reg Edwards]

It is dangerous to connect two different types of power transistor in
parallel. First one goes because it takes most of the load and then the
other.

 

[mikem]

I reading this thread, I never saw the filter capacitor mentioned.
To get 13V at 20+A starting from a 17Vrms tranny, a fullwave Si bridge
and leaving a little to drop across the series regulator transistor,
the capacitor ripple voltage would have to be very low, less than
~3V.

C*V=I*t; solve for C.

V = 3V (ripple drop)
I= 20A
t=8.33ms (60Hz line, full wave rectifier)

C= (20 * 0.0083)/3 = 0.0055 FARAD!

You better have a 60,000uF 25V capacitor in there, do you?

MikeM

 

[Albert]

You better have a 60,000uF 25V capacitor in there, do you?

No, it's a 6500uF at 18 volts. As I said in the original post, the
transceiver sounded chirpy at high power and sometime the PLL would go
out of lock and latch. Although it is a 50 watt output transceiver,
I'm sure the quirky behavior can be attributed to the under design of
the power supply. I'll put a larger cap on the shopping list, thanks
for pointing this out!!




-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

 

[Watson A.Name \"Watt Sun]

budgie wrote:

On Mon, 26 Apr 2004 10:14:18 -0400, Albert <pass@pass.com> wrote:
[snip]

Schottky diodes will "buy" you something less than a volt IF you get a suitable
high-current type, but a 40A device is unlikely to gain you much at 25A - you'll
be looking at 80A or bigger devices to get the lean volt drop @25A.

I was thinking about this the other day as I pulled another heatsink
with dual diodes off an old AT power supply. Since the cathodes of
those dual 25A schottky diodes are connected together, why not just
connect the two anode leads, and make a single 50A diode. Since they're
on the same header and heatsink, if one hogs a bit more of the current,
the other will get heated up also, which distributes the amperage more
evenly than if they were two separate diodes.

Since there are often a pair of these on the same heatsink, one for the
high current 5V and one for the high current 12V, I could parallel them
again and make some kind of arc welder. Hah-hah, just kidding. But I
could get some seerious current with them paralleled. They might even
work okay for the app mentioned above.

 

[Watson A.Name \"Watt Sun]

Tweetldee wrote:

[snip]

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.

That's exactly what seemed to have happened to an old HP power supply I
picked up from Ebay recently. The thing wasn't putting out any current,
and the pass transistors were burned out. Someone had subbed an
inadequate transistor in there, and changed some resistor values.

Good judgement comes from experience, and a lot of that comes from bad
judgement.

Last time I checked, judgment didn't need an e after the G, at least not
in the U.S., anyway. But the saying rings true.

 

[Watson A.Name \"Watt Sun]

Albert wrote:

[snip]

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.

Has anyone tried to do this? Series aiding secondary to boost the ACV.
(view with courier font)


o-----+--+
120VAC | | +-------o
| )|| |
120VAC Pr'y | )||(
12VAC 3A Sec'y | )||(
| )||(
| )|| | 132VAC
| | | 3A max.
o--+--)---+ | to PS
| | |
| +--------+
|
+--------------------o


One time I used a similar setup to make a 220 VAC guitar amp run off 120VAC.

 

[Watson A.Name \"Watt Sun]

Reg Edwards wrote:

It is dangerous to connect two different types of power transistor in
parallel. First one goes because it takes most of the load and then the
other.

Same thing happens when they're matched. ;-)

 

[Watson A.Name \"Watt Sun]

Tweetldee wrote:

"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.

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!!!!!

Maybe they're a Sziklai pair?

http://www.ampslab.com/c200cfp.htm