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Old 28th June 2012, 10:51 AM   #1
hozone is offline hozone  Italy
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Default select power for bridge rectifier

hello,
i've a simple question.
i would like to know how to estimate the power for a bridge rectifier.
let's suppose a 230v to 21v / 9a single phase power trasformer.
does the rectifier need to be 9a, or more?

thanks!
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Old 28th June 2012, 10:54 AM   #2
DF96 is offline DF96  England
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It depends on what comes next: capacitor, choke? Duty cycle? Soft start arrangements?
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Old 28th June 2012, 11:11 AM   #3
MarianB is offline MarianB  Romania
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That 9A characteristic of the transformer is not the maximum possible current of it but rather the maximum aloweble for the diameter of the conductor with wich the coils are made, and as we know that when charging a capacitor from 0V ( or somewhere near that ) it acts for a little time as a shortcircuit, that means the rectifier would have to cope with much more current than those 9A, current wich if i am not mistaken is given by the capacitor ESR, ofcourse that the bigger the cap after the rectifier, the bigger the shock on it.
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Old 28th June 2012, 11:21 AM   #4
DF96 is offline DF96  England
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Yes, 9A for a transformer secondary usually means 9A continuous resistive load will not overheat the windings. You might only get 3A continuous DC from a rectifier and big capacitor.
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Old 28th June 2012, 11:33 AM   #5
hozone is offline hozone  Italy
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i've now disassembled the power board.
i've see that it comes with a 35w rectifier, strange?
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Old 28th June 2012, 11:47 AM   #6
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35A bridges are cheap enough to use them almost by default. So the designer does not need to think to much about peak currents etc.
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Old 28th June 2012, 02:01 PM   #7
hozone is offline hozone  Italy
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thank u,

i'm doing reverse eng. of a power supply board - i hope my meter test are right.
i do not understaind some components (a lot ), so if someone can reply me it would be great!

what's are there for?
1) c2, c1, c10
2) c9?
3) 0.01 ohm 3w resistor is a filter?
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Old 28th June 2012, 02:49 PM   #8
MarianB is offline MarianB  Romania
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I think that the joining point between C1 and C10 goes to some ground point or case or somthing like that, C2 i think filters out the rails noise, and C9 filters the high freq noise, as for the power resistor it might be some current limitter or sensor or something like that but i cannot be sure ultill i can see the entire power unit, maybe you could take a high resolution pic and post it here.
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Old 28th June 2012, 02:57 PM   #9
hozone is offline hozone  Italy
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here is the picture,
i'm pretty shure c1 and c10 goes from ac to - dc output of the rectifier :S

another general question, can i use 35v 4700uf capacitors (those are rated 50v), transformer is 21v ac out.
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Last edited by hozone; 28th June 2012 at 02:57 PM. Reason: picture missing
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Old 28th June 2012, 03:45 PM   #10
MarianB is offline MarianB  Romania
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I will tell you this:

-It is usualy a good idea to use filter caps with theyr voltage at least 20 or 30% higher than what they will be subjected to.
-21Vac will be about 19,6V after it has been rectified but not filtered because about 1,4V drops on the rectifier.
-19,6V will be about 27,6V, say 28Vdc after the filter caps because they rise up the voltage at peak level of the wave ( given a sinus wave ).

Now you can decide for youre self if 35V caps are ok for this power unit

PS; Take another pic, both from the top and bottom face of the board ( not from the side way ) with the whole unit if possible in it.

Last edited by MarianB; 28th June 2012 at 03:48 PM.
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