Hi,

I'm doing a rectifier and I have to choose the storage capacitors... In the datasheet there is the following information :


I'm not sur to understand it. Does it mean that the value of the capacitor is dependant on frequency ? And for example if i'm working at 120 KHz, the value of the capacitor is equal to the nominal value multiply by the coeffecient associated to the frequency (in this case the coefficient is 1) ?

Thanks,

Have a nice day !

 

hi leo,
Do you mean 120Hz , not 120kHz.?
Do you have a link to the documentation where you got that clip.?
E

 

Yeah, I m sorry, It is 120 Hz. I have an other question : At the power on of the board, the current into the capacitor will be very high because the capacitors are discharged... Will they support this inrush current ? Or is there something to do to prevent this inrush current a power on ?

Here is the link to the datasheet : http://www.farnell.com/datasheets/2...806.657545816.1546109600-915965338.1546109600

 

hi,
The initial inrush charge current can be limited by the impedance of the voltage source.
Some PSU's do have a low value series current limiting resistor.
The current ripple specifications are on page #2 of your PDF, Note: they are in milli-Amps, not amps.
E

 

I don't know the impedance of my voltage source. The output impedance comes from a transformer that I "salvage".

(What do you mean by PSU ? Power Supply unit ? )

I don't understand the ripple current on page 2. At DC voltage, there is no variation on voltage, so no ripple current according to the formula i(t) = CdUc(t)/dt. Can you please bring me to the light ?

And did I do a mistake about my first question ?

Thank you

 

hi,
If you do not know the meaning of ripple current in a capacitor, read this link tutorial.
Its the current that flows thru the capacitor at a given frequency.
E
https://passive-components.eu/ripple-current-and-its-effects-on-the-performance-of-capacitors/

 

I may not know what is ripple current. I will so ask you what is ripple current @ Vdc (frequency = 0) ? and I will ask you too how the ripple current do no depend on load connected to capacitors. If you find informations about this on page 2 ...

 

With DC voltage on a capacitor there is no ripple current only a very low level leakage current.

It would be a good idea if you posted a circuit so that I can understand the problem.

 

You are worrying too much. Typical when we select the capacitor size for a rectifier circuit we usually use this formula

C = Q/V = ( I*t)/V

where:

I - load current
t - discharge time 1/100Hz (10ms) for a full wave rectifier or 1/120Hz for countries with 60Hz mains.
V - capacitor ripple voltage

And buy capacitor.

And the ripple current of capacitors is the allowable rms current that can flow through the capacitor.
If your current will be larger than this you decrease the capacitor lifetime.

 

Thank you for your answer Jony