[Amps] An inexpensive choke for a resonant filter

Angel Vilaseca avilaseca at bluewin.ch
Wed May 16 01:45:24 EDT 2007


I see from your simulation that a 2 H inductance is typical for a HV 

I wonder if a HV transformer from a microwave oven cold be recycled in a 
HV power supply, using its secondary winding a a choke. It's big and 
it's free!

There is also this nice HV capacitor in a micowave oven power supply. I 
wonder what the resonance frequency would be if wired in parallel with 
the transformer secondary. Or a couple of them in parallel.

The magnetic shunts should be left in the microwave oven transformer. 
They put them there to avoid current peaks, so I guess they would very 
conveniently damp any overcurrent peaks at resonance.

Both units, transformer and capacitor are HV units, and designed to 
carry quite a bit of current, so using them in this way should be quite 

The main point is: what would the resonant frequency be? I think I am 
going to try to measure this this week-end.

Vy 73

Angel HB9SLV

Jim Tonne a écrit :
> Tom/Angel/Peter:
> I went ahead and fired up my LTspice and entered a
> schematic so I could see for myself what kinds of stress we might see 
> on a capacitor placed in parallel
> with a filter choke.
> I used parts values that I thought were believable if
> not absolutely real-world.  (No suggestions or other
> feedback from the list, remember!) 
> The primary (mains) voltage was 120  at USA frequency of 60 Hz.  
> Transformer primary inductance was 10 millihenries, second was a pair
> of 1 henry windings with .5 ohms of resistance.
> The filter choke was 10 henries, .1 ohms resistance.
> The filter capacitor  was 10 uF, load 1000 ohms.
> Voltage across the load was a bit over 1000 volts
> so we had a one-kilowatt system.  The added
> capacitor was .15 microfarad.
> Ripple without the capacitor across the choke was
> about 25 volts peak to peak, nearly sinusoidal.
> With the added capacitor it dropped to about
> 5 volts peak to peak.
> I found in my circuit that the circulating current in
> the choke/capacitor pair was not at all high.  But one thing that 
> really surprised me was the voltage across those two items at 
> turn-on.  The voltage across the capacitor (and also across the
> choke) is the full output of the rectifier.  If there are
> any transients at turn on then the situation would be
> even worse.  Any transients coming in on the power
> line would also appear right across that added
> capacitor. 
> The only advantage I saw in putting the capacitor
> in the circuit was that reduction of several dB in the
> ripple level.  Notching out the second harmonic of
> the mains seems to be fairly easy but the other
> harmonics are not particularly attenuated;  the higher
> ones actually seem to be increased in level.
> But I would place money that the cause of the failure
> of that added capacitor is voltage stress, particularly
> at turnon or if there are transient voltages on the
> power line.  I would think that in a choke-input
> power supply the choke would more easily tolerate
> such transients;  capacitors aren't so forgiving as
> regards over-voltage.
> - JimT

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