DC capacitor voltage

Discussion in 'The Projects Forum' started by innernerd, Nov 22, 2008.

  1. innernerd

    Thread Starter New Member

    Nov 15, 2008
    I am trying to help repair an antenna rotating device associated with Grandpa's HAM radio system. He opened the case and believes that the 155uF 50VAC capacitor may be bad. I don't have a meter with enough range to verify if it is bad or not, we were just going to replace it and move on from there. It has no signs externally of heat/burn/failure of any kind. It did come out of a fairly dated piece of equipment.

    All of the caps at my local RS are rated DC, not AC. Is there a way to determine what DC value cap would suffice to replace this AC rated one? TIA

  2. SgtWookie


    Jul 17, 2007
    He has a non-polarized electrolytic cap, used as start/run caps for AC motors.

    If you could find two 330uF 50V polarized electrolytics, you could connect their two negative leads together, and then connect the two positive leads where the original cap was. That would effectively create a 165uF 50v non-polarized cap.

    However, Radio Shack doesn't carry a 330uF cap rated for 50v.
    They do carry 10uF 50v non-polarized, but you'd have to buy 15 of them.

    I suggest that you check with places that repair air conditioning, small motors, etc. As long as the voltage rating is equal or higher than 50v, it'll work - as long as the capacitance value is close; say 140uF to 165uF.
  3. awright

    Well-Known Member

    Jul 5, 2006
    The problem is that motor start/run capacitors with ratings below 270 or 330 VAC are pretty rare. Motor start capacitors are intended to be in the starting condition for only a second or two as the motor comes up to speed and motor run caps are usually of much lower capacitance than 150 uF. The average motor start cap AND motor start winding will burn out if the circuit does not open within a short time. Of course, the loading of an antenna rotator motor is MUCH less than a heavy duty equipment motor, so a start capacitor might work fine. However, it is likely to be much too big to fit in the position of the original cap.

    What evidence is there that the capacitor is defective? If it looks OK, what makes your grandpa think the problem is the capacitor? That may well be the case, but random replacement of components that show no signs of failure is not a very sophisticated approach to equipment repair. Have you checked the controller contacts? Have you measured the AC voltage to neutral on each end of the capacitor while it is being given a command to rotate? Across the capacitor? These can be performed without circuit disassembly.

    If you have an analog ohmmeter you can perform a crude test of the capacitor (removed from the circuit) by applying the ohmmeter leads to the capacitor terminals. You should see a dip of the pointer toward zero ohms followed by a climb back toward infinite resistance. Play with different resistance ranges to get the best indication. The numbers don't matter.

    Reverse the capacitor terminals each time you touch the probes to the terminals. This doubles the impulse of the charge/discharge of the capacitor. (You will actually not see any response at all the second time you touch the terminals without reversing them because you have already charged up the capacitor from the ohmmeter's internal battery.)

    What you are doing with this test is using the ohmmeter's internal battery to charge the capacitor. When you first touch the probes to the discharged capacitor, the ohmmeter sees a short circuit for a short time that is determined by the internal range resistors of the ohmmeter. This is the dip toward zero ohms. Then, as the capacitor charges up to the battery voltage (at a speed determined by the range resistors), you see the pointer rise up toward infinite resistance. If it stalls at some moderate resistance (less than several megohms) the capacitor has internal leakage and should be replaced. A good capacitor should approach the highest resistance your ohmmeter can read. If it dips to zero and stays there, the capacitor is shorted and should be replaced.

    I'm not necessarily recommending it, but All Electronics in Van Nuys CA has 150 uF 100 volt non-polar capacitors for $2.50.


    I wouldn't be surprised to find that Mouser electronics or Digi-Key, both mail-order electronics suppliers with no minimums, carried a suitable capacitor.

    My hesitation, based upon lack of actual knowledge, is that a non-polar capacitor is not necessarily the same thing as an AC rated capacitor. I think there may be some issues of tolerance of the heating effect of AC current passing through a capacitor loaded by a motor winding. However, this one may be a good bet since an antenna rotator never operates continuously and heating may not be an issue. The 100 volt rating would be equivalent to about a 70 volt AC rating, so you would have a little reserve.

    I'd compare the size of this replacement to that of the original, both for fit inside the rotator case and for equivalence of heat dissipation capability. If the replacement has at least as much surface area as the original, it should be capable of dissipating at least as much heat as the original.

    Have fun.

    Last edited: Nov 23, 2008