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Old 5th June 2003, 07:17 PM   #1
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Default 40A power supply....

i want to build a simple power supply to car amplifier for a car store display..... in the pic it is a power supply for 1 amp....
it is out a 40A...in 16.8VDC........ i already build it and try this....
but if i up the power - of the amplifier (music) the led of the amp... start to blinker...

why ??? 40A no enough ??? or the 4700uf is not enough ???

this is a normal amp.... 1000W peak... the fuse in the amp is 20A x2 (if is metter)
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Old 5th June 2003, 07:51 PM   #2
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Under heavy load your circuit will only put out 10.5V DC, the RMS voltage minus the rectifier drop.

A real 12V 40A supply would have a 34VCT transfomer and a two diode bridge feeding a regulator. 13.8V out(12V nominal) + .7V diode drop + 2.5V for the regulator = 34VCT

A switching supply would have different requirements, usually beyond DIY.
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Old 5th June 2003, 08:11 PM   #3
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Your transformers will probably be fine, and i guess the amp won't mind the extra voltage, maybe 17V or so when unloaded. But you need a bigger capacitor, you need one at least ten times as big.
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Old 5th June 2003, 08:56 PM   #4
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Quote:
Originally posted by megajocke
Your transformers will probably be fine, and i guess the amp won't mind the extra voltage, maybe 17V or so when unloaded. But you need a bigger capacitor, you need one at least ten times as big.
yes the amp won't mind the extra voltage - 15-17V

soo u say that i need a 47000uf ? or more?

and i i build a 80A like the pic but with 4 transformer...
and i connect them 611000uF (4700uF x 130)

it will be good???
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Old 6th June 2003, 12:11 AM   #5
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I use a one farad capacitor on my test bench for car audio amps.....and the transformers look too small in my beliefs......I have a 1500kva transformer and it wants to stall when burning in some serious amplifiers.....I got around this by charging up some Gel-cell marine batteries..but thats not the answer to your question

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Old 6th June 2003, 01:01 AM   #6
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Quote:
Originally posted by Dj BASS AMP


yes the amp won't mind the extra voltage - 15-17V

soo u say that i need a 47000uf ? or more?

and i i build a 80A like the pic but with 4 transformer...
and i connect them 611000uF (4700uF x 130)

it will be good???
hehe... You'll need way more than 47,000uF of capacitance, here. In fact, the equation to determine just how much capacitance you will need is:

C = I/(dV*f)

Where C is farads, I is amps, f is 120 hertz and dV is ripple in volts peak-to-peak. What amount of ripple your amplifiers can tolerate depends heavily on the switching power supply inside them, but let's make it 5%, or about 600mV. The capacitance needed will be... 555,555uF. It will be absolutely necessary to obtain this amount of capacitance through paralleling several smaller capacitors in order to split the current up among them. A single "farad" capacitor, like the type so popular in car audio applications, will not have the ripple current rating (usually between 3 and 5 times the average current is necessary!) to cope with this duty.

Note that a 12.6V secondary voltage will be perfectly adequate as long as there is plenty of capacitance and strap copper connections to carry the current!
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Old 6th June 2003, 08:56 AM   #7
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Originally posted by jeffreyj

. The capacitance needed will be... 555,555uF. It will be absolutely necessary to obtain this amount of capacitance through paralleling several smaller capacitors in order to split the current up among them.
555,555uf it is good for 80A power supply??

and what better.... 1F - car capacitor.... or 1F (from a lot of 4700uf)
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Old 6th June 2003, 10:53 AM   #8
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I don't know where you guys learned to push electrons, but the way I learned it it goes:

12VAC goes to 17V peak every 8.3mS(on 60hz), but it also drops to 0V every 8.3mS too. The Root Mean Square of this is 12V. While the cap can store energy from the 17V peaks and release it during the 0V periods, the capacitor creates no energy, it only stores it to be released later.

Diodes drop voltage across them, about .7V nominal, more at high currents. I was being charitable when I only subtracted 1.5V for a four diode bridge. What kind of diodes are you guys using that have no losses?

The power factor on a cap input filter with a hugh cap is really poor, around 0.7, this means that your two 250VA transformers will pull 500W in and only put 350W out. So at the peak of 17V with a 1 Farad cap you will get less than 20A out, even though the transformer is rated at 40A RMS. And I forgot to subtract the rectifier loss, so it will be more like 15V at 20A

One other thing: If you use a hugh cap you also risk blowing the rectifiers. Even though the thing will only put out 20A the peak charging currents every 8.3mS will only be limited by the DC resistance of the transformer wire, the hugh cap will only draw current on the very peak of the sine wave and will act like a dead short.

Maybe a quick trip to the local community college book store would be in order?
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Old 6th June 2003, 11:48 AM   #9
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Quote:
Originally posted by djk
The power factor on a cap input filter with a hugh cap is really poor, around 0.7, this means that your two 250VA transformers will pull 500W in and only put 350W out.
I have measured the power factor of a diode bridge feeding a big cap from the ac mains with this fancy Voltech PM1000 digital meter meant specifically for this kind of measurement, and it measured 0.6, an even lower figure. Based on that, your 500VA's worth of transformers is only going to be able to put out 300 watts. Bridge losses again are extra.
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Old 6th June 2003, 11:56 AM   #10
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Quote:
Originally posted by djk
I don't know where you guys learned to push electrons, but the way I learned it it goes:

The power factor on a cap input filter with a hugh cap is really poor, around 0.7, this means that your two 250VA transformers will pull 500W in and only put 350W out. So at the peak of 17V with a 1 Farad cap you will get less than 20A out, even though the transformer is rated at 40A RMS. And I forgot to subtract the rectifier loss, so it will be more like 15V at 20A

One other thing: If you use a hugh cap you also risk blowing the rectifiers. Even though the thing will only put out 20A the peak charging currents every 8.3mS will only be limited by the DC resistance of the transformer wire, the hugh cap will only draw current on the very peak of the sine wave and will act like a dead short.
i dont ask u if it is peak or not or what learned...

i ask what will be better.... 1F - car capacitor.... or 1F (from a lot of 4700uf)
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