"Jeff Liebermann" wrote in message
...
"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.

*** That's interesting that you only see the decimal point as a blur. Are
you by any chance using an LCD monitor in non-native resolution ? I say this
because the point is perfectly sharp on my CRT monitor, and there has been
quite a debate going on here from time to time about the relative merits of
CRT over LCD when it comes to fine detail, and photographic colour
rendition. Over here, the decimal point went in the centre when I was a kid,
but by the time I was at senior school, it had been moved to the bottom.
This is one reason that we were always taught to preceed any such value with
a zero. Over here now, the decimal point tends to be omitted so as not to
cause confusion with spots of fly-crap on the page. Thus, a 0.22ohm
resistor, would be written as 0R22 or just R22. 22 ohms would be written as
22R. Likewise caps - 4.7uF = 4u7 6.8nF = 6n8. A better form of
nomenclature, I think.



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.


*** Ah, OK ! I see where you're coming from now. Valid point - in theory.
However, my data shows a max base current for the device of 7 amps. Bear in
mind also, that as long as the current sharing resistors have not gone open,
any short circuit current drawn, will be spread among the three 2N3055s. As
the 7805 can only supply a maximum of around an amp before going into
overcurrent foldback, this will be only represent around 330mA per
transistor base. They should be able to handle this all day, without so much
as a whimper, let alone a grunt. So yes, I agree that the circuit may well
produce around an amp of short circuit current, but I do not agree that this
will blow out the 2N3055s' B-E junctions.



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.


*** There is probably just about enough decoupling around to stop it
oscillating under fault conditions, but had it have been my design, I agree
that I would probably have put a bit more in. The regulator, properly
heatsunk, should be able to cope with overcurrent foldback, which keeps the
device within its SOA, all day.

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.


*** I don't believe that the B-E junctions will blow - see above


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.

*** The use of a 7805 nominally fixed regulator is, I agree, an odd choice.
I would not recommend attempting to use one of these in a variable
configuration. However, the alternative LM317 specified is, AIR, a genuine
adjustable regulator, which goes down to its internal reference voltage of
1.2v, and up to around 35v, so by the time you had factored in drops in the
series pass element, you would get down to nearly zero output, and up to
somewhere near what you were putting in.

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.


*** Slight error in the math there ! 3 x 0.22 in parallel, is 0.07 ohms,
not 0.7 ohms, thus at 5 amps, the drop across them is 0.35v, and about 1.4v
at 20 amps



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.

*** Yep, I'll conceed that one !! Your thinking is much clearer than mine.
In mitigation, I'll just say that it was in the very early hours that I was
sitting here thinking about it ... !! ( but still no excuse ... )



One more. At 20Amps, 4700uF is inadequate filtering. I'm too lazy to
do the numbers. It needs a series resistor or choke.

*** That may or may not be true, depending on the application. Many loads
will not mind a dirty output. I agree that if it were my design, I would
probably put better filtering in, with larger caps, but there will be a
degree of electronic smoothing achieved, even with this poor design, by the
basic 78xx or LM317 regulator element. AIR, these devices exhibit around
70dB of ripple rejection. However, I wouldn't suggest that this circuit
would achieve anything like that figure, because any good regulation or
ripple rejection at the bases of the series pass elements, will be worsened
by a factor of their gain.


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.

*** I don't really think that the design is fundamentally unsafe per se,
and I reject your contention that the design has no short circuit
protection - see above. It may oscillate under the right ( wrong ?? )
conditions, but I think that this is fairly unlikely, given that there are
decouplers in the right places. I agree with your analysis of the diode and
transformer specifications. I agree that the potential voltage regulation is
poor compared to some other designs, but not necessarily, that it falls into
the "miserable" category for low to medium demands. I dispute that it will
blow the 2N3055s if the fuse fails or is removed - see above. I agree that
we cannot judge the design in terms of specifics, without knowing the
ultimate requirements of it, nor was I trying to, but that does not preclude
judging its validity as a standalone circuit, suitable and adequate for
*some* applications. Actually, if you think of it more as what it is - a
variable voltage source - rather than what it's not - a properly regulated
power supply, then it has many valid applications supplying non-critical
loads. A few that spring immediately to mind are minidrill speed controller,
model railway speed controller, sump pump speed controller, garden pond pump
speed controller, low voltage lighting intensity controller, gel battery
charger and so on.

Arfa