I am taking 12V from Power card to my backplane card to use for Hard disks. I have been advised to put decaps of 330 uF on 12V supply. Can I use 220 uF or even lesser value. What is the criteria for selcting the value 7 what other parameters should I check??


Unfortunately, there only correct answer to this is "read the datasheet". Lacking a datasheet, you need to have knowledge of the current draw characteristics of the specific hard disks you're using. Lacking that, you're left with trial-and-error.

Assuming you're in the trial-and-error boat, put the 220uF capacitor on there and see if it works. If you get strange behavior, increase the capacitance. You can't really go too high on a decoupling capacitor, so it's safer to err on the high side.

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  • \$\begingroup\$ Excess capacitance can cause problems in some cases. If there is a fuse, the inrush current could blow a fuse. In some situations, boards can be inserted into a powered backplane. If the boards that are inserted have large capacitance, they may draw so many amps from the backplane, so fast, that the backplane voltage droops and causes the whole system to crash. I don't think there is enough information here to answer the question. OP should ask the person who gave the advice to explain it. \$\endgroup\$ – mkeith Oct 29 '14 at 6:55
  • \$\begingroup\$ The Datasheet doesn't state about the capacitor to be placed. The problem is that in 330 uF & 12 V , the electrolytic are too big for my board size. I want to reduce the size of the caps. I tied for tantulum but not much difference. \$\endgroup\$ – Oshi Nov 3 '14 at 9:21
  • \$\begingroup\$ I don't know what your size limitations are, but you can use several smaller capacitors in parallel. That's actually beneficial in a way because the total ESR is reduced by the parallel resistance equation. \$\endgroup\$ – Dan Laks Nov 3 '14 at 15:41

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