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Voltage Multiplier  Capacitors
Voltage Multiplier  Capacitors
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Old 13th October 2019, 11:36 AM   #1
cautious1 is offline cautious1
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Default Voltage Multiplier Capacitors

Have made a 6 times voltage multiplier power supply for a 6cm5 amplifier. Output stage is done but i have a question. The last two capacitors in the multiplier are rated 450 volts. When i first turn the amp on the B+ reaches 505 volts DC , for few seconds. Will this damage the Capacitors ? And also will the two last capacitors each have 505 volts or will it be shared ? Thank you
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Old 13th October 2019, 11:38 AM   #2
ubergeeknz is offline ubergeeknz  New Zealand
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Circuit diagram, please Voltage Multiplier  Capacitors
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Old 13th October 2019, 11:52 AM   #3
Jazid is offline Jazid  United Kingdom
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Given your forum name I'd have to ask: Is it worth the risk?
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Old 13th October 2019, 01:46 PM   #4
Mark Tillotson is offline Mark Tillotson
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Diode-capacitor multipliers (Cockcroft-Walton) have constant voltage rating along the chain, so for long chains the device voltage rating will be less than the total voltage - basically everything's in series and the voltage rating depends only on the input AC voltage.
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Old 13th October 2019, 05:12 PM   #5
kodabmx is offline kodabmx  Canada
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Voltage Multiplier  Capacitors
If it's hitting 505V for a few seconds it'd within the "surge voltage" rating of all but the worst capacitors. You should be fine.
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Old 13th October 2019, 07:09 PM   #6
Eli Duttman is offline Eli Duttman  United States
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Quote:
Originally Posted by Mark Tillotson View Post
Diode-capacitor multipliers (Cockcroft-Walton) have constant voltage rating along the chain, so for long chains the device voltage rating will be less than the total voltage - basically everything's in series and the voltage rating depends only on the input AC voltage.
Several voltage multiplier topologies are available. The 1/2 wave serial multiplier described has substantial stage to stage losses. Cost is its principal advantage. The uploaded 1/2 wave parallel topology is much better for a B+ supply, with far smaller losses. In the parallel case, increasing voltage stress, stage by stage, on the caps. is present.

Lots of good info. is available on Voltage Multipliers Inc. web site. The upload was extracted from that site.
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Old 14th October 2019, 07:38 AM   #7
cautious1 is offline cautious1
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Thank you Kodabmx and and others... The power supply voltage is the same as in the Valve Heaven Lamington 2 . Not sure how to post circuit of it sorry. My main concern is the high voltage at start on the last two Capacitors.
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Old 14th October 2019, 11:13 PM   #8
MarcelvdG is offline MarcelvdG  Netherlands
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Assuming it's 505 V with nominal mains voltage, that the mains voltage tolerance is +/- 10 % and that nothing is regulated, it could occasionally become 555.5 V.
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Old 14th October 2019, 11:37 PM   #9
JMFahey is offline JMFahey  Argentina
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Iīd use 2 x 350V rated capacitors in series for the last stages; thatīs what tube guitar amp makers routinely do.

Not "audiophiles" by any means, but solid Businessmen, if they do that itīs because it allows them avoid expensive warranty claims, so Iīd follow their advice.

Running stuff to the limit and trusting tolerances is not a good recipe.

Hey, speaking from own experience too: just last week a customer complained one of my 300W Bass amplifiers was smoking big time.

Asked for gut pictures and found that one 4700uF x 63V supply cap had blown its top and steamed out with mere 60V supply, go figure.

Mind you , they are a generic brand (Suntan) and always use them with no trouble ... until last Friday that is.

So in principle: working too close to ratings is "barely enough"; surpassing them is bad engineering, even if sometimes you can get away with it.
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Old 15th October 2019, 12:17 AM   #10
ubergeeknz is offline ubergeeknz  New Zealand
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If you do this it's usually recommended to put balance resistors across each cap in series. One paper on the subject from Illinois Capacitor recommends

R (in megaohms) =10/C (in uF)

Eg. for 47u

R = 10/47 = 0.21 Meg = 210k

220k near enough

Rating for 225v gives 0.23W. Better use 0.5W or more to have a good margin.

By this formula, larger caps need lower resistance and therefore burn more power and need a higher rating.

If you can match the capacitors well and overrate them (eg. Use 2 x 450v caps) you can probably get away with higher value balancing resistors as getting the balance exact won't matter as much.

Keep in mind also that total capacitance is halved when you put two like caps in series. So to make one 47u you'll need 2 x 100u .. etc
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