Since you mentioned that, why does the front of the SB-220 say "2KW LINEAR
AMPLIFIER" if it was designed as a 1kW input amp?
I've often wondered that...
Joe, N3JI
----- Original Message ----
From: Tom W8JI <w8ji@w8ji.com>
To: amps@contesting.com
Sent: Friday, July 14, 2006 11:01:33 PM
Subject: [Amps] Transformers
> The Heathkit 220 and a few other models had around 15%
> regulation which is right on the edge of
> saturation (they ran a high flux density to save on weight
> and cost).
The SB220 transformer was designed for a kilowatt INPUT DC,
which was the legal input when it was designed. This was
about 450 mA at about 2200 volts. It was a good conservative
design for that power level. At rated power on a good power
line it sags less than 10%.
The problem is most people seem to think it was designed for
1000-1300 watts CW output, and they expect a 1000 watt INPUT
power design to be stellar at two or three times the rated
CW dc input.
>They don't saturate, but under full
> load they're cutting it pretty close.
A regular power transformer certainly does not work the way
that statement might lead us to believe.
Maximum flux density, which means closest operation to
saturation, occurs with NO load. As load is increased flux
levels do not increase. Increased current, because of
resistive losses in the primary circuit, actually causes the
transformer's flux level to decrease. When a transformer is
designed the highest primary voltage under no load is used
to set flux density at a safe level.
The actual mechanism inside a transformer is the secondary
develops a counter-MMF. This opposing flux would reduce flux
density, but primary current increases in order to try and
maintain the **same** flux density. When mains and primary
resistances carry more current from increased load, the
primary voltage drops slightly. This REDUCES flux density,
moving the transformer further from saturation. Not closer
to it.
If the transformer had a separate secondary winding with
constant current you would actually see the voltage on it
decrease with any increase in load on the secondary with
variable load. This voltage decrease is because flux density
decreased.
73 Tom
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