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 28th February 2003, 06:43 PM #1 Evaas   diyAudio Member     Join Date: Apr 2001 Location: Montreal QC capacitance 101 I got my grimy hands on a bunch of surplus caps and I want to measure them to make sure they work as advertised. I don't have an LCR meter and I can't buy one. So I figured I could measure them by discharging across a known resistor and counting the time to discharge. My problem is that I've had my head in the digital world for so many years that I don't know if I'm doing this the right way . Another thing that adds doubt is that nobody else seems to have done this, while there's been lots of talk about lcr meters. here's how I did it: RC equals one time constant, which is roughly equivalent to 63% of discharge. I have a 12.5 v supply so if I charge to 12.5v then 4.6v would represent one time constant. So I charged it up to 12.5, then discharge using a 150 ohm 5% resistor, and count the seconds until it gets to 4.6v as measured on my multimeter. This gave some strange results as seen below in discharge time and calculated capacitance: Cap 1: 12s = .08 F Cap 2: 10.5s = .07 F Cap 3: 11s = .073 F Cap 4: 10.5s = .07 F Sounds good except that the spec for these caps is 60,000 uF. How am measuring above spec? My multimeter could be off, but this is a 17% error. I did all of the readings at least twice, and found similar readings for other sets of caps. what gives? thanks, Evan
 28th February 2003, 07:38 PM #3 SY   On Hiatus     Join Date: Oct 2002 Location: Chicagoland OK, well I'm old. Two things: first, e-lytics do tend to measure high. Second, the capacitance is a bit higher before they are formed. You didn't say, but I'll bet they haven't had anything like their rated voltage across them for quite some time. And when e-lytic caps are used at less than WVDC for a time, the value does tend to drift upwards. In any case, if you're going to use these for a PS application, some extra uF aren't going to hurt you. __________________ "You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
 28th February 2003, 08:02 PM #4 Evaas   diyAudio Member     Join Date: Apr 2001 Location: Montreal QC traderbam - good analysis, and you brought up one point that I hadn't considered, which is delay of the meter. In fact I'm using a cheap analogue meter and I have no idea how accurate it is. I think analogue may be better for this application because its easy to watch the needle and anticipate when its going to hit the right spot. My resolution is better than 1 second whcih I was able to show because when I repeasted the test for each I didn't look for the unit number on the cap until after I took the measurment (an aside is that I found all of the big caps have numbers printed on the top which makes it really convenient to do things like this. its a nice touch). so it was a blind test now to your point - I don't know how much delay there is in the needle speed, although it doesn't have to move very fast with this resistor. I could increase resistance and see if it changes anything, and that would take meter delay out of the equation. I measured the resistor and got spot on 150 as best as I could tell, but it fell on a part of the meter where its hard to interpolate exactly, and of course I'm sure the meter aint perfect. 20% error rating is nice to know too, and as long as my procedure is within reason, then I'm happy. Sy - you are correct in that they've probably been sitting in the shop for a long time without a charge. I'm going to see if I can pick up a cheap powersupply today and charge em up better. Its very interesting to hear that C creeps up if they haven't been charged for a while. Thats the kind of thing I was hoping for in this thread and this group never ceases to amaze One more thing is that they are not from the same lot - they're the same manufacturor and same ratings, but slightly different model number (2 of each) and one set is heavier than the other. thankfully cap #1 & #3 belong to one set while #2 & #4 belong to the other (they have the most similar measured C) The fact that I got em for a buck fifty a piece makes the difference in model number not so significant I also measured some others and this is what i got: - at least one pair came in under spec but I don't know if I should be pleased or not rated 14,000 uF: 1: measured 16,000 uF 2: measured 14,000 uF rated 47,000 uF: 1: measured 44,000 uF 2: measured 42,000 uF I'm happy because it seams at least that I haven't lost too much basic knowledge. just don't ask me to do anything more complicated unless its all 1's and 0's I'd buy a better meter but I'm applying to grad school so why should I buy a meter when I can use one in the lab at school for the next seven years or so...

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