Re: what's wrong with this circuit?

"Arfa Daily" <arfa.daily@xxxxxxxxxxxx> hath wroth:

http://members.ozemail.com.au/~pebarhug/radios/2N3055schem.jpg

*** 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 ?

Dell P1110 19" viewable flat screen CRT. I can't afford an LCD
monitor at this time.

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.

I'm familiar with the merits of this discussion. I usually recommend
that customer purchase 1600x1200 LCD displays so that running the
display had 800x600 does not result in a pixel roundoff error.

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.

Agreed. That was my problem. In the USA, the decimal point is always
near the bottom of the line. I thought the spot was a smear. Please
note that literally *ALL* my previous posting have the leading zero to
avoid confusion. 0.22 or 0R22 would be fine. R22 is not because it
is easily confused by the resitors reference designator. If the
common usage has been anything other than the letter "R", I would also
do it this way, but I've had far too much trouble with mixing values
and reference designators on schematics than losing a decimal point.
The capacitor examples might work as it's unlikely to confuse U47 (an
integrated circuit) with 4U7, a cazapitor. I've done it many
different ways, as demanded by various companies drafting standards.

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.

http://www.onsemi.com/pub/Collateral/2N3055-D.PDF
Y'er correct. I should have looked at the data *** first. I didn't
realize that the 2n3055 can handle 7A of base-emitter current. 333mA
is not going to blow up the 2N3055's. However, I find it a rather bad
design that can blow the fuse, and still produce substantial output
current.

*** 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.

I've dealt with my share of 3 terminal regulators in various designs.
Depending on the construction, load impedance, lead lengths, etc, I
can sometimes make them oscillate. 1uF is not enough. I usually use
0.1uF in parallel with 10uF.

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

I agree. It will not blow up.

*** 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.

Agreed. LM317 would be a better choice. It will go down to
approximately 0 volts.
http://www.national.com/ds/LM/LM317.pdf

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

Oops. I hate it when that happens. That's not as horrible regulation
as I miscalculated. However, it's still not as good as it might be if
the 2N3055's had been inside the feedback loop.

*** 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 ... )

Well, I made more mistakes than you have. My excuse is that I was
constantly getting interrupted while writing my reply and didn't have
time to do much more than a spelling check.

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.

The LM317 will certainly help with the ripple reduction. However, the
ripple will appear at the collectors of the 2n3055's which does not
have as much ripple rejection as the LM317. The ripple won't be huge,
but it will be present.

It's a bit more important than just minimizing the ripple. There's
the problem of ripple current. The input filter capacitor conducts
lots of current on each half cycle. I once repaired an Astron 60A
linear DC power supply. The problem was that running at almost full
load, the single 250,000uF 25VDC (not sure of values) had a
sufficiently high ESR that the screw terminals literally melted on the
capacitor. At 5A to 20A, this is not a problem. However, even at
20A, the 4700 uF capacitor might get hot. The LM317 will clean up any
ripple that's left, but the cazapitor still has to supply the power
between cycle peaks.

*** I don't really think that the design is fundamentally unsafe per se,

I do. Any circuit that continues to supply power after the fuse
blows, and that does not have short circuit protection, is in IMHO
unsafe.

and I reject your contention that the design has no short circuit
protection - see above.

Ok. Turn the LM317 version to full output at perhaps 25VDC. Now,
short the output to ground. Will the fuse blow? Maybe depending on
the value selected and whether the xformer can supply the necessary
power. The fuse really belongs in series with the output voltage (as
well as adding a fuse on the 117VAC input). When the current goes to
the limit, it is sure to blow because the 2n3055's will supply the
necessary power to blow the fuse. It's not so clear whether the fuse
as shown will blow. If the LM317 goes into current foldback
protection mode, it won't blow. The output voltage will also drop to
about Vbe plus whatever the LM317 outputs. However, put a big filter
capacitor load on this thing, and it will take time for the LM317 to
complain. There's probably enough power left during this time to blow
the fuse. I can't really tell without values or bench testing (or
modeling). I guess you could call this load dependent protection.

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 can make it oscillate with an inductive load. The 1uf is strictly
for improved transcient response, not filtering or stability.

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.

Agreed. It's not as horrible as I thought. It's still quite bad and
could have been done much better.

I dispute that it will
blow the 2N3055s if the fuse fails or is removed - see above.

Agreed. The 7A of base-emitter current is sufficient to prevent
destruction. I assumed a much lower value.

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.

All of these will function except the gel cell battery charger.
Charging a gel cell battery is not a trivial exercise and should not
be done with an ordinary power supply. Go over voltage for just a
small amount of time, and the battery is toast. The other
applications are not particularly critical and can probably tolerate
such a power source.

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

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