How can you test a filter cap (can...etc...)??
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5150loveeddie
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How can you test a filter cap (can...etc...)??
How can you know if an old can is still good? How do we test them??
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rockstah
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Dude
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If you are going to be testing electrolytic caps you really need two meters; a capacitance meter and an ESR meter. Using a capacitance meter alone will tell you jack about the state of the cap. Be aware that a capacitance meter is really nothing more than a pulse generator, it doesn't actually 'charge' the capacitor under test in order to preform the test. A capacitor with a low voltage leak will read higher than its rating. Keep in mind that capacitors can have a large tolerance and also that a leak can still occur at higher voltages than the cap meter can provide. You also need to have the capacitor out of circuit in order to prerform the test.
Unfortunately if our capacitor has passed this test does not gurantee its in good working order. Let me give you some basics on electrolytic capacitor construction;
Electros are similar to batteries in that they work by chemical action. Its made up of two aluminium foils (usually) and separated by an absorbent material similar to paper and soaked by a liquid called an electrolyte.
As part of the manufacturing process, a current is passed through the electrolyte which forms an oxide coating on the positive foil (forming). The oxide coating is a dielectric which is what enables a capacitor to hold charge. Over time, the electrolyte dries up and the equivelant series resistance (ESR) goes up until in the worst case it becomes open circuit.
ESR basically means the capacitors given resistance to an AC current.
Each capacitor has an ESR rating, smaller caps are higher in ESR, bigger ones lower, also the voltage rating of the cap modifies this figure as well.
In any case we are talking low ohms here, roughly 20R to 0.04R.
As ESR goes up, so does the working temperature of the capacitor, a dry cap can often be easily found as it will be quite warm or even hot! This in turn speeds up its demise as it helps in drying out electrolyte. Gas is also generated by this process and it is quite common for capacitors to break their rubber seal in which case you will see some goo leakage at its base. At other times the seal will hold and it will simply explode with one hell of a bang and a similar mess! Treat capacitors with respect, especially old ones, they could easily take out an eye if they pop.
I might point out here that all capacitors leak current to some extend, otherwise they would of course keep their charge forever. This leakage is actually crucial to their operation as it maintains the dielectric oxide coating by continually forming it. If you've ever wondered if its ok to replace an electrolytic capacitor with a higher voltage one of the same capacitance then remember that you will be reducing its leakage current by doing so. If you go too high in voltage rating the capacitor will eventually break down due to a loss of dielectric. A figure of 75% of a capacitors rated voltage is considered optimum.
Getting back to testing capacitors... A capacitance meter will not always find a dry capacitor! They might actually measure totally ok and be as dry as a fart. Don't consider this as isolated cases - it happens quite often! I have changed hundreds of capacitors in my time and my weapon of choice when testing caps is an ESR meter. 99% of the time you can measure them in-circuit and a dry cap will show up like dogs balls. Dry caps will have an ESR many times its rated ESR. If you get very low readings on caps you suspect should be reading higher due to their age, chances are that they are leaking. Certainly a capacitor with a reading of 0 or close to would be worth examining further. Pull it out of circuit and test it with the cap meter. 90% of the time you will find faulty caps with the ESR meter, for the rest of the time you will find them with the cap meter.
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For testing other sorts of capacitors, polyester, ceramic etc - use a capacitance meter, the ESR meter doesn't apply to these. A cap meter also comes in handy for checking values of small capacitors when the numbers have rubbed off, or for when you have had too many beers and can't tell a nanofarad from a picofarad - it will convert the values for you as you change the range.
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For under $100 you should be able to get a capacitance meter and an ESR meter. I recommend the Dick Smith "ESR and Low Ohms Meter" (kit).
Forgot to mention... always make sure that the cap is fully discharged before connecting these meters to it!
Unfortunately if our capacitor has passed this test does not gurantee its in good working order. Let me give you some basics on electrolytic capacitor construction;
Electros are similar to batteries in that they work by chemical action. Its made up of two aluminium foils (usually) and separated by an absorbent material similar to paper and soaked by a liquid called an electrolyte.
As part of the manufacturing process, a current is passed through the electrolyte which forms an oxide coating on the positive foil (forming). The oxide coating is a dielectric which is what enables a capacitor to hold charge. Over time, the electrolyte dries up and the equivelant series resistance (ESR) goes up until in the worst case it becomes open circuit.
ESR basically means the capacitors given resistance to an AC current.
Each capacitor has an ESR rating, smaller caps are higher in ESR, bigger ones lower, also the voltage rating of the cap modifies this figure as well.
In any case we are talking low ohms here, roughly 20R to 0.04R.
As ESR goes up, so does the working temperature of the capacitor, a dry cap can often be easily found as it will be quite warm or even hot! This in turn speeds up its demise as it helps in drying out electrolyte. Gas is also generated by this process and it is quite common for capacitors to break their rubber seal in which case you will see some goo leakage at its base. At other times the seal will hold and it will simply explode with one hell of a bang and a similar mess! Treat capacitors with respect, especially old ones, they could easily take out an eye if they pop.
I might point out here that all capacitors leak current to some extend, otherwise they would of course keep their charge forever. This leakage is actually crucial to their operation as it maintains the dielectric oxide coating by continually forming it. If you've ever wondered if its ok to replace an electrolytic capacitor with a higher voltage one of the same capacitance then remember that you will be reducing its leakage current by doing so. If you go too high in voltage rating the capacitor will eventually break down due to a loss of dielectric. A figure of 75% of a capacitors rated voltage is considered optimum.
Getting back to testing capacitors... A capacitance meter will not always find a dry capacitor! They might actually measure totally ok and be as dry as a fart. Don't consider this as isolated cases - it happens quite often! I have changed hundreds of capacitors in my time and my weapon of choice when testing caps is an ESR meter. 99% of the time you can measure them in-circuit and a dry cap will show up like dogs balls. Dry caps will have an ESR many times its rated ESR. If you get very low readings on caps you suspect should be reading higher due to their age, chances are that they are leaking. Certainly a capacitor with a reading of 0 or close to would be worth examining further. Pull it out of circuit and test it with the cap meter. 90% of the time you will find faulty caps with the ESR meter, for the rest of the time you will find them with the cap meter.
//----------------------------------------------------------------------------------------------------------------------------------------------------------------------
For testing other sorts of capacitors, polyester, ceramic etc - use a capacitance meter, the ESR meter doesn't apply to these. A cap meter also comes in handy for checking values of small capacitors when the numbers have rubbed off, or for when you have had too many beers and can't tell a nanofarad from a picofarad - it will convert the values for you as you change the range.
//----------------------------------------------------------------------------------------------------------------------------------------------------------------------
For under $100 you should be able to get a capacitance meter and an ESR meter. I recommend the Dick Smith "ESR and Low Ohms Meter" (kit).
Forgot to mention... always make sure that the cap is fully discharged before connecting these meters to it!
Last edited by Dude on Mon Dec 26, 2005 10:17 am, edited 2 times in total.
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Flames1950
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VelvetGeorge
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Dude
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The Dick Smith meter will measure caps from 1uF to 10,000uF. Although the table on the front of the meter only shows voltage ratings up to 250V, you can always make an educated guess for higher voltage caps or consult the capacitors datasheet and if not available, cross reference datasheets of similar caps which will give you an idea of the ESR value for a new cap of that value.
ESR meters are usually expensive pieces of test equipment as you found out George, but the DSE will do just about anything they will do at a fraction of the cost, which is probably the reason the kit has sold by the thousands.
Here's a couple of sites that sells the DSE ESR meter:
http://www.flippers.com/esrktmtr.html
http://dseau.resultspage.com/search.php ... y=10&I1=Go
ESR meters are usually expensive pieces of test equipment as you found out George, but the DSE will do just about anything they will do at a fraction of the cost, which is probably the reason the kit has sold by the thousands.
Here's a couple of sites that sells the DSE ESR meter:
http://www.flippers.com/esrktmtr.html
http://dseau.resultspage.com/search.php ... y=10&I1=Go
Last edited by Dude on Thu Dec 29, 2005 4:48 am, edited 1 time in total.
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VelvetGeorge
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JerryP
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I use this one Cap Analyzer. It's a great tool. I usually change electrolytic caps when they get 10-15 years old. They are gonna fail, that's a given, why take a chance on smoking a power transformer, choke, or other components you'll never be able to replace (with the same vintage piece)?
Jerry
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