The resonant choke (L input filter) for power supplies can indeed
operate at resonance, with a parallel capacitor that is rated for the
'tank' current and voltage. If the choke is tapped (as i had them
manufactured by SNC for the Broadcast Electronics FM-3.5A and B
series transmitters - circa 1984) it can be for 50 or 60 Hz. The
resonator works at the ripple frequency for the single phase supply,
and it needs to be chosen to work when the choke L is at maximum.
This is when the DC current is low, i.e., for SSB during idle time.
Choke inductance will decrease as DC current increases. You can
measure this with a DC supply and an inductance meter or bridge. I
suppose you would want a 'stiff' choke for a resonant choke system,
not a swinging choke. Otherwise it will be fighting your network.
Bleeder current alone and tube AB1 or B idling current being the only
load at the one end of the curve. At the heavy load times, a standard
choke is adequate to keep constant current flowing throughout the
rectification cycle. Check the critical inductance calculations in
any handbook and see that it depends on the load current. Thats why
old fashioned L input supplies had big bleeders as well. Better
regulation. If the current is discontinuous over the cycle, the
filter cap will charge up to peak, and the power supply soars. This
is horrible regulation, no load to full load.
I would recommend that any resonant choke system be followed with a
second LC smoothing filter, it can be relatively small such as a few
Hy and uF. Or the filter cap should be chosen larger. Calculate what
sort of ripple suppression you need and work through Ohms law for the
AC ripple. Check the harmonics up to mid audio band. You may be
suprised at how bad it can be, if above the ripple frequency.
Remember that the resonator is only working on the 100 and 120 Hz,
and it passes the higher frequencies with ease to the load.
A few weeks ago I spoke with a broadcaster who had one of the rigs I
mentioned above. He told me that he was horrified to see the plate
meter pegging when he removed the exciter drive to the PA. It dawned
on me that the capacitor had been there since 1984, and was probably
leaky or changing value. I recommended replacing the 0.97 uF
polypropylene 2500 VAC capacitors. Two are used in series as the C
across the choke. That was it.
Hint>>> you can get HVAC polypropylene capacitors as replacement
parts for the voltage doubler for microwave ovens. They are oil
filled as in old filter caps and motor start caps, but they can
handle the high circulating current and ripple v across them. The old
timer at SNC who developed my choke told me that Heathkit or Johnson
had tried to use a standard oil/paper cap in the 1960s and had
reliability problems.
73
John
K5PRO
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