mysterious CAP setting on my DMM? - Page 1
Forum section: electronics / beginners

author: dentaku
author_rank: Frequent Contributor
timestamp: December 05, 2013, 10:29:52 pm
content: I have a regular hardware store meter made by Mastercraft (52-0052-2).It has a setting named CAP and I have no clue how to use it.The display shows that it measures in nF and uF but I can't see how a multimeter can do this. It seems to measure nothing

author: PA4TIM
author_rank: Super Contributor
timestamp: December 05, 2013, 10:40:26 pm
content: It is maybe a bit unconventional but have you thought about reading the manualhttp://www.tnt-net.ca/ve6kq/mastercraftdmm.htm

author: Dave
author_rank: Super Contributor
timestamp: December 05, 2013, 10:43:39 pm
content: Quote from: PA4TIM on December 05, 2013, 10:40:26 pmBlasphemy!How have you tested this function on your meter? Have you tried connecting a capacitor to he input of the multimeter?

author: dentaku
author_rank: Frequent Contributor
timestamp: December 05, 2013, 11:15:10 pm
content: Yes I have.It just gives me random values that have nothing to do with the particular capacitor.Quote from: Dave on December 05, 2013, 10:43:39 pm

author: dentaku
author_rank: Frequent Contributor
timestamp: December 05, 2013, 11:18:56 pm
content: I read the manual obviouslyThis is what it saysCapacitance Measurement1. Discharge the capacitor being tested before starting the measurment.2. Set the function/range switch to the CAP position.3. Insert the black (negative) test lead into the "COM" jack and the red (positive) test lead into the V/? /F/ / jack. NOTE: The multimeter will be in auto-ranging only.4. Touch the test lead tips across the devise under test. The red test lead to the positive and the black test lead to the negative.5. Read the value and unit shown on the LCD. If OL is displayed, the value of the capacitor is over the maximum range of the multimeterI guess mine just doesn't work in that setting. I've never seen it do anything useful.Quote from: PA4TIM on December 05, 2013, 10:40:26 pm

author: marshallh
author_rank: Unknown
timestamp: December 05, 2013, 11:24:36 pm
content: Probably won't work above 10uF. Polarity matters for electrolytic/tantalumOr your meter could just be crap.

author: trackman44
author_rank: Regular Contributor
timestamp: December 05, 2013, 11:25:01 pm
content: First of all, what capacitor are you testing? Second, have you tried to measure other capacitors of the same value (or different values) to the point where you'll get a stable reading from your DMM?Will

author: sleemanj
author_rank: Super Contributor
timestamp: December 05, 2013, 11:42:33 pm
content: Quote from: dentaku on December 05, 2013, 11:18:56 pmWhat value cap are you testing it with.The larger the capacitance, the longer the test will take.On my VC99 meter for example, measuring 100uF takes about 7 seconds to produce a result, before which time it will just displays 0 (of course, depends on your meter....) which if you were not expecting that you might think "this thing isn't working", when it is working, it's just taking a while because you're measuring a "large" capacitance.

author: dentaku
author_rank: Frequent Contributor
timestamp: December 06, 2013, 02:20:15 am
content: OK, I just tested out a few more and you're right. It's just quite slow with caps larger than 10uF and it doesn't work at all on 100uF or larger ones.You can see that it's not a feature I've really ever cared about. I was just curious.I just connected it to my oscilloscope and it seems to charge the cap to a certain voltage (looks like 1.5V) then discharge it to 1V, charges again then discharge half way then back up again.I'm assuming it's timing how long it takes to do this and just solves for C because it already knows T and R?This IS the Beginner forum so I'm asking beginner questions here.Quote from: sleemanj on December 05, 2013, 11:42:33 pm

author: c4757p
author_rank: Super Contributor
timestamp: December 06, 2013, 02:23:19 am
content: Quote from: dentaku on December 06, 2013, 02:20:15 amBasically. More likely, knowing Delta-T, Delta-V and I, it solves I = C dv/dt for C. This allows it to use the same current source that it uses to test resistors, so the capacitance function can be implemented entirely in software using existing hardware.

author: dentaku
author_rank: Frequent Contributor
timestamp: December 06, 2013, 09:49:16 pm
content: Nice.At least I wasn't way off on that.Quote from: c4757p on December 06, 2013, 02:23:19 am