test equipment measuring it's own guts? - Page 1
Forum section: electronics / testgear

author: paulbt
author_rank: Regular Contributor
timestamp: December 21, 2020, 08:08:45 am
content: Hi everyone!Recently I remembered about a doctor during the USSR era named Leonid Rogozov who performed surgery on himself during an expedition in Antarctica because he had appendix problems and he was the only trained medical guy there. I find this very astonishing, so I was thinking if this is applicable to our test equipment we used every day.I know that in order to troubleshoot a broken scope, you need another good scope (I consider this fact almost a paradox), but I am mostly sure I saw/heard sometime ago that you can troubleshoot a broken channel with the other good one of a dual channel scope.What about multimeters? Can it measure it's own battery? I tried this with a Fluke 115 at my workplace and it was strange and interesting at the same time. I was hoping to see some ~9V reading, but it didn't happen. No harm/damage was inflicted, the Fluke is still working fine. I never studied a DMM's full schematic, but this little experiment gave me a reason to do it.So please try this with one or more DMMs and tell us what you saw. A picture or a small video would also be nice.

author: BravoV
author_rank: Super Contributor
timestamp: December 21, 2020, 08:10:52 am
content: There is a chance, that one mistake, like reversed polarity of the probes, will kill it instantly.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: December 21, 2020, 01:55:43 pm
content: the vast majority of handheld multimeters have the COM (negative) input terminal sitting somewhere between the +ve and -ve terminal of the battery. for instance, in a fluke 77 the COM terminal is approximately 3 volts below battery +ve.so to measure the battery voltage, connect JUST the positive probe to the meter VOLTS terminal, and probe first the +ve end of the battery, then the -ve end.on the fluke 77 i have here in front of me, the results are:battery +ve terminal measures 3.12vbattery -ve terminal measures -5.81vthis means the battery voltage is 3.12+5.81 = 8.93 volts.DO NOT EVER connect the COM terminal to anything internal to the multimeter, as some meters may be damaged by this. also remember, that if the battery is low, then the readings you get may be in error (artificially high). similarly, if the meter has any sort of fault, the readings may be incorrect.cheers,rob :-)

author: alm
author_rank: Super Contributor
timestamp: December 21, 2020, 02:03:59 pm
content: Isn't any kind of built-in self test for analog circuits an automated example of this? Like a power supply might set a channel to its max voltage, and then use the built-in ADC to measure if it's within expected values.If not designed for it, the only way I see is with multi-channel instruments (scope, frequency counter), or instruments that are effectively multiple separate instruments in one, like a bench power supply with panel meters. You could in theory use those panel meters to measure inside the instrument. But of course you have to be mindful of the lack of galvanic isolation in most cases. And you'd have to be pretty limited om resources to do something like this instead of grabbing a DMM.