select power for bridge rectifier

[hozone]

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!

 

[DF96]

It depends on what comes next: capacitor, choke? Duty cycle? Soft start arrangements?

 

[MarianB]

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.

 

[DF96]

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.

 

[hozone]

i've now disassembled the power board.
i've see that it comes with a 35w rectifier, strange?

 

[Juergen Knoop]

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.

 

[hozone]

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?

 

[MarianB]

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.

 

[hozone]

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.

 

[MarianB]

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.

 

[hozone]

thanks for cap explanations, about this, have i to consider my caps subjected to 19,6V or 27,6V? i suppose it is 27,6 but i'm not shure

i have attacched pictures of the board.
i must admit it is a pool cleaning robot power supply
electronic that you see on board is broken somewhere, i suppose it attends as a timer, or for sending istructions to robot (go ahead, go back...), anyway my robot has a logic self implemented, so it only needs +28v~ volts to work, this is the reason i would like to replace all those components and make my power supply board and an atmega timer

maybe for my purpose i can just forget 100nf caps, transient voltage suppressor, and 0.01 resistor.

i double checked c1 and c10 and those two goes to negative output of the rectifier, i've search and find a design that looks like this, i've found one here Nanoo - DIY LM3875 IC Non-Inverting Chip Amplifier (Gainclone)
i've asked the author the reason for those two capacitors.

 

[MarianB]

I thought as much, well C1 an C10 bypass 2 of the rectifiers diodes, it actually should be 4 caps, 1 on paralel on each diode, they reduce the diodes swithcing noise, it makes the voltage sort of more clean.

That voltage suppressor makes sure that voltage in for the board cannot go higher than it's specification, it's a protection you could do without if you want, but if you want to make an entirely new PSU than a regulated output would be best i think.

 

[hozone]

thank you! now i've understand c1 and c10!

the caps should be 20 / 30% voltage rated more of the input voltage 19,6V or 27,6V?

 

[MarianB]

30% more than the voltage they supply in the circuit after they filter it, that is about 28Vcc, so they should be of minimum 35V, coosing a higher value doesn't help much, it's just a protection measure.

 

[hozone]

can i also ask you about the varistor current?

ref. datasheets S07K300 pdf, S07K300 description, S07K300 datasheets, S07K300 view ::: ALLDATASHEET :::
actually the varistor is tvr 14470, which is like S14K30
S14K30 -> imax 8/20
s = 1000A | Wmax (2 ms) = 9,0J | Max. clamping voltage i = 10 A
i have some S07K30
S07K30 -> imax 8/20
s = 250A | Wmax (2 ms) = 2,0J | Max. clamping voltage i = 2,5 A

my transformer as i said before is rated 21v, 9a

have i to consider for the varistor Max. clamping voltage i > 9 A, or imax 8/20
s > 9 A?

 

[MarianB]

Why do you need a varistor in the first place?

 

[hozone]

to protect ac overvoltage.
i've never use it until now, and because i see you are skilled i would like to ask you my doubt about varistor. i've read some documentations, wiki etc.. but i can not understand about the current.

 

[MarianB]

As i see in the datasheet TVR14470 has a maximum clamping voltage of about 775V and curent of 50A that means you cannot use that suppresor/varistor on supplyes bigger than that, and this one allows up to 300Vac-rms to pass, it suppresses everything above that, but i do not think it is of much use in youre case, you need something like 30Vac for example, or at least that is how i view the problem, but you can forget all about this ( and avoid a possible headache ) by using a Dc stabiliser after the rectifier, that way you have no worry cus no matter what the mains variations are the machine only gets the voltage you let it to do, the only thing you need to let me know is how much current that machine/robot requires.

 

[hozone]

i've measured 2 amp on 26v ac more or less with motors running

 

[MarianB]

Ok but you sayd that it is 28Vdc, or something like that, right?