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#122609 - 12/03/05 05:17 AM PSU problem  
Trumpy  Offline

Joined: Jul 2002
Posts: 8,223
SI,New Zealand
(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).]

Tools for Electricians:

#122610 - 12/03/05 08:32 AM Re: PSU problem  
gideonr  Offline
Joined: Dec 2004
Posts: 161
Edinburgh, Scotland, UK
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!

#122611 - 12/03/05 11:04 AM Re: PSU problem  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England

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?

#122612 - 12/04/05 06:08 PM Re: PSU problem  
aussie240  Offline
Joined: Jul 2005
Posts: 223
Blue Mountains, NSW, Australia
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.

#122613 - 12/05/05 04:08 PM Re: PSU problem  
Paulusgnome  Offline
Joined: Jun 2004
Posts: 56
Christchurch, New Zealand

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

Mark aka Paulus

#122614 - 12/06/05 09:44 AM Re: PSU problem  
IanR  Offline
Joined: Dec 2004
Posts: 328
Palm Bay FL USA
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).

#122615 - 12/07/05 03:21 AM Re: PSU problem  
kiwi  Offline
Joined: Dec 2004
Posts: 354
christchurch new zealand
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. [Linked Image]

I myself am a model employee and only read and post on ECN from home. [Linked Image]

HA HA Monson ! Buuuuuuuuuurn!!!

#122616 - 12/08/05 08:26 PM Re: PSU problem  
Trumpy  Offline

Joined: Jul 2002
Posts: 8,223
SI,New Zealand
Children, children,
What's going on here?. [Linked Image]
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.
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. [Linked Image]

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

#122617 - 12/10/05 08:37 PM Re: PSU problem  
Trumpy  Offline

Joined: Jul 2002
Posts: 8,223
SI,New Zealand
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:

[Linked Image]

Second is the transformer:

[Linked Image]

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:

[Linked Image]

Next two pics are the power transistors:

[Linked Image]

#122618 - 12/10/05 08:45 PM Re: PSU problem  
Trumpy  Offline

Joined: Jul 2002
Posts: 8,223
SI,New Zealand
Carry-over post to add rest of pics/info.


[Linked Image]

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

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

[Linked Image]

Finally, here is the original nameplate from this unit:

[Linked Image]

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.

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