[AMRadio] Everyone know this but me?


Donald Chester k4kyv at charter.net
Sun Aug 14 15:43:23 EDT 2016


> From: AMRadio [mailto:amradio-bounces at mailman.qth.net] On Behalf Of
> Rob Atkinson
> There's a lot here I am not getting.
> 
> When you write "buck/boost transformer" I picture a single winding with a
> common and line pair of taps and another tap above or below the line tap
> that goes to the load.  The "lower voltage winding" would be the same
> winding because there's only one winding.

The buck/boost transformer would be low-voltage high current, perhaps a
filament transformer, with separate primary and secondary windings.  The
primary is connected directly to the a.c. mains outlet.  The LV secondary is
wired in series between the "hot" side of the line and the load, while the
neutral (or the other hot if it's wired for 230V) is connected to the load.
In other words, the low voltage output from the filament winding is wired in
series with the output from the a.c. mains , so that it either adds or
subtracts from the mains voltage, depending on polarity.

A second way to hook it up would be to connect the hot side of the filament
transformer primary so that it is in parallel with the load, in series with
the filament winding, instead of directly to the hot mains wire.  In that
case it is made to function as an autotransformer.  I always preferred the
former configuration, but they should each work equally well but give
slightly different output voltages, so it might be well to try both
configurations to see which one comes closer to the desired net output
voltage.


> SOLA makes these devices that hold the v. to the load at a constant amount
> while the line v. can move up and down.  I can't remember how they work,
> only that they hum loudly and give off a lot of heat.  Is this the kind of
device
> you are talking about?

Those things are inefficient and tend to be acoustically noisy.  They have
two secondaries; one close to 1:1 turns ratio for the load, and the other a
HV winding with enough capacitance connected in parallel with the winding,
to form a tuned circuit at line frequency.  The unloaded resonant circuit
pulls a lot of current at each peak of the cycle, enough to drive the core
into saturation.  The saturated core is what  limits the voltage from the
1:1 turns ratio winding. One problem is that the output isn't a pure sine
wave but maybe more like a square wave, and in some equipment power supply
circuits this may cause weirdness or other problems.

I have a small one of those, rated at 75 V-A, which is actually very quiet.
I use it to  run my 75A-4.  It was manufactured more recently, sometime in
the 1980s, designed to run a desktop computer in an office setting, so they
evidently went out of their way to make it quiet.  A Dayton flea market
find.

> Well, 21 amps @ 120 v. is around 2500 watts so that should run  a couple
100
> watt rigs and a receiver or two but I don't know where the .25KVA comes
> from.

Using a 120v primary/10v 20A secondary filament transformer as an example,
since the filament winding is in series with the power mains, it is
supplying full current (20 amps), but only at its rated output voltage ( 10
volts), the load on the transformer is .200 KVA (10v times 20 amps) ; it's
supplying only a small fraction of the total power delivered to the load.
Connected to boost mode, the above configuration would boost the 120v line
to 130 volts. In buck mode, it would reduce it to 110v.  The load would be
pulling a net  total of either  2.60 KVA (in boost mode) or 2.20 KVA (in
buck mode), but the buck/boost transformer is not supplying the full load.
It's just suppling enough voltage (at 20 amps) to correct the voltage
delivered to the load.

Don k4kyv



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