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Acceptable leakage of NP caps


Alexander

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Depends on what spec they were built to. You'd have to do a search on "how to read a capacitor", but what you're interested in is not leakage, but tolerance. Leakage will change the value of the capacitor because it changes the amount of time it takes to charge up to its nominal value. Once the capacitor deviates from its tolerance spec, usually a percentage from nominal value, then it's time to replace it. For instance, some capacitors have a 20% tolerance, 10% tolerance, etc. .

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On ‎8‎/‎2‎/‎2017 at 8:32 PM, avguytx said:

Non polarized maybe?

Sorry, yes non polarized electrolytic caps.

 

The  NPE caps are 18uf 10% @100v. they measured a bit over 20uf, just outside of spec., but my tester could be a little off too as it has 2% variance .

 

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17 hours ago, djk said:

The ESR is more important than the absolute value.

If it isn't too much to ask would you give us a quick primer on level of importance in measuring caps?   Or even a link(s) if I am asking too much? The LCR I got did include ESR and if I remember they were rather low but what are good,bad ranges?

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You need to know how to interpret the data presented. I will explain my understanding and reasoning for not changing out the old Aerovox caps. When time permits I can do an actual write up in a separate thread if anyone is interested. I didn't use an LCR meter I used a signal generator and an oscilloscope.

 

 

Let us talk about the 2uF caps first. I forget where the actual crossover point for the tweeter is in the AA crossover but I will use arbitrary numbers for the example. The tweeter is only handling the higher frequencies, looking at the ESR of the 2uF cap at 100Hz tells us nothing. In fact higher ESR at this frequency will only help us IMHO because it will attenuate more lows going to the tweeter. The Xc (capacitive reactance) of a 2uF cap at 2kHz is 40 ohms. Do you really think a cap that has an ESR of .4 ohms at 2kHz is going to make a difference? It won't. ESR will decrease with increase in frequency so test with the lowest frequency for the application. When testing the 13uF cap you may want to now test down to 500Hz. The Xc of a 13uF cap at 500Hz is 24 ohms. Do you really think .5 ohm of ESR is going to make a difference? I don't. The actual value of capacitance will make more of a difference. For example if the cap actually measured 17uF now the Xc is 18 ohms which will change the crossover point much more than .5-1 ohm of ESR. I think the best way to test these caps is with a signal generator and an oscilloscope at the lowest frequency for their application. The output amplitude will be a result of the combination of BOTH ESR and capacitance without having to worry about either separately. Use a series resistance with your signal generator and measure the amplitude across the cap.

 

To summarize what I am talking about and what I did;

 

A perfect 2uF cap at 2kHz should be 40 ohms. Take a 1v signal and shoot it through a 1k resistor in series with the cap. You should measure ~40mV across the cap. If you don't  have ~40mV either the capacitance is off or the ESR is off or both.

 

A perfect 13uF cap should be 24 ohms at 500Hz. Shoot a 500Hz signal though the same 1k series resistor and measure across the cap. It should read ~24mV. If you don't get 24mV across the cap either capacitance is off or ESR is off or both.

 

I hope this makes sense and helps.

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