FC5687 wrote:
On the other hand, the DC resistance of the plate winding (HT winding)
can play an important part in determining final output voltage,
regulation, and even the maximum safe input capacitance of the filter
network. The effects of this are easy to simulate using a program like
Duncan Munro's "PSU Designer II".
Hi Fred!
If I order a power transformer for the HT supply with a low DCR on the
secondary winding, is it necessary to have a low "ACR" on the primary winding?
Cheers,
Tube747
No, you don't need to worry about it. As far as your circuit is
concerned, you can lump the primary and secondary DCR's, as well as all
other losses, into a single equivalent resistance on the secondary side.
In other words: Measure open-circuit voltage, then put a reasonable load
onto it and measure the loaded voltage. Subtract loaded voltage from
open-circuit voltage, and divide by load current. This gives the
effective series resistance on the secondary side, which is the value
you'd put into your simulator.
Transformers are usually rated in terms of output voltage at a given
maximum current. It's very rare that a manufacturer would bother
publishing the DC resistances, and other loss factors, because it would
not be meaningful to very many end users.
The thing you have to watch out for, that trips up a lot of people
building power supplies, is that the DC current you can get out of a
transformer (using a capacitor-input filter) is somewhat less than the
maximum rated AC current. This is because of the high current spikes
required to recharge the capacitor; since power and therefore heating is
proportional to the square of the current, there will be a lot more loss
during these high-current spikes.
I stay with reasonable input capacitor values (usually around 100 uF)
and choose a transformer with 50% higher rating than the DC current I'll
be drawing, so far that rule of thumb has always worked out. However, if
I'm using Hammond transformers, I'll push them closer to the limit
because they are more conservatively rated.
Cheers,
Fred
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