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Old 7th June 2003, 01:33 PM   #11
djk is offline djk
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"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)"

I'm sorry you don't seem to understand Circlotron or myself, we are trying to help you benefit from mistakes we have made in the past:

Your transformer is too low a voltage, you're wasting your time and money.

To make the most of the bad situation you're in I would re-wire the two transformers you have as 12-0-12 and use a pair of Schottky diodes, no filter cap, and float a marine-grade battery(lead-antimony, not lead-calcium) across the supply.
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Old 7th June 2003, 04:34 PM   #12
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I don't think the voltage of the transformers are going to cause any problem, but the are a bit too small... but if you don't need 40A continously they will probably be fine for testing your amp. But you will need a huge cap
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Old 8th June 2003, 11:41 AM   #13
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"megajocke: I don't think the voltage of the transformers are going to cause any problem, but the are a bit too small... but if you don't need 40A continously they will probably be fine for testing your amp. But you will need a huge cap"

Hey, megajoke, what kind of living do you make as a stand up comedian?

For those of us too lazy to think, and have no common sense, there is a free on-line power supply designer:

http://www.duncanamps.com/psud2/index.html

If you input a 12V transformer with 0.1 ohms resistance, and a 1F cap with 0.001 ohms resistance, with a bridge rectifier, and a 0.5 ohm load, you get:

18.95A load current
27.8A transformer current
9.5VDC

The ratio of load current to transformer current is .68, the old fart (that's me) guessed .7, I also guessed it would only have 10.5VDC out, I was wrong: it only does 9.5V (I didn't calculate the loss due to the DCR of the wire in the transformer).

And by the way, the 20A rated transformer will burn up if you run it too long at 27.8A !

"But you will need a huge cap "

Tell me another one!
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Old 8th June 2003, 12:37 PM   #14
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I re-did the calculations for two 250VA transformers with 7% no-load to full-load regulation, and a cap with an ESR of 10 milliohms, and it comes up:

27.7A load current
41.7A transformer current
11.08VDC

and a .664 load/transformer current ratio.

The rectifier model on this program is a little weak, it has too high of a value for Ifsm(the program model has 999A for Ifsm where a real bridge will be between 300A~400A)and no way to edit for actual diode drop vs current. I think in reality the rectifiers will have higher losses and the output voltage will be a little lower. The other problem is that software guys usually input 25*C specs instead of numbers at real operating temperatures.

It is still a very good tool for a DIY project.
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Old 8th June 2003, 08:06 PM   #15
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I never said he would be able to get 40 amps out of those transformers! Where did you get that from?

What I meant was that those 27.7A should still be enough to power his amp as long as he doesn't play it too loud.
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Old 8th June 2003, 11:27 PM   #16
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The 27A is only at 11.08VDC, after the rectifiers heat up that will be even lower.

He said this is for a store display, think he will have a problem with the voltage going from 11V to 17V depending on the load. A lot of head units will not like it, the display panels will go dim and bright, some units will shut-off wth the low voltage.

In reality this would not be able to run a 100W stereo amp at full output.

Unlike you, I have actually built several high current supplies for car stereo displays and test benches.

But you are welcome to make your own mistakes.

I prefer to learn from the mistakes of others.

Do what you like.
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Old 8th June 2003, 11:40 PM   #17
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djk....people are just trying to help here but it seems you think were idiots on the subject...you brought up some valid power correction factors so leave it at that and not start slamming other members...were all on the same team here

DIRT®
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