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Old 9th August 2013, 10:02 PM   #1
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Default Choke input supply. Cap question.

I am planning a choke input power supply using a 1200vct power transformer. It sims in PSDII to have a B+ of ~540V.

The choke supply does not look like it rings or has a big voltage spike at startup. Can I safely use a 600V cap in C1 position?

Can you over cap a choke input supply?

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Old 9th August 2013, 10:27 PM   #2
piano3 is offline piano3  United Kingdom
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I would choose a cap with a voltage rating to withstand the worst case scenario that the supply became inadvertently unloaded
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Old 9th August 2013, 10:38 PM   #3
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Without using series caps with sharing resistors, I could never afford enough 800V electrolytics.

Those 800V caps are something like $50-75/ea.
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Old 9th August 2013, 10:56 PM   #4
piano3 is offline piano3  United Kingdom
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Use the series caps then (which is what I have done a number of times) and you will be running them conservatively (=long life!!). 450v caps are cheap after all.
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Old 9th August 2013, 11:04 PM   #5
piano3 is offline piano3  United Kingdom
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Anyway, 800V may not be enough; 600x sqrt2 is nearly 850
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Old 10th August 2013, 03:00 AM   #6
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Unless a draw is present, a choke I/P filter charges up to the peak voltage, just like a cap. I/P filter does. Good regulation, at approx. 0.9X VRMS, sets in, when the critical current is drawn. A good approximation for the critical current, in mA., is given by V/L. Placing a bleeder resistor across the 1st filter cap. that guaranties the critical current is drawn is a common practice. For a 10 H. choke, a 10,000 Ω resistor is an appropriate bleeder part. If the bleeder is present, a 600 WVDC 1st filter cap. will not fail. The bleeder needs to be high wattage.

If you refer to an older version of the ARRL handbook, you will find that LCLC is recommend for critical current (choke I/P) filters.
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Old 10th August 2013, 03:23 AM   #7
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It may not be just the first filter cap that needs high voltage rating (if you want to avoid large power ballast resistor or some form of power zener). And don't forget that mains may go at least +5% on what you may presently be measuring (especially in the middle of the day if all your neighbours are exporting solar power).

450V large electro's (eg. 470uF) are soooo common due to smps, so if you have the room and need that level of filtering then it can be quite cheap. Alternatively, a much smaller value cap can be used (ie. cheaper) because you have choke input for starters and the ripple current spec is unlikely to be exceeded.

If your screen and preamp demand is small, then you could try to run them off the mid-point.
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Old 10th August 2013, 03:50 AM   #8
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Thanks guys!

It is a 500mA+ western electric power transformer and a Hammond 193Q 10H @ 500mA choke. I suspect the ripple to be fairly low off the choke before the caps. I have even considered a group of five or so 10uf 800V poly caps.

I have another 7H choke for the driver tube supply.

It is for a quad of KT88s, so there should be enough current.
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Old 11th August 2013, 01:54 AM   #9
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The Hammond 193Q may not be up to serving as the 1st inductor in a HIGH current choke I/P filter PSU. Unless an inductor is explicitly rated for choke I/P service, it is necessary to derate its current handling capacity. Noise and overheating are what occurs, when an inductor is "in over its head".

You may be able to get a handle on things by installing a 0.47 μF. "fudge factor" cap., in what would be the 1st position of a CLC filter. Use a film snubber rated for at least 2500 WVDC. Truly NASTY kick back spikes can be present.

If vacuum rectification is to be employed, the 5R4 is only common tube whose PIV rating is "tall" enough. You will need a pair of 5R4s to handle 500 mA. of B+. Another possibility is pair of 6CJ3 damper diodes. If you use 6CJ3s, dedicate a filament trafo to them and tie the cathodes to 1 end of the heater. Make sure that the filament trafo you use has an outstanding hipot rating.
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Old 11th August 2013, 03:54 AM   #10
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Hi Eli,

What makes a choke suitable for input stability? I'm OK with dropping a small film cap across the PS. Maybe even up to 5-10uF. Depending on my goal B+.

I looked at Sowter because they have some rated for choke input, but the main difference is the static DCR being a bit higher.
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