Help identifying a capacitor 10/25
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  1. #1
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    Help identifying a capacitor 10/25

    I can't for the life of me figure out what kind of capacitor these are, circled in red, it's a polarized capacitor labeled 10/25 or 10/35.
    I don't get the nomenclature
    it's for a power supply.

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    That the purpose was unattainable, if not foolish, was not so important,
    Dr. Alvin Weinberg on the Aircraft Reactor Experiment
    http://energyfromthorium.com/ http://thoriumremix.com/2016/

  2. #2
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    Re: Help identifying a capacitor 10/25

    That usually designates a 10uF (micro Farad) at 25 VDC (35 VDC) electrolytic. Those are in a position to be used as "bulk" caps, meaning that they are not stressed too badly and anything at least that uF and voltage should work fine. Bigger on either won't hurt. Look for a bulged rubber cap where the wires come out, a bulge on the can, or any signs of corrosion indicating the electrolyte is leaking. It doesn't have to be bulged to be damaged, and they're cheap, so replacing them all is probably good. If they're in an area that sees a lot of heat, look for a higher temp rating, something like 105c.
    Digikey (min $25), Jameco, Mouser, etc...

    Hope that helps.
    Good luck.
    Last edited by boroko; 01-11-2017 at 12:56 AM.
    A red Buzz Box, a Hobart MigMan, a 330 Aircrafter, and now a Syncrowave 250 (sort of).

  3. #3
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    Re: Help identifying a capacitor 10/25

    Thanks Boroko, I have a very limited knowledge of how capacitors function in a circuit. I understand that sometimes they are used for voltage spike suppression, and on a Mig welder they are what makes it constant voltage. But what the units mean and why you'd select a particular size on a chip is beyond me. Again thank you.
    That the purpose was unattainable, if not foolish, was not so important,
    Dr. Alvin Weinberg on the Aircraft Reactor Experiment
    http://energyfromthorium.com/ http://thoriumremix.com/2016/

  4. #4
    Join Date
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    364

    Re: Help identifying a capacitor 10/25

    Boroko is dead on. FWIW the ones (C114, C117, etc) marked 100N are 100 Nano-farads. A Nano is one millionth of a millionth. In circuits like this electrical designers like to use a combination of high capacitance (bulk) capacitors to filter out surges and low value capacitors to filter out high frequency noise hence it's common to see high value and low value capacitors in parallel.

  5. #5
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    Re: Help identifying a capacitor 10/25

    Like FlaJoe said, the position and values of those big capacitors is to help supply a smoother consistent voltage to the regulators and give some reserve, allowing them to hold their specified output voltage closer. The small ones are to catch transients that confuse the regulators as they try to adjust quickly to spikes. Capacitors are analogous to a balloon that collects electricity: when the voltage wants to go up, it stores more, if it drops, the balloon tires to maintain pressure (in this case, voltage).
    The ac from the transformer first goes with one wire positive compared to the other, and then reverses polarity and that wire becomes negative compared to the other, in a sine wave pattern. The 4 diodes are configured as a bridge rectifier. While saying they convert the ac to dc is correct, they direct all of the positive arcs to one wire and all of the negative to the other. To visualize it, imagine a bunch of camel humps lined up together. The bulk caps help to smooth the areas between the humps and make it closer to pure dc. Maybe more of an explanation than you wanted, but it helps to understand when you're looking for problems.
    If you have a multimeter, first you measure the dc output of each regulator to the circuit ground. They should be very close to their rated voltage. If you want to look a bit deeper, set your meter to read ac and if the caps are bad, you will likely it as a a small ac voltage. That's how much variance is getting through the regulators.

    Hope this helps.
    Bo
    Last edited by boroko; 01-12-2017 at 01:25 AM.
    A red Buzz Box, a Hobart MigMan, a 330 Aircrafter, and now a Syncrowave 250 (sort of).

  6. #6
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    Re: Help identifying a capacitor 10/25

    Quote Originally Posted by boroko View Post
    If you want to look a bit deeper, set your meter to read ac and if the caps are bad, you will likely it as a a small ac voltage. That's how much variance is getting through the regulators.

    Hope this helps.
    Bo
    It's not so much that variance is getting through the regulators as it is that the pass transistors in the regulators are switching off and on at a very high rate. When they're on, they pass almost the full input voltage to the output but when they're off, they pass no voltage. The result is that the output fluctuates between those two voltages and the bulk capacitor on the output "averages" out the output.

    FYI, You can use the AC voltage function of a DVM as a rough check of functionality of the output capacitors but it will only work if there's a load on the power supply, otherwise the caps will simply charge to the PEAK voltage of the output and stay there. The only thing that no load testing will tell is that there is SOME capacitance in the circuit. Testing with a load will discharge the peak voltage out of the cap and then you'll be able to see the amount of AC voltage at that point. Ideally it should be none but a very small AC voltage is normal.

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