"Arfa Daily" hath wroth:
There is a decimal point on the schematic. That's zero point two two ohms.
That makes the Rt 0.07R. That makes the voltage drop across them, at 5 amps,
0.35v , not 35v ...
I'll take your word for it that there's a decimal point. It looks
more like a blur or a smear to me. In the USA, the decimal point goes
at the base line of the lettering. Other countries have it in
different places. I guess this one is in the middle. Anyway, as I
indicated in my analysis, 0.22 ohms is a more reasonable value.
How do you arrive at a maximum output of 1A ? Whilst the
7805 can deliver a maximum output of 1 amp, this figure is multiplied by the
current gain of the 2N3055's. Assuming a ( poor ) gain of 20 on them, that
would result in an output current of some 20 amps with 1 amp of drive to
their bases. That is the whole point of having external series pass
transistors. The output current is drawn from the collector-emitter circuit,
not the base-emitter circuit, so there is no reason why the base-emitter
junctions should fry.
The 1A was my guess as to the short circuit output current with the
fuse blown. Short the output to ground with the fuse blown and the
current gain doesn't even enter into the picture. 100% of everything
the 7805 can deliver will go through the base-emitter junction.
With the fuse not blown, the output current will go to whatever the
xformer and diodes output, minus 0.6V for emitter-base, and whatever
drop is across the 0.22 ohm resistors. That should blow the fuse,
followed by blowing the emitter-base junction.
Of course, the 7805 has current foldback, which will limit the amount
of current that it can supply to safely reduce dissipation. It may
take a while to blow up. Meanwhile, it will probably oscillate
merrily.
The output is current limited and protected by the fuse feeding the series
pass transistors' collectors.Admittedly, this is not very elegant, but it is
protection, no matter which way you look at it.
Not if blowing the fuse also causes the emitter-base junction to blow
up from excessive current. I could supply a better analysis if I knew
the values of the xformer voltage and current and what value of fuse
is specified.
Interestingly, the range of output voltage is rather odd. Using the
7805 example, the 270 ohm resistor and 5K pot form a divider with:
5V * 5000 / (270 * 5000) = 4.74 V
Therefore, the maximum output voltage is 9.74 V (minus the Veb drop in
the 2N3055. I'll call it 11 volts maximum output. The range of
output voltages is 4.4V to approx 11V.
At full current (5A), the three 0.22 ohm resistors appear as a single
0.7 ohm resistor for a drop of 1.1 volts. Therefore, the output
voltage will vary over a range of 0 to 1.1 volts depending on the load
current. This is not what I would call good regulation. It's 4 times
worse at 20Amps.
The 7805 is not an LDO regulator, so we'll need a few volts drop
across it. My guess is about 5 more volts. Therefore, the xformer
and full wave center tapped bridge need to supply 32V center tapped at
5Amps. That's a fairly large xformer. At 20Amps, it's a fairly huge
transformer.
It doesn't make any difference to the size of the transformer, if you have
one 20v 20A winding, or two 20v 10A windings, series'd and grounded at the
junction. It's still 400vA either way
With full wave center tapped, you're only using one half of the
xformer secondary at a time. Therefore, each *HALF* of the secondary
has to supply the full current and full voltage for half a cycle. Half
a cycle later, the other half of the xformer is doing the work, while
the first half just sits there. To supply my calculated 16 volts of
DC from the full wave center tapped system, each *HALF* of the
secondary would have to supply 16VAC at 5Amps for a rating of 32VAC CT
at 5Amps.
To do the same thing with a full wave center tapped arrangement, the
entire secondary is used each half cycle. Therefore the xformer
rating would be 16VAC at 5A or half the size. Ignoring slight
efficiency differences, and a larger physical size, the xformer rating
for both devices would be about 80VA, but the center tapped version
would be about twice as physically large due to the doubling of the
secondary windings.
One more. At 20Amps, 4700uF is inadequate filtering. I'm too lazy to
do the numbers. It needs a series resistor or choke.
I agree that this is not a *good* design, and will suffer from poor dynamic
regulation due to the current-dependant drop across the current sharing
resistors, but it is at least functional, and a simple useable design to
produce an adjustable, reasonably high current output. Depending on what it
is needed for, it might be quite adequate, and its shortcomings, of little
or no consequence.
I beg to differ with you conclusions. The design is unsafe, has no
short circuit protection, may oscillate, uses an inefficient xformer
design, has improperly selected diodes, has miserable voltage
regulation, and will blow up the 2n3055's if the fuse is removed or
blown. Since the application has not been specified, neither you nor
I can judge if the design is adequate.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558