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Author Topic: Resonant choke HV PSU questions  (Read 6929 times)
K1ZJH
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« Reply #15 on: December 20, 2012, 07:59:50 AM »

Basil

You can download a program called PSU Designer II from the Internet. I found it to be very useful to determine how the parts I have on hand will work in a power supply.

Be careful with capacitor input, it can stress early high voltage power transformers
that intended for use with choke input supplies.
 
Pete
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W0BTU
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« Reply #16 on: December 20, 2012, 09:30:14 AM »

Actually resonating the choke can be very nasty, as Tom, W8JI, has graphically told us.

It sure can. Tom was almost killed.

The voltage and current in a resonant circuit like that can be breathtakingly high. Read this, and decide if you still want to resonate that choke:
http://lists.contesting.com/_amps/2007-05/msg00115.html
The interesting stuff starts at about paragraph 7 there.

I built a resonant choke circuit in an 800 VDC power supply for my Collins 32S-3. But I would never do that in a PS that puts out any higher voltage than that.
« Last Edit: December 20, 2012, 09:34:12 AM by W0BTU » Logged

G3RZP
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« Reply #17 on: December 20, 2012, 12:50:07 PM »

'Tuning' and 'resonating' chokes in PSUs are very different  things. The principle is not for the unwary, or dare I say it, for someone who has got an Extra Class callsign by passing a 'vote for Joe' exam without understanding the basis for the electronics behind it all.
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W0BTU
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« Reply #18 on: December 20, 2012, 01:01:05 PM »

'Tuning' and 'resonating' chokes in PSUs are very different  things.

What exactly do you mean by that? Sounds like you know something I don't.

My circuit was an ordinary pi-net filter, and the series choke has capacitors in parallel with it to tune it to about 120 Hz.

Just looked at the old schematic (1979). It's a 7 henry choke with .25 uF/2400 v in parallel.

EDIT: The values above may not be accurate. I used an audio signal generator and a VTVM to find the correct value capacitors.
« Last Edit: December 20, 2012, 01:09:37 PM by W0BTU » Logged

N2EY
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Posts: 3879




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« Reply #19 on: December 20, 2012, 01:21:39 PM »

'Tuning' and 'resonating' chokes in PSUs are very different  things.
What exactly do you mean by that? Sounds like you know something I don't.

In a choke-input filter, the choke must have more than a certain amount of impedance for the filter to behave properly. At high load impedances the choke needs to have lots of impedance.

It turns out that for a 60 Hz full-wave supply, the minimum inductance needed is the load resistance in kilohms. IOW a 25,000 ohm load needs a 25 H or larger input choke.

However, choke impedance can be increased by tuning with a parallel capacitor. But if the combination is tuned exactly to the ripple frequency, the voltages across the choke can be enormous! So the combination is usually tuned to a frequency somewhat different from the ripple frequency.


73 de Jim, N2EY
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2E0ILY
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« Reply #20 on: December 20, 2012, 01:54:26 PM »

I have been linked to an old article that gives a means of testing these chokes. It suggests putting the US 115V AC 60Hz mains of the period across the choe, and assuming the choke's DC resistance is less than 100 ohms (mine is 35.2 Ohms) using this formula:

Inductance in Henry = (line voltage) X (1000)
                               --------------------
                               (377) X (Current in mA)

Dividing 377 by 60 cycles = 6.28

6.28 times 50 cycles = 314

I used 314 in my equations. At 115 V across the inductor I get 28H, at 200V I get 27H, and at 240V I also get 27H

Current drawn was 13.08mA, 23.3mA and 28.09mA respectively.

I then put the choke on my Fluke RCL meter (PM6303A) which I believe tests with a 1kHz signal. This showed it as 44H

Plugging 1kHz into the equation instead of 50Hz gave 38H , but just how relevant a test the latter was I am not sure.

When the Hirschmann choke arrives, which has a proper spec tag rivetted on it, showing it as 4 Henry, I'll do the same test and see
how the figures stack up. More for interest and education.
« Last Edit: December 20, 2012, 02:12:46 PM by BASIL » Logged

Best regards, Chris Wilson.
W0BTU
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« Reply #21 on: December 20, 2012, 01:58:14 PM »

'Tuning' and 'resonating' chokes in PSUs are very different  things.
What exactly do you mean by that? Sounds like you know something I don't.

In a choke-input filter, the choke must have more than a certain amount of impedance for the filter to behave properly.

Understood, but this was not a choke-input filter.
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N2EY
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« Reply #22 on: December 20, 2012, 03:50:22 PM »

'Tuning' and 'resonating' chokes in PSUs are very different  things.
What exactly do you mean by that? Sounds like you know something I don't.

In a choke-input filter, the choke must have more than a certain amount of impedance for the filter to behave properly.

Understood, but this was not a choke-input filter.

Completely different situation, then.

73 de Jim, N2EY
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G3RZP
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« Reply #23 on: December 21, 2012, 05:36:17 AM »

Basil,

That is a way of measuring the choke's impedance, but you haven't got DC superimposed, which is what you need for a meaningful measurement.

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2E0ILY
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Posts: 131




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« Reply #24 on: December 21, 2012, 05:58:09 AM »

Basil,

That is a way of measuring the choke's impedance, but you haven't got DC superimposed, which is what you need for a meaningful measurement.




I am a bit wary of how to do this. Do I need to impose the full working DC voltage on it? Or will a far lower 9and safer) level suffice? My other concern is loops, assuming I use a mains powered DC supply to inject the DC. How would you do this please?
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Best regards, Chris Wilson.
G3RZP
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Posts: 4539




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« Reply #25 on: December 21, 2012, 08:09:13 AM »

Use a Hay type Bridge - google that. Then you use a DC power supply to put enough current through the choke, You might need to get up to a hundred volts, depending on the resistance of the choke, and you'll have a similar resistance in series with the choke as one of the bridge arms. Then bypass the DC supply with capacitor and transformer couple the AC signal for the bridge in series with the DC.
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