(Thread moved from Non-US Area)

I have a PSU (Power Supply Unit) that has a 230V input.
Now so I'm told, this thing was running a 13.8VDC 2metre Ham transciever.
The tranny in the PSU has an output of 38VAC and the wires in the secondary are very heavy (suggesting a high VA rating).
The transciever itself required 9A on full TX (transmit), it is FM not SSB or anything like that.
OK, the PSU has been modified about 5 or 6 times from what I can see, judging by the age of the wiring.
What I am willing to know is this:
It has 2 Series Pass Transistors, I guess to maintain a high current to the output.
Not having hooked the original radio set up to it, to save damaging the thing, is there a way that I can re-wire this PSU to give 13.8VDC.
One other thing is the fact that it has rectifiers that I've never seen before.
Naturally there are 4 of them and they are made by Toshiba (Reference No. 6CC11) they are a pink/orange colour.
There is also a 4700uF 75V Electrolytic Capacitor in parallel with the Diode bank.
I'll upload some pics tommorow and let you folks have a look at what I'm talking about.
Any help would be cool though.


[This message has been edited by Trumpy (edited 12-22-2005).]

38 volts ac rectified would give you just over 50 volts dc on the capacitor. The rectifiers are 6A 150V type and are obsolete. Not sure what this supply was meant for, but the transformer voltage is way too high, and the current rating of the diodes to low for 13.8V 9A out.
Are the pass transistors 2N3055 type? They are often used in this kind of equipment.

Basically, you'd need to change the transformer and the diodes, so pretty much all you'd be left with is the case!

Mike,

Are the two series pass transistors wired in parallel?

Are you certain that the 38V secondary on the xfmr doesn't have a center tap? The diodes could be wired to give two sets of full-wave rectification that way?

Sounds awfully innefficient starting out with a 38v transformer....there's going to be a huge amount of power dissipated in the series transistors. Assuming a 9A load, a rough calculation suggests about 330W of heat!
If possible it would be a good idea to remove secondary turns from the transformer to get down to about 18v. If the transformer has a centretap it would actually be about the right voltage.

Mike,

If I understand correctly, you have a power supply with a transformer that has a secondary voltage of 38VAC.
As noted above, rectifying 38V will give you a DC voltage of more than 50V.
Linear regulators will be hopeless at dropping the output voltage down to 13.8V. You will waste three times as much power as you use in your load.
A switchmode buck converter is probably the best way here, although the complications of rolling your own are considerable enough to challenge the inexperienced. It may be possible to find a kitset that will suit.

Back in the old days before cheap switching electronics, the solution would have been to place an SCR in the primary circuit to regulate the output of the PSU by chopping in to the mains waveform in the same manner as a dimmer does. This needed fearsome output filtering to eliminate the harmonics, but it was the standard fare for large rectifier sets such as were used in telephone exchanges.

Best of luck,
Mark Monson

Allthough a switcher has been suggested, that seems like a very complicated way to go if you are rolling your own. Unless the tranny is a center tapped unit, my advice would be to put it in your junk box for something else and find a suitable transformer(About half the voltage).

Get Back to work Monson ! You haven't got time to be solving other peoples problems !

I'd have the computer nerds remove Explorer from your computer but you'll probably just go ahead and use "Firefox" or something.

Trumpy, Marks advice is invaluable, but I must ask that you disregard any of his posts made during business hours. He's meant to be working ! I know this because we both work for the same company.

I myself am a model employee and only read and post on ECN from home.

HA HA Monson ! Buuuuuuuuuurn!!!

Children, children,
What's going on here?.
Sorry to hold out on you guys, I've had 5 days of 20 hour days starting last Sunday.
Call it a new sub-division that had to be "wired" before the foot-path contractors got too nasty with us.
Anyhow,
I'll commit tomorrow afternoon to taking this PSU completely to bits and having a good gander at it, from what I can see there are 2 seperate secondary windings.
I'll keep you guys posted.

[This message has been edited by Trumpy (edited 12-08-2005).]

OK guys,
Here we go.
{I apologise for the rather poor quality images, but me and my camera are having a really bad hair day}

First up are the diodes I was referring to:



Second is the transformer:



Now, at the bottom of the pic, is the Primary winding, from left to right is Neutral, 220V and 230V tappings.
The top terminals from left to right are the first set that give some other voltage and the ones currently being used are the ones that give 38VAC.
They are two seperate windings as the pic below shows:



Next two pics are the power transistors:

BDY20:

Carry-over post to add rest of pics/info.

2N3055:



Oh and by the way Paul, these two trasistors are in series.

Here is a board that was in the case as well:



Finally, here is the original nameplate from this unit:



Been a while since I've seen any Pye gear around the place here.

I've still yet to get with it and put some test gear on this circuit.
But I have a feeling that the secondary windings have been swapped over, there looks to be a lot of cut wires in this power supply, either someone got mad with it one day or there has been some sort of a butcher inside the case.

Yep, it certainly looks as though somebody has been "at it."

No doubt the home-made part on the perf board is a basic zener regulator which is then controlling the series-pass transistor(s).

When you say they are in series, do you mean that the output current is actually flowing in series through both (i.e. emitter of one goes to the collector of the other) or that they are wired as a Darlington pair?

Paul,
When I was originally having a "look" at the circuit, it suggested a Darlington connection between the Power Transistors.
Would the BDY20 and the 2N3055 make a Darlington pair?.
I lent my latest Semiconductor reference to a fellow Ham and never got it back!.
{If you are reading this Ted, give the thing back!}
I have tommorow off and I'll get the test gear out and give it a good test.
What I want to know is, is there a bad winding on the Primary or either of the Secondary sides.
If there is, it's only fit for the rubbish heap.
I have a 7kW dummy load here, that I can use to "soak test" the transformer.
I also have a Variac that I can hook up to the Primary side, with a Fluke True RMS meter monitoring the actual voltage.
I will Megger it first though, to Earth.
Until then, I'll keep you posted.

[This message has been edited by Trumpy (edited 12-11-2005).]

Hi Trumpy,
They definately would need a darlington configuration to drive the 2N3055/A that hard. Its beta is only spec'd @ 20/70. You might need almost .5A of base drive to drive your output on transmit. Your PD is 115W max (infinate heat sink and all that) so your input/output differential voltage is way too high. It sounds like it might have started out as a full wave center tap configuration that was converted to a full wave bridge in error, giving you too much voltage at your input filter. It's easy enough to make right if the pieces parts haven't let all of their snoke out.
Joe

http://www.national.com/ds.cgi/LM/LM723.pdf

Hello again Trumpy,
I just wanted to add a link to a datasheet for what's got to be about the most popular IC in the world for driving high current outputs like yours.(linear) Just about any serious chip maker has made them for decades. Of course, motorola always had to be different and stick 1's in front of things such as MC1723CN or similar. Your application would typically use something like you see in figure 4. Yours would feature your darlington network in place of the external NPN transistor shown. The supply for the 723 would be zener limited while the collectors of the darlington would be going directly to your input filtering. This chip has provision for high current shutdown and foldback current limiting. I would guess that you HAMs would like stuff like that. Maybe even an output fuse and an SCR crowbar circuit. I mean, if you're going to have a dorky little perf board in there anyway, you might as well put some goodies on it.
Joe

A possible way around this problem: buy/acquire a car battery & mains battery charger. You'll have no dramas about the battery supplying the required current & also as a bonus, works in a blackout. May save you time & money trying to adapt something.

You can look at the schematic of the Astron supplies over at repeater-builder; those are LM723-based designs.

http://www.repeater-builder.com/astron/astron-index.html

/mike