| Electronics > Beginners |
| Test capacitors without an ESR meter? |
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| lisafig:
--- Quote ---You can buy a very simple but very efficient one for less than 10$ on ebay --- End quote --- But @JacquesBBB! can you test the cap in-circuit with these? |
| vk6zgo:
--- Quote from: cbmeeks on September 21, 2016, 03:30:59 pm ---Is there a way to get a good estimate on the life of a capacitor without an ESR meter? Especially something a little more accurate than "it's good" or "it's bad". Like something that would estimate a cap being within 10% of specs or 80% of specs, etc. Thanks! --- End quote --- Back in the day, we didn't have ESR meters. An LCR bridge will tell you the capacitance----if that is very low, you don't have to mess around with ESR, just change the thing! In later years, I was lucky enough to work where we had a digital LCR meter. It measured capacitance, inductance & resistance. As well as basic LCR. It could also determine the "Q" of an inductor, & what it called "loss" in a capacitor (basically another name for ESR). I also had an adaptor for a DMM which read capacitance. Over time, I noticed that this device would show a cap with high ESR as being low in capacitance, so that was useful if someone else was using the LCR meter. Fixing stuff up for work, it took more time to remove, test, & resolder electrolytic caps with stubby leads than to just fit new ones, with leads you could cut to length, & make nice new solder joints. Time being money, & electros being cheap, it was my usual practice to change them "scattergun" style. Being able to test unusual ones was very useful,though. |
| MrAl:
Hi, You can test electrolytic cap ESR by energizing it with a square wave through a resistor and monitoring the current through the resistor and the voltage across the cap with a scope. The ESR is detectable during the first few instants of the square wave rise (and fall). That's because ESR is nothing more than a resistance, so you will see the voltage across the cap jump up by a very small amount for a 'good' cap, then it will start to ramp up (or down). During the first instant, if the voltage rises by 'v' volts and the current is 'i' amps, then the ESR is: ESR=v/i and note that the current 'i' does not change much with a good cap because there is a resistor in series with the source and for a small voltage change across the resistor and cap the current is relatively constant. With very bad caps you'll see 'i' change too, but then the ESR becomes less interesting because you know it's a bad cap right away. The other thing you can test is the capacitance using: dv=i*dt/C but this is done AFTER the first few instants after the voltage across the cap starts to ramp up (or down). If you take two readings v1 at time t1 and v2 at time t2 (during the ramp) you then have: dv=i*dt/C v2-v1=i*(t2-t1)/C and so you can estimate C as: C=i*(t2-t1)/(v2-v1) In the past i have measure several caps after having problems with the devices they had been installed in. For example a computer powe supply where the electrolytic cap values and ESR changed markedly causing the computer to shut down for apparently no reason. After the caps where changed, the power supply worked normally again. I found that both the cap ESR rises very significantly and the capacitance went down a lot. Both of these things cause problems with the ripple which is detected as either over or under voltage and so the power supply shuts down. I have some graphic depictions of scope shots too if you want to see them. |
| spec:
There are three checks you need to make on an electrolytic capacitor: * Capacitance value * Leakage current (at full working voltage, if possible) * ESR |
| rhb:
The BSIDE ESR02 Pro comes in a nice very solid case. I just put a 20% 220 uF electrolytic in mine. BSIDE 229.0 uF 0.31 ohms Vloss 1.4% Peak Atlas ESR70 233 uF 0.16 ohm no Vloss measurement. The BSIDE was $20. It also tests resistors, inductors and transistors. For those I bought a Peak Atlas DCA75 and LCR45. So probably 15x as much for the Peak Atlas kit. In all fairness, the DCA75 also provides curve tracing if plugged in to a PC which the BSIDE does not. Also the BSIDE measures the turn on Vbe whereas the DCA75 measures the saturation Vbe and Vce. I'd bought the Peak Atlas stuff because it has a good reputation and at the time all the Chinese testers were bare boards without a case. Both the DCA75 and the BSIDE report the transistor type, leads and hFe. |
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