Linear Power Supply Design Question

Just a few questions on upgrading my old linear power supply. I haven't
touched electronics in years, I forgot quite a bit...

Schematic here:
http://img230.imageshack.us/img230/6576/alimpage47fg.jpg

Originally rated 0-30V/0-3A, I want it to go to 0-50V/0-5A.

The 'new, improved' power section consists of a 400VA toroid
transformer, 35A bridge, 7400 uF 250 VDC filter capacitor, a TIP31C
driving a bank of 5 parallel 2n3773 (with 0R22 emitter resistors)
mounted on heatsinks with 110v fans at both ends. Following my first
unsuccessful upgrade I fused the output to 5 amps to minimize
fireworks.

The transformer supplies 53 VAC when both its secondary windings are
connected in series. With this configuration the DC voltage at the
collectors is 73.5 V *but* I'm currently using a single secondary
winding giving me approx. 35 VDC output.

Will the circuit survive with 53 VAC on input? I'm asking this because
I don't want the thing to blow up again like it did the first time when
I was setting the max amp limit with 53 VAC. (Should have used a
fuse!!) My first attempt at this upgrade involved adding 2 2n3055, they
shorted out at about 4 amp.

It works great now with only half the supply voltage and I think it
should work with full voltage because of the 2n3773s, though I'm a
little unclear on the concept of 'secondary breakdown'. And I know the
heatsinks may run a bit hotter but they're huge and well ventilated.

Thanks for all your comments.

 

Secondary breakdown was in fact the first thing I thought of when you
mentioned what you wanted to do.

In short it reduces the allowable dissipation of power transistors when
operating at higher collector-emitter voltages. Regardless of temperature.

I assume you have the data sheets. See where the 'kink' is on the I vs V
allowable dissipation graph. 2N3773s are definitely better than 2N 30055s
in this regard though. They don't need derating for secondary breakdown
below 100V so no problem there.

Graham

 

2N30055?


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

allowable dissipation graph. 2N3773s are definitely better than 2N 30055s

I understood this to mean 2N3055, just a typo. The power supply worked
well with the 53 vac secondary, though I felt the 3773 were getting a
tad warm for a 'small' test load, a car headlamp, 3.5 amps at 12 vdc.
So I'm back to using one 26 vac winding. Next time I'll be more careful
in choosing the transformer, a toroid I picked up cheap on eBay. I also
think I'll rarely need to go over 30 vdc for my projects. Anyhow, it's
fun to get back in the swing of things after twenty something years
away from electronics.

Speaking of eBay, I'm overjoyed by the old analog FET VOM I picked up.
Quite the piece. And it works very well.

Soon I'll be trying that Manhattan construction method for a few
projects. Anyone have any tips on making the round pads without them
disintegrating? I tried several types of paper punches, the first one
produced concave pads, the second one caused the pad to delaminate, and
the punch I got from the dollar store broke in my hand as I squeezed
it.