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Old 3rd August 2007, 10:33 PM   #1
nhuwar is offline nhuwar  United States
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Default Voltage Multipliers

Hi I'm going to use a full wave voltage doubler on a 200 vac transformer to get my 400vdc I need for my driver tubes.

The thing I'm not sure of is the size of the 2 caps in the doubler.

The current draw isn't much as you can amagine.

I am just not sure on the capacitance so any help would be greatly appreciated.

Thanks so much, Nick
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Old 3rd August 2007, 10:47 PM   #2
ilimzn is offline ilimzn  Croatia
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With a voltage doubler it's fairly easy - it gets quite tricky with tripplers, quadruplers etc.
Think of it this way - if you did not need a voltage doubler, you would have some value of cap X.
With a voltage doubler, the two caps are connected in series, and should, together, form the same capacitance X. Since the total capacitance of two equal series connected caps is half that of one cap, your two caps should each be twice X. This way, when connected in series, they will again form a capacitor of capacitance X end to end, which is what you need.
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Old 3rd August 2007, 11:02 PM   #3
nhuwar is offline nhuwar  United States
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Ok That make sence. I had built the circuit and used to caps rated for 450vdc with the series capacitance of 1100 microfarads.

When I applied power to it it took about 90 seconds to get to full voltage.

This is a long time and I'm sure it was to hi8gh a capacitance.

I have 2 oil paper caps with a capacitance of 1 micro farad a piece I was thinking it would get to voltage pretty fast.


Would this work decently with a filter choke on the output?

Thanks
Nick
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Old 4th August 2007, 10:52 AM   #4
EC8010 is offline EC8010  United Kingdom
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If you use PSUD2, you'll find that it can model a voltage doubler, so you can play to your heart's content before soldering.
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Old 4th August 2007, 04:43 PM   #5
ilimzn is offline ilimzn  Croatia
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What EC8010 said - my post was only about the relationship between the capacitances of a doubler and regular power supply wit the same output voltage and current. The actual value depends on the current you need and ripple voltage you can tolerate - neither of which you mention in your question.
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Old 4th August 2007, 09:48 PM   #6
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1uF is far too low for use in a doubler but 1100uF series capacitance is exceptionally high.

Try a pair of caps of around 220uF each to start with, using PSUDII to model it. Use a load that is realistic for your requirements and be sure to allow enough time for the voltage to reach its final level - about 1000 ms simulation time shoud do, I think. You may want to include a reporting delay, to reduce the run-time.
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Old 5th August 2007, 04:14 AM   #7
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I would like to know the same thing, about how long it takes to charge caps. Would putting a smaller value in paralell help? Seems a common value on old amps was 100-150uf. MC240 used 220uf, If I am not mistaken.
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Old 5th August 2007, 09:33 AM   #8
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The time taken for the caps in a voltage doubler to reach their full charge depends on the source resistance of the power tranny, the values of the doubler caps and the drain due to the load, but 0.25 to 1 sec would be fairly typical.

Quote:
Would putting a smaller value in paralell help?
I'm sorry, I don't understand your question. If you mean making up a large value by strapping smaller values in parallel, then yes, you can do that if you need to.

As to finding out what caps you actually need, and how long they will take to charge up, PSUD2 is as good a way of finding out as any. You need to give PSUD2 some fairly accurate data about the components that will make up the power supply, though, to get a reliable result. It needs to know the nominal voltage and DC resistance of the power tranny secondary, the capacitance and DC series resistance of each cap in the PS, the inductance and DC resistance of any choke you may plan to use for filtering and the equivalent resistance of the load on the PS.

You might prefer to use a current sink as the load in PSUD2, although I think it's less realistic than a resitive load, because the current drain will not be constant as the PS ramps up to its final voltage, and you can get spurious 'error' warnings.
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Old 5th August 2007, 11:45 AM   #9
swamp is offline swamp  United Kingdom
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Quote:
Originally posted by nhuwar
When I applied power to it it took about 90 seconds to get to full voltage.

This is a long time and I'm sure it was to hi8gh a capacitance.

Would this work decently with a filter choke on the output?

Thanks
Nick [/B]
Could this method be applied as a HT delay in a tube amp. I have a spare tx that outputs 110V AC but the circuit I am thiking of feeding needs about 200 ish. If I used large caps in the doubler would this act as a simple delay for the HT to let the heaters come on first or would I still need a timer delay circuit. Maybe a daft question ..
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Old 5th August 2007, 03:49 PM   #10
nhuwar is offline nhuwar  United States
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Thanks everyone I didn't know psud2 would sim multipliers




Thanks

Nick
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