[Amps] Power factor and choke vs resonant-choke input supplies
w8ji at w8ji.com
Fri May 11 03:32:06 EDT 2007
> First thing: See if your choke winding has a gap. That's
> the big ticket
> hurdle, because without a gap, it can't swing.
Actually with a gap, it swings less. Virtually any iron core
choke will swing some amount with current, even those with a
gap. But the gap makes the inductance be a bit more stable
and the gap can actually make the inductance higher at a
given current than the same core without a gap!
The reason it "swings" is flux saturation and the
magnetizing curve of the core material. When the core
saturates any further increase in current causes the flux to
increase like the additional flux is in air. That effect
doesn't occur sharply in a typical core material, and the
inductance change is smoothed even more by an increasing gap
until eventually with all air it won't change at all..
The wider the gap the less the effective inductance changes
with current. It is more and more like an air path
regardless of flux density (current).
Even a resonant choke needs a bleeder, although it needs a
lot less bleeder current than a choke without a parallel
capacitor. That's because the effective inductance is much
higher. The effective inductance increases when a capacitor
shunts a choke, provided the capacitor's reactance is not
less than half the reactance of the choke at a particular
You always want to set the resonance slightly on the high
frequency side of the ripple frequency. This is so the choke
does not "pull through" resonance as the load current
You CAN use a swinging choke for a "tuned" choke input. This
is because the capacitor, regardless of value, always
increases the effective reactance. The exception is when the
capacitor's reactance is less than half the inductive
reactance at a given load current. What Gary is referring to
is a resonant filter that is designed to minimize ripple
voltage in a fairly constant load supply rather than improve
regulation over widely varying loads.
Circulating currents in the capacitor and choke can be
extremely high. The choke peak voltage is no worse than a
regular choke for the same effective reactance, except I've
seen a case where the choke created extreme voltages.
I had a resonant choke in a full wave bridge rectifier
supply using a pole pig transformer. It was an attempt to
use a pole pig with a pair of small Eimac triodes. My
intention was to come up with 3700 volts dc from a pole pig.
I used the formula in Bill Orr's Handbook to determine
values. My components were VERY conservative. The filter
capacitor was a power line power factor correction
capacitor, so it would handle many kV.
When I powered this thing up my 3-500Z's arced from anode to
grid. Thinking I had gassy tubes, I pulled the anode caps.
When I powered up again a large bypass cap shattered. Then I
clipped a HV probe across the HV line, stood back a couple
feet on a rubber matt, put one hand in my pocket, and I
turned on the toggle with the other. The television CRT 2nd
anode HV wire in the supply blew in two and it came out the
open door and attached to my wrist. I wound up with first
through third degree burns all up and down my arm. I had a
charred hole the size of a quarter in my wrist.
Something about that supply caused a huge HV transient when
powered up. I found later I had the choke very slightly on
the low side of resonance. I suspect the 120 Hz pulses
banging the resonant circuit set up extreme voltages. I
converted to a grounded CT (the line voltage adjustment taps
on the pole pig were at the center of the winding) and never
Experimenting later I found you always want to have the
resonance set slightly higher than the ripple frequency so
the choke pulls away from resonance under increased load
current, not into resonance.
Since that time I've been very selective about where and how
I use tuned filter chokes. I'm always amazed when I turn one
on and nothing explodes, and I don't die.
By the way that whole thing gave me great insight into how
my brain works. I looked at my wrist, saw the hole, and I
saw the burns up my arm. I though "gosh, that surely stopped
Then I watched the second hand on a clock move and carefully
noted I could still think, hear, move, and see. I was
frustrated I never paid attention to how long a human could
live with no heartbeat or with seriously flawed heartbeat.
After the second hand swept around and there was no change
in my ability to think, I assumed my heart was still running
at enough capacity to get help, so I made sure the power was
off and I told my wife not to panic but we needed to get
some medical help.
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