ECN Forum Forums Photo Discussion Forums Photos for General Discussion PSU problem

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