BM235 Capacitance Reading Oddity - Page 1
Forum section: electronics / testgear

author: rediv
author_rank: Newbie
timestamp: September 15, 2020, 03:24:04 pm
content: When measuring a 100nF cap, the display alternates between "131.0nF" and then switches to "0383nF" (note that there's no decimal in the second value but does have leading zero).I replaced the batteries and the behavior persists. Any ideas what this second value is? Thanks.

author: bob91343
author_rank: Super Contributor
timestamp: September 15, 2020, 08:31:02 pm
content: What method of measurement does this unit use?

author: wraper
author_rank: Unknown
timestamp: September 15, 2020, 08:34:58 pm
content: Probably this cap is faulty or you are measuring it within circuit. And what multimeter does is switches ranges automatically back and forth due to this. In neither range you get an accurate reading due to parasitic effects. In some cases you may need to set range manually to get proper reading.

author: indman
author_rank: Super Contributor
timestamp: September 16, 2020, 06:18:32 am
content: Quote from: rediv on September 15, 2020, 03:24:04 pmFind a few more similar capacitors and measure them. What will be the result?This DMM, like most others, measures capacitance based on its charge / discharge, so a faulty leaky capacitor will have such erratic results.

author: rediv
author_rank: Newbie
timestamp: October 07, 2020, 09:45:06 pm
content: Thanks to all.I measured 3 identical SMD caps on a partially populated board where there was no circuit around the caps. I did try manually setting the capacitance range. So I'm pretty confident that the measurement technique was OK. What confuses me is that the display was cycling between a capacitance value and some digits with no units.I did have a bench DMM handy at the time and the caps measured (on the board as above) within spec.Other ideas?

author: wraper
author_rank: Unknown
timestamp: October 07, 2020, 10:06:02 pm
content: Quote from: rediv on October 07, 2020, 09:45:06 pmUsing conductive flux for soldering.

author: bob91343
author_rank: Super Contributor
timestamp: October 08, 2020, 07:08:08 am
content: You can't measure capacitance. What you can measure is the effect the capacitance has and then compute what capacitance would have that effect.It's not like measuring a piece of wood, which has a definite dimension. All capacitance measuring devices rely on electrical performance, not on physical attributes.So if you are getting strange results, the measurement technique is flawed, or there is something going on that you don't understand.The best way to measure capacitance is to remove it from the circuit and put it into a bridge which will balance it at one frequency. For corroboration, measure at some other frequency.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 08, 2020, 10:50:47 am
content: most meter(s) use a time constant charge to get and display a value, testing in circuit with a meter like this wont help, normally you use an lcr meter to do that, some of them can test in circuit and have a serial and parallel mode to helpI have a DE-5000, it does the job 90-95% of the time, but in special rf circuits, it can't do its job properly, once the part is outside the pcb, its fine

author: bob91343
author_rank: Super Contributor
timestamp: October 08, 2020, 08:33:26 pm
content: I have several instruments for measuring capacitance. Each has its place. First I have the cute cheapie Mega328 gadget that measures everything. Not too good for capacitors a lot of the time.I have two General Radio bridges, an old 1650A (at 1 kHz) and a 1658 (1 kHz and 120 Hz). I have a nanoVNA that is good from 50 kHz up. I have a Boonton RX meter with limited capacitance range (HF and VJF). I have two Boonton Q meters that cover a wide frequency range. I have been known to devise my own method for oddball parts such as very high capacitance or very poor quality capacitors.There is no one device that can replace all of these units. It depends on the application and you don't just say 'let me measure this capacitor' unless you know something about the part and how it will be used.The handy Chinese digital units are fine for common parts but you are doing yourself a disservice if you don't understand how they work. Understanding is vital to knowing how to interpret the readings. Measuring charge or discharge time confuses the unit if the part is leaky. If measurement frequency is high, one needs to account for ESL. You can't get a useful reading if you are measuring anywhere near self resonance.

author: Wytnucls
author_rank: Super Contributor
timestamp: October 09, 2020, 06:53:55 am
content: If you're sure that the capacitors are not in circuit and that your connections are not corrupted, there may be something wrong with your meter.I would measure a loose electrolytic cap out of circuit to confirm it.Most meters make use of the formula I=C dV/dt.A constant current (uA) flows through the cap while the voltage rate of change is measured, then the cap is partly discharged to repeat the process.If you have an oscilloscope, the waveform can be observed for integrity.The discharge rate can be less than the charging rate, which could explain the weird temporary higher capacitance shown by your meter, if it is faulty.