[Amps] Power factor and choke vs resonant-choke input supplies

Tom W8JI 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 
looked back.

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 
my heart".

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.

73 Tom


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