multimeters and impedance? - Page 1
Forum section: electronics / testgear

author: deth502
author_rank: Frequent Contributor
timestamp: December 21, 2013, 11:11:27 pm
content: now, this is probably a dumb question, but i had always assumed that a high impedance was better as it affected the circuit under test less. but lately ive seen a few "higher end" budget meters ($75-100 range) promoting that they are low impedance, like it was something special to be proud of, something to justofy making you pay an extra $40 over the model below.so, hi-z or lo-z. what are the pro's and con's of each?

author: Monkeh
author_rank: Super Contributor
timestamp: December 21, 2013, 11:16:38 pm
content: A lot of meters now have a second, low impedance range to eliminate coupled voltages in mains wiring.

author: SArepairman
author_rank: Frequent Contributor
timestamp: December 21, 2013, 11:23:15 pm
content: which basically means a high impedance meter might pick up some relatively high voltage, often known as a ghost voltage, in mains wiring.So you might see 70 volts on the meter, but in reality this 70 volts is of extremely low power and irrelevant from a power & safety standpoint.it is useful in power electronics to help determine if the voltage you are measuring has any whoompff behind it.

author: Monkeh
author_rank: Super Contributor
timestamp: December 21, 2013, 11:24:37 pm
content: Usually a good indicator of shitting wiring practice, though.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: December 21, 2013, 11:43:17 pm
content: Quote from: deth502 on December 21, 2013, 11:11:27 pmHi Z is great as you mentioned in most situations. That being said in certain environments the high incidence can be a drawback. For example, in situations like electricians work, where the DUT is long wires which can couple easily to induced noise the high z can result in erroneous readings. This is because it doesn't take much charge to create a higher voltage with a high impedance. So a low z is used which allows the induced charge to dissipate preventing erroneous readings.Sent from my EVO using Tapatalk

author: cs.dk
author_rank: Unknown
timestamp: December 22, 2013, 10:14:11 am
content: How much does the Low-Z meters load the circuitry?A regular multimeter can indeed measure "nothing".I've run into this problem many times, when fault-finding on cars. My multimeter claims ~12,5-12,7V so it should be fine, but the device still don't work. Hook up a test-lamp, a bulb, or any other load to the circuit, and the voltage drops to 0V. Time to look for broken wires, bad connections, etc.Can a Low-Z meter do this sort of measuring, so i don't have to use a test lamp all the time?

author: Rerouter
author_rank: Super Contributor
timestamp: December 22, 2013, 11:11:28 am
content: Depends a little on how low the impedance is, but generally yes, noting that a test light is generally 100-1K ohms, and from a very quick search it looks like some multimeters lowZ modes are around the 3-10K mark, so looks very much like it would accomplish the same task

author: SeanB
author_rank: Super Contributor
timestamp: December 22, 2013, 11:13:27 am
content: Tpyically it is a 47k resistor or something around that mark, or a PTC thermistor that draws about 10mA irrespective of voltage ( within limits, typically they max out at 250VAC) so that there is a load on the lead. Helps to tell if a wire is disconnected or broken in a long wire run where you might get close to the full mains induced into the disconnected lead but where any load drops the voltage close to zero.

author: Wytnucls
author_rank: Super Contributor
timestamp: December 22, 2013, 02:42:12 pm
content: Here is a good Fluke article on the subject:http://www.newark.com/pdfs/techarticles/fluke/DIDM.pdf

author: calexanian
author_rank: Super Contributor
timestamp: December 23, 2013, 03:45:12 am
content: I carry a set of probes in my box with a 300 ohm resistor tied across them near the meter end to handle cross talk and capacitive coupling in 24 volt control systems. Also its handy in checking batteries as they can show false normal voltage. Put any load on them and they drop.

author: PA4TIM
author_rank: Super Contributor
timestamp: December 23, 2013, 09:30:04 am
content: Quote from: Monkeh on December 21, 2013, 11:16:38 pmA lot of metersI have over 30 meters and only one has a low impedance option (1K for VDC) and that is an old analog military meter (AN/USM 223). I know Fluke has some meters for Industrial/mains use have that option .My favourit meters have a > 1G impedance in the low VDC ranges, VAC is 10M there

author: Jon86
author_rank: Frequent Contributor
timestamp: December 23, 2013, 09:33:27 am
content: Why not just add a resistor in front of where you're probing? Or inside the multimeter for that matter.

author: Rerouter
author_rank: Super Contributor
timestamp: December 23, 2013, 10:19:04 am
content: because you then form a voltage divideryou can however get for instance a 9M resistor and put that in front of it noting that the reading is divided by 10, my older analog multimeter does the same with a 999M high voltage probe to divide by 1000,

author: calexanian
author_rank: Super Contributor
timestamp: December 23, 2013, 03:55:15 pm
content: Quote from: Rerouter on December 23, 2013, 10:19:04 amThe purpose of this is to measure voltage in systems where you may have a floating or otherwise coupled voltage. Having a shunt resistance is critical for those applications. Its more of an electrician thing. Also we encounter it in longer runs of multi conductor control lines. It is intended for mains voltage and below. AC or DC. Here in the states where we like our grounding In a 100 foot run of direct burial multi conductor with 120 VAC on it you can float the adjoining conductors up to a common up to over 100 volts through capacitive coupling at the multi meg ohm range of the volt input of a modern multimeter. Plenty enough to be confusing to a young player. By adding a small shunt resistance you can bring that under control.Just my experience, but heck, my brain squeaks when I walk so what do I know?