In the picture you can see the type of capacitor im dealing with. The specific one is actually the third in the picture, but I took the picture of the easier to see one. The one that has failed is a 5000MFD segment, and smooths out the positive ripple from a 4 diode AC rectifier.

The whole thing is part of a electric organ.

Would this sort of thing be a suitable replacement ? (disregarding the capactance difference) http://www.jaycar.com.au/Passive-Components/Capacitors/Electrolytic/4700uF-25V-RB-Electrolytic-Capacitor---105oC/p/RE6244

Or does it need to be big and chunky like in the picture? (or is the one in the picture like that because it is old?)

enter image description here

  • \$\begingroup\$ The one in the picture appears to be a 3 section capacitor, one of 3500 uF and two of 1000 uF. Are the markings on the one you intend to replace similar? If so, you will need multiple single capacitors to replace it. Also, explains why the capacitor is large. \$\endgroup\$ – Barry Apr 1 '15 at 13:16
  • \$\begingroup\$ The one in question and not pictured, is a 5000, 1000, 1000. But im just disconnecting the 5000 leg and substituting that part of it, placing the new cap underneath the existing ones. all are 25VDC \$\endgroup\$ – Hayden Thring Apr 1 '15 at 13:45

There are three main factors to consider when selecting a capacitor for replacement:

  1. Capacitance
  2. Voltage
  3. Effective Series Resistance (ESR)

The capacitance is easy to match, and you probably want to match this as closely as possible, though capacitance can vary wildly from device to device, with environmental factors, and with time. I assume "MFD" here stands for "MicroFaraD" not MilliFaraD (that would be 5 Farad!), so 4700µF would be a reasonably close match.

For the voltage you need to have at least the same voltage rating. It doesn't usually matter if you go over, since it's a limit to the voltage it can handle. The voltage for a power supply capacitor should be above the maximum ripple voltage peak it has to handle. That can be higher than the voltage of the supply.

The ESR for a power supply is more critical. You need it to block as much DC current as possible while at the same time allowing as much of the 100 / 120Hz ripple noise to pass right through as possible. So you want the ESR at 100 / 120Hz (note, after full wave rectification the line frequency is doubled) to be nice and low.

The advantage of the big cans is that they give a large surface area for the capacitor plates, which in turn keeps the ESR down. Using a physically smaller capacitor may increase the ESR, but given the advances in technology and capacitor construction, that may well be somewhat offset.

So what's the worst that can happen if your capacitor isn't up to the job? Assuming you have the right voltage, all that you may notice is an increase to the background hum from the speakers. Of course, if the voltage of the capacitor is too low you may want to stand back as it explodes and fills the room with vaporised electrolyte and shredded paper and aluminium foil. That's always a laugh.

  • 1
    \$\begingroup\$ You could add the ripple current rating, which is related to the ESR. Ripple current * ESR^2 gives a power loss in the capacitor, which heats it, and (if it's too small to dissipate the heat) can destroy it, another way to get a laugh. So the replacement should have similar or better (lower) ESR, and similar or better (higher) ripple current rating. \$\endgroup\$ – user_1818839 Apr 1 '15 at 13:42
  • \$\begingroup\$ Thanks, but what about "Ripple Current" rating, as it driving quite a large organ, could this be a factor ? The voltage will be right, as it states 25v dc on the cap and i can get that, and yes it is 5000uF \$\endgroup\$ – Hayden Thring Apr 1 '15 at 13:48
  • \$\begingroup\$ How would i determine the ESR and Ripple ratings of the existing one ? \$\endgroup\$ – Hayden Thring Apr 1 '15 at 13:50

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