[derkiow]

I have a 68volt rectified output that needs a bus capacitor of 7200uf , 91volt and 5amp ripple ,, if i can;t get the right size capacitor which way is it best to go, up or down in size (uf) derkiow

[Al_The_Man]

I work it out that at 68v a 7200µf will produce 5% ripple at ~4.4amps load.
at 10amps you would need around 16,000µf for 5% ripple.
It depends on your load and the amount of ripple you want to design in at max current.
One down side with going too large on capacitance is it requires an increase in transformer VA.
Al.

[john-100]

Hi derkiow

I just had a look at RS components

and found item 381 - 9194 8200 uF 100V 18A @ £34-45 screw terminals

with some values pcb mounting types are less expensive

if for some reason you need to use a number of capacitors to make up the required value

I would use 100 V capacitors in parallel that add up to the value needed

[derkiow]

So is it safer to go up or down would 6600uf do or would i be better going to 8800uf .The reason i ask is that have some 2200uf capacitors already ,,,,and thanks for the replies derkiow

[MrWild]

UP Always up. The more the better.

[Al_The_Man]

UP Always up. The more the better.

Except if you go too high and your transformer is tightly sized, you can go into saturation when running at maximum current.

So is it safer to go up or down would 6600uf do or would i be better going to 8800uf .The reason i ask is that have some 2200uf capacitors already ,,,,and thanks for the replies derkiow

What is your estimated maximum current?
Al.

[derkiow]

i have three drives at 5amps so that will be 15amps and i have a 1kva transformer 45v @ 22amps my idea was to use 4 times 2200uf for each drive..derk

[Al_The_Man]

For a 15a max load at 68VDC you would need 25,000µf for 5% ripple.
100% rating would be 1.3kva and 60% duty cycle would require a .8kva supply.
So it looks like you could go up to 25,000 for 5% ripple.
Al.