Rookie RMS DMM question - Page 1
Forum section: electronics / beginners

author: boulderjoe
author_rank: Newbie
timestamp: February 09, 2012, 03:54:08 am
content: Hey All- I have a question about RMS DMMs. I have an Extech 22-816 RMS DMM (sold through radioshack). The readings are all over the place until it gets a strong signal, which makes trouble shooting things a pain!. With the cables attached to nothing it will range from 800 volts to 0, then back, all in random order. When hooked up to a power source it will eventually hit the right voltage, but it takes time.So before I pick a new meter up (for eletronics hobby, arduinos, blinky leds, those types of things if you have a suggestion; budget up to $150), is that random auto ranging normal?Thanks for the help with the rookie question.-Joe

author: IanB
author_rank: Super Contributor
timestamp: February 09, 2012, 04:02:37 am
content: Most of the higher performance multimeters show random display readings on DC volts ranges when the probes are free floating. This is because the high impedance input (usually 10 M or more) allows the meter to measure voltages induced in the leads by stray electric fields. The reading should settle down once the probes are attached to a circuit.To avoid this problem you can buy a very cheap meter. The cheaper meters are less sensitive and cannot measure such small signals.

author: boulderjoe
author_rank: Newbie
timestamp: February 09, 2012, 04:05:46 am
content: Thanks for the answer, that makes sense.If I have the probes attached to a LED or other part, when their is no power going to the part, is exhibits the same behavior of numbers all over the place. Is that normal in RMS DMMs too?Thanks for the info!-Joe

author: vk6zgo
author_rank: Super Contributor
timestamp: February 09, 2012, 04:14:58 am
content: I doubt if there has been a DMM which doesn't have a 10M input impedance,since they were first made.My old Fluke 77 doesn't exhibit this behaviour,or the far more recent Fluke I used in my most recent job.I can understand a DMM reacting to randomly picked up signals if locked to a very sensitive range,but not otherwise.

author: IanB
author_rank: Super Contributor
timestamp: February 09, 2012, 05:06:43 am
content: Quote from: boulderjoe on February 09, 2012, 04:05:46 amNo, it doesn't seem quite normal. The voltages should only be small when the probes are free floating. A fraction of a volt, say. As soon as you attach the probes to an unpowered part like and LED the display should quickly go to zero.You might want to check how it measures a known voltage like a battery. You could also check that both probes are properly seated in the sockets. With some meters the probe plugs need a slightly firm push into the meter sockets to engage properly.

author: IanB
author_rank: Super Contributor
timestamp: February 09, 2012, 05:08:24 am
content: Quote from: vk6zgo on February 09, 2012, 04:14:58 amI think Fluke do some special display damping in their meters. Because some people get concerned about the wandering values they do something to squelch the display reading until a circuit connection is detected.

author: AlphZeta
author_rank: Regular Contributor
timestamp: February 10, 2012, 03:18:24 pm
content: This type of behavior is not that uncommon among auto-ranging DMMs, and in some it's more pronounced than others.But most auto-ranging DMMs have a manual override option so that you can lock the range at a particular resolution to prevent this type of behavoir.

author: boulderjoe
author_rank: Newbie
timestamp: February 12, 2012, 03:27:44 am
content: Quote from: AlphZeta on February 10, 2012, 03:18:24 pmThanks everybody for the info. By locking it in to the lowest range (which is all I deal with) it works good enough. that was the trick.Thanks!-Joe