"Jeff Liebermann" wrote in message
...
"Peter" hath wroth:

http://members.ozemail.com.au/~pebar...N3055schem.jpg
A fellow hobbyist told me it was not a good circuit arrangement as it had
poor regulation with the NPN pass transistors. Can someone explain to me
why?

Argh. That's an awful design. The lack of voltage ratings on the
output, xformer, and cazapitors should be a clue.

1. The output voltage isn't regulated. The feedback stops at the
base of the 2N3055 xsistors. The regulation is further mangled by
having 3ea 22 ohm resistors in series with the output. The voltage
drop across these resistors will vary with the load current, which is
not exactly my idea of voltage regulation. Ummm, doing the math:
E = I * R = 5A * 7 ohms = 35 volts
The 7 ohms is from 3ea 22 ohms effectively in parallel. In order for
this piece of junk to output any voltage at 5A, the regulator input
voltage will need to be 35 volts plus the regulators dropout voltage
and Veb. It won't at all work with the 22 ohm resistors.

2. In normal operation, the 22 ohm resistors will cook. For example,
if this power supply really does mangage to output 5A then the
dissipation in the resistors will be:
P = I^2 * R = 5^2 * 7 = 175 watts.
Not even close.

3. The output isn't current limited or protected. Short the output
to ground and you get 1A (output current limit of the 7805) through
the base-emitter junction of the 2N3055. The usual value for such
emitter follower load equalizing resistors is prehaps 0.22 ohms. Were
those used in this design, the current would smoke the EB junction on
of the 2n3055's.

4. The diodes listed are all plastic parts that will get rather hot
near their current limits. For anything over about 8 Amps, one really
should use diodes with heat sinks.

5. Using a full wave center tap xformer means that the xformer will
be twice as large as a smaller xformer with a diode bridge.

Start over.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Many thanks, guys for all the responses. They've helped clarify my
thinking.

Peter