On 16 Mar, 23:11, "Arfa Daily" wrote:
wrote in message
oups.com...
On 15 Mar, 11:45, "Arfa Daily" wrote:
wrote in message
roups.com...
On 14 Mar, 17:22, "Arfa Daily" wrote:
wrote in message
groups.com...
On 14 Mar, 12:41, "powerdoc" wrote:
On Mar 14, 4:05 am, "N Cook" wrote:
wrote in message
egroups.com...

If you see high failure rates at 80% rated voltage, something's wrong.
Perhaps the circuits are such that voltage fluctuates. Or maybe theyre
low grade caps.

It's something that I've seen for 35 years with all grades of caps ...

I'm puzzled why I havent and everyone else here has.


If it's just a preamp, the current demand on it is not going
to be any more than a couple of hundred mA tops,

yes, probably less.

which will not load any
reasonably rated transformer anywhere near into copper losses IMO.

Eh? A small lower power transformer is going to have poor regulation,
ie relatively high winding R, and you've got a peaky current waveform
being drawn. Vdrop in the copper will thus be significant.

Why a " peaky " current waveform ?

The load is a bridge rec + reservoir, so it only charges the reservoir
caps at the peaks. Most of the time i=0, and at peak i= several times
average. Copper losses have a bigger effect with peaky waveform on a
low power and thus poor regulation transformer.

I think that this is a highly debatable way of looking at it. If the cap is
of a sufficiently large value, the charging 'peaks' on each cycle should be
small, once the cap has gone through the initial charging phase over the
first few cycles after power up, otherwise you have significant ripple,
which I'm sure you would agree, is not the case with most properly designed
power supplies. The cap does the averaging, so the current demand on the
transformer, is pretty much constant rather than 'peaky'.



With a low current demand circuit such as
a preamp, the current drain from the transformer should be pretty
constant,

averaged over each cycle yes, but instantaneously its the other way
round.

See above

ok, explanation time.

What you say is true for a large high power good regulation
transformer, but things are different with 6 watters.

Opening my catalogue at the 6w transformer section shows regulation of
25% for all the 6 watters.

What this means is that when delivering no load V_out is 25% high, and
at full load that 25% is dropped across the transformer, it is copper
loss. And this is true for a sine load.

Now, along we come with a BR+reservoir load, which only eats at the
peaks. Trouble is, peak current is several times tf rated current, and
thus copper loss V_drop is several times 25% of V_out_rated. What this
means is that peak i is reduced, and conduction occurs over a wider
angle than is ideal. It also means V on the reservoirs falls due to
copper loss.

The end result of this is even poorer regulation on the higher side of
the Vregs.


the main rectifier resevoir caps taking care of supplying any transient
requirements. Even a 'small' transformer at 18-0-18 is likely to have a
current rating of at least 500mA per limb,

no... thats a 9w transformer. Why would one fit a 9w tf to a 3w app?

OK, maybe an overkill, but we already agreed that the curent demand of this
item is likely below a couple of hundred mA, so maybe a 5 or 6 VA tranny,
which is a typical size that would likely have been fitted originally. Even
at this level, I still contend that on a reasonable quality tranny, copper
losses won't be significant.

Quality is nowt to do with it. 25% regulation is standard for a 6w
tranny. If you build one with lower R wire, it can pass more i and has
higher power rating.

From same catalogue:
12VA 10%
100VA 9%


and with the low demand of this
type of circuit, I would not expect to see barely a small drop due to
copper
losses. The poor regulation will ensure that the output voltage is high
on
the nominal design figure, and will likely remain so.

an old fashioned inefficient way to do things. Cheap volt regs make
such practices unnecessary today.

I wasn't suggesting that this was a good thing. What I was trying to say is
that if a designer decided that say 18v AC was required to arrive at the DC
level he needed on the back side of his bridge or whatever, then he would
have to take account of the fact that a cheapo small tranny with poor
regulation, would be likely to produce a significantly higher level than
that calculated and, because of the very light loading, it would be unlikely
that this value would drop to what was actually required, as a result of the
copper losses that you are fond of

you need to read up on transformers & psu design

... Cheap voltage regs by no means
mitigate the potential problems of this

no, it worsens things, as today we use minimum power transformer with
bad regulation and sort it out with a low cost regulator.


as, first off, we come back to the
level of voltage that you are throwing across the resevoir cap before we get
near the regulators. Secondly, these monolithic voltage regs are quite
inefficient, being shunt types, so dissipate quite considerable amounts of
power, which is why it is important to keep the input voltage as low as is
practical, above the required overhead for correct regulation.

sure, just basic cost and energy efficiency


If a circuit
is designed for a particular input overhead, based on what the calculated DC
*should* be, and then that DC turns out to be 15 or 20% higher due to poor
transformer regulation, this is going to significantly increase the
dissipation in the regulator, which might mean that the calculated
heatsinking that was required, is no longer sufficient, which could lead to
the regulator starting to go into thermal foldback, which completely wrecks
any stabilization that it was bringing to a rail.

If thats the situaion then the designer doesnt know what theyre doing.


What I'm
saying is 28v on a 25v nominal tranny output is bad, higher, if you think
that it might be, is even worse.

I dont know you think that. You'd be hard pressed to find a 6w tranny
with regulation as good as that.


Multiply that up by 1.4 to get to the peak voltage
and you will be just about at 40v across the caps.

You're ignoring the effect of loading plus winding resistance. R has
more effect on a peaky waveform.

I don't think I am. I am employing years of practical experience with
this
sort of thing.

exactly. If you work through the theory + numbers you'll see what
youre doing creates results that work fine until mains sags, then they
go wrong. A designer has to make circuits that tolerate the usual
overvoltage and undervoltage limits, whereas when repairing this is
optional in practice.

I agree, but there are limits, and sags of the sorts of level that you are
suggesting are pretty significant, and much worse than I would have expected
over most of the civilised world. I see many many group amps and hifis for
repair, all of which employ some kind of regulators, and most of which use
78 and 79 series ones, which as you rightly say, are cheap. Most group amps
have semiconductor front ends these days, employing opamps, run very
typically from +/- 15v rails, derived from 78 / 79 regulators. It is
*exceedingly* rare for the input to these to be in excess of +/- 25v. In
practice, even if the regulators did drop out of tight control for brief
periods of excessive power line sag, it is unlikely to have a significantly
noticable effect on the performance of the opamps, and I think that most
designers would be prepared to accept occasional poor regulation on these
occasions, as a trade against excessive regulator dissipation in the vast
majority of circumstances.

Whats the real load of an opamp based preamp? More like 10s of mA.


If we were talking about a power amp, then yes, factors such
as transformer regulation and copper losses have to be taken into account
for voltage sag calculations, but in low demand power supplies, it's more
relevant to look at it from the opposite angle, and work out how much
higher
the output voltage will be than expected.

Surely it should be as expected, else you've miscalculated.

No, because the real world calculations will not match the theoretical
calculations,

only if you screw up the calcs


because I still maintain that in cases of very light
transformer loading, the copper losses will *not* be significant.

Its not possible.


NT