They are ideal for building FWB and FWD C-filter anode supplies for SSB
linear amplifiers because they have low-R primary and low-R secondary
windings, plus, thanks to the Grade-5 Hipersil core, they have
insignificant core-loss.
- Since low-R results in high inrush current, a step-start is probably
not a bad idea. For example, a Dahl 68-pounder primary looks like
about 0.19-ohms when the secondary begins charging a discharged
capacitor. Thus, without a step-start resistor and relay, with 240v
input, an ESR of 0.19-ohms, and a mains R of 0.1-ohms, a 68-pounder
produces 240v / 0.29-ohms - or c. 800A of inrush-I -- which is hardly
good news for a 40A power contactor.
Because the winding R is so low, for SSB service - where the typical
duty-cycle is only c. 30% during a monologue, a smaller than expected
Dahl transformer will do the job. For example, a Dahl, 3000V-rms,
1.5A, 4500kva, 68-pounder will power an 8170 or 8171 SSB amplifier that
produces a PEP of 14 big uns.
For heavy contesting, the interior to exterior heat transfer of a Dahl
transformer can be improved by filling the air spaces in the windings
with fiberglass laminating resin. This (potting) procedure typically
costs little and can be done in the home workshop without a vacuum
chamber. Also, potting pretty much stops transformer-paper moisture
absorption. For details, see:
http://www.somis.org/D-amplifiers3.html
potting transformers.
Another consideration with a Dahl transformer is that due to the
recurrent peak currents being c. 10x the average current - for a
C-filter, one needs to compensate by subtracting 4 AWG sizes from the
National Electric Code specification. For example, the recommended Cu
wire gauge for a 40A circuit is #8, but for a Dahl transformer with a
C-input filter, #4 makes for better regulation due to lower E = I x R
drop.
One nicety with a Dahl transformer is that it is possible to custom
order the transformer with an 18Vrms, 0.5A winding so one can FWB it
and power a 24vdc step-start relay.
end
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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