Re: Power Transformer VA question

"flipper" <flipper@xxxxxxxx> wrote in message
news:vj5mb59q51q1r4bg2n1dgps1r5b9eve38o@xxxxxxxxxx
On Wed, 23 Sep 2009 15:58:14 +0100, "Ian Iveson"
<IanIveson.home@xxxxxxxxxxxxxxxx> wrote:

Is the RS chart any use?
http://www.davidbridgen.com/rects.htm

Those are the 'normal' formulae I mentioned, although
that
particular
collection is optimistic. For example, it says DC for a
FWB into a cap
filter is 1.41 VAC and I assure you it won't be that
high
due to diode
and IR drop on the AC peak. I think a more realistic
number is closer
to 1.22-1.25.

They also say Idc is .62 Iac but the more common factor
I
see is .55.

I'm close enough that using .62 would 'solve the
problem'
by math
alone but since I don't trust the 1.41 I don't trust the
.62.

I'll just say, up front, I'm tempted to take the Sowter
page because
'tube' transformers is their business.

On the other hand, the transformer I'm using wasn't built
for tubes.

That's the problem with 'rule of thumb' formulae, seldom
are the
defining assumptions, nor the 'whys', given so I just
don't know if
they apply. It's not too much of a problem if you can just
slap in
something bigger but it's further compounded if you're
doing something
'unusual'.

Note that the RS data is intended for a particular set of
transformers, and is not complete, probably, because the
definition of "regulation" is superfluous. RS will have
lifted it from the suppliers' data***, I guess.

Well, the ones who do state something say it's at full
load so the
'regulation' is moot, from this aspect anyway.


So it may well be that nominal diode drop and actual
transformer regulation at full load have been taken into
account. Or maybe they're all high voltage secs and diode
drop is insignificant.

If they were universal (forgetting the diode drop that
won't
be a problem in your case), regardless of winding
resistance
of the transformer, then you could just pretend your
series
resistor is part of the winding. Then, when you've worked
out VA for the transformer/resistor assembly, you can
apportion it between the winding and the resistor, pro
rata.
The A is common and the V is split in proportion to R.
Assuming that the AC current shown is true rms.

I'm not sure it's that simple because the Rs are not the
same nor do
they take into account core losses.

What I did do was move the winding resistance 'out' of the
transformer
model, where spice doesn't let me look at it, and put
external
resistors where I can measure power. I'll note that I'm
not sure it's
valid because spice does some funky things with the
waveform and I
also don't know if the winding ohms I'm using was measured
or simply a
swag (probably a swag to make the model simulate the rated
regulation)
as it wasn't an issue with the last amp and I no longer
have one in
hand to check. But, anyway, maybe it's useful as a
'relative'
comparison.

Measuring the 'whole ball of wax', loaded B+ and heaters,
with
rectified AC on the heaters, straight resistive load, I
got about 19
watts total winding R heat (note, no core loss
simulation). With
heaters on B+ (using a 550 ohm series resistor so it comes
out 100mA
DC) it jumped to 39 Watts. Note, that's not only the 'DC
factor' but
an additional 4.5 Watts of load burned off in the 550 ohm
resistor.

With the 'alternate' 150 ohm-22uF filter into the 100
ohm-heater
string it came to 23 Watts. Note, there's an additional
(vs AC
resistive load) 3.7 watts burned off in the 150 ohm
resistor.

I emphasis again, as absolute values those numbers make so
sense at
all (and neither does spice as it flat ignores those
watts) but maybe
they give a relative indication and if they do then it
looks like my
alternate filter gets back most of the heat.

It's also visually 'better'. Instead of the typical
multi-amp cap
charge spike there's a smooth 'snow cap' of a sine wave
(meaning not
the whole sine) peaking at 300mA vs 141mA with a pure
resistive
100mArms load.

Btw, when I say spice ignores those watts I mean spice
says the power
delivered from the power line is equal to the load watts.
The watts
dissipated in the transformer, whether internally modeled
or external
resistors, don't show up in the power delivered. We have a
19-39 watt
hand warmer that takes no power line watts to make warm.
Neat, huh?


Another way to think about it.

Assuming that the rms current output rating of the
transformer secondary is directly related to its VA rating,
and noting that the rms current taken by the heaters is the
same whichever circuit you use, the significant difference
between candidates is how much ripple current is passed by
the cap.

So, if you can measure that ripple current with the series
resistance after the cap, and then with it before, such as
to give the same rms heater current, then the difference
between the two cap-shunted ripple currents should give you
the difference in winding current.

Transformer rating presumably takes into account all losses
at rated output current, or rated VA, which are usually both
given.

Although I can't use my SPICE program directly to sum over
time, it occurs to me that I can easily build an rms meter.
Otherwise I can't imagine how you are unable to simulate
this circuit successfully. It'd be a doddle with my SPICE.
Maybe I'll do it for fun. What's your winding resistances?

Another big advantage of using a resistor before the cap is
that it will reduce the ripple, and maybe the noise, on the
HT supply.

Sowter trannies will possibly have lower winding resistance
and more iron.

Ian





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