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Old 13th May 2005, 04:36 PM   #1
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Default you guys are smart.. :D help :(

basically....

I have 1000VA of transformers.... 9volts AC....

once rectified, how much capacitance to i need to keep the DC up with full load on the transformers, which is 80amps or so...

the mains is 50Hz... DC output from the rectifiers is around 12.5volts with a small load (lets assume 12volts under full load, they are big transformers)

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Old 13th May 2005, 04:39 PM   #2
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What's your application? Caps only reduce ripple.
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Old 13th May 2005, 04:55 PM   #3
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its to run a car amp, in the house....

edit: and maybe to jump start a car if it has a flat battery
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Old 13th May 2005, 05:33 PM   #4
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is ripple the right word?

I mean.. with a small load, and no capacitors, the DC voltage drops to 9 volts... while the AC stays constant.... ripple??
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Old 13th May 2005, 06:09 PM   #5
tiroth is offline tiroth  United States
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Download PSU Designer and find out. I suspect the answer is that you need a heck of a lot more voltage to maintain 12V at 80A. If you are ok with a lot of rail sag, then the requirements aren't as difficult.
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Old 13th May 2005, 06:16 PM   #6
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Quote:
Originally posted by tiroth
I suspect the answer is that you need a heck of a lot more voltage to maintain 12V at 80A.

I'm abit confused by that statement...
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Old 13th May 2005, 06:19 PM   #7
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ok... how about YOU download it, and tell me the answer
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Old 13th May 2005, 06:31 PM   #8
simon5 is offline simon5  Canada
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The easy way out would be to buy a supercapacitor from a car audio shop. I guess 1 F should be enough. I would also put two smaller ones in parallel... maybe a 1 mF and a 1 uF.

You could also use Maxwell Ultracapacitors...
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Old 13th May 2005, 06:34 PM   #9
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Quote:
Originally posted by simon5
The easy way out would be to buy a supercapacitor from a car audio shop. I guess 1 F should be enough. I would also put two smaller ones in parallel... maybe a 1 mF and a 1 uF.

You could also use Maxwell Ultracapacitors...

yeah, but would 1 farad actually be enough? or would 2 farads be better? or, is 1 farad overkill?

THATS my question
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Old 13th May 2005, 07:12 PM   #10
tiroth is offline tiroth  United States
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Ok. 4.5-0-4.5 tranny, full wave rectified into 1F of capacitance. Assumptions: 1.1m source resistance, 2m capacitor series resistance. (better than you will see in reality!)

Load: 150m (80A draw at 12V)
RMS voltage: 6.6V
Ripple: 321mV p-p

Load: 300m (40A draw at 12V)
RMS voltage: 8V
Ripple: 232mV p-p

Load: 1.5R (8A draw at 12V)
RMS voltage: 10.2V
Ripple: 88mV p-p

That's what I meant by "you need more voltage."
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