Can't make sense of digital multi-meter readings. - Page 1
Forum section: electronics / testgear

author: SteveDavies
author_rank: Newbie
timestamp: March 30, 2025, 05:04:59 pm
content: Hi, Could someone explain these multi-meter readings please? I'm used to analogue, where you multiply the reading by the maximum value that it's set to. The current was the same in both settings.Thanks, Steve,

author: HKJ
author_rank: Super Contributor
timestamp: March 30, 2025, 05:28:05 pm
content: It might be easier to understand the reading if you use the correct input sockets.10A input socket is only for 10A range

author: bdunham7
author_rank: Super Contributor
timestamp: March 30, 2025, 05:30:45 pm
content: You have the red lead in the wrong jack for the second reading and that is causing it to be 100X too low. There is likely a 0.01R shunt for the 10A range and a 1R shunt for the mA range, thus the 100X difference. The meter is just measuring the voltage across the shunt and then displaying it with the decimal where it thinks it should be. Having the mA range switched and on the same jack as V and R is less common, but sometimes found in older or cheaper meters.

author: SteveDavies
author_rank: Newbie
timestamp: March 30, 2025, 05:52:12 pm
content: So if we ignore the 200m setting, I still don't understand why there is a '20' under the decimal point and is it giving me a reading of 0.08 x 10A = 0.8A? Thanks.

author: mikes
author_rank: Regular Contributor
timestamp: March 30, 2025, 06:05:52 pm
content: Quote from: SteveDavies on March 30, 2025, 05:52:12 pm10 A is a range maximum, not a multiplier. DMMs move the decimal point as appropriate. 200/20 indicate the maximum whole unit reading. i.e. 200.0 or 20.00. Yes, even though the maximum allowed is 10.0. There is no "10" indicator and the meter may in fact read more than 10.00 A before being damaged.

author: Aldo22
author_rank: Super Contributor
timestamp: March 30, 2025, 06:44:20 pm
content: In other words, your first photo shows 0.08A (80mA) DC and in your second photo, the leads are connected incorrectly.The “20” is only the automatic measuring range -> Additional information.You don't have to calculate anything. In the 10A range, the displayed value is in A, in the 200mA range in mA.More modern DMMs also display the unit.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 30, 2025, 06:51:21 pm
content: The 10 A range may for a short time (like a few seconds) still read up to 20 A. There may also be just not indicator for a 10.It is anyway rather rare to have the extra indicator for the range in the display. This is especially the case with a more manual ranges.

author: SteveDavies
author_rank: Newbie
timestamp: March 30, 2025, 07:27:16 pm
content: Thanks for all your help guys

author: Fungus
author_rank: Super Contributor
timestamp: March 31, 2025, 10:58:13 am
content: Quote from: SteveDavies on March 30, 2025, 05:52:12 pmSimple: It doesn't have a "10" in the LCD so they used the "20" instead (it's a 2000 count meter so itdoeshave a 20).(It might measure more than 10A, maybe even 20A for a second or two...)The reading is 0.8A.

author: Aldo22
author_rank: Super Contributor
timestamp: March 31, 2025, 11:10:38 am
content: Quote from: Fungus on March 31, 2025, 10:58:13 amWhy?I think it's 0.08A in the first photo.The second one is useless.

author: Fungus
author_rank: Super Contributor
timestamp: March 31, 2025, 12:09:12 pm
content: Quote from: Aldo22 on March 31, 2025, 11:10:38 amI meant 0.08A.I dunno where the other zero went.On the second pic the lead isn't connected so anything goes. The '8' is pure coincidence.

author: bdunham7
author_rank: Super Contributor
timestamp: March 31, 2025, 02:13:22 pm
content: Quote from: Fungus on March 31, 2025, 12:09:12 pmI don't think it was a coincidence. The lead is not hanging loose, it is connected in the wrong place. That results in a wrong reading, not a random reading.

author: AVGresponding
author_rank: Super Contributor
timestamp: March 31, 2025, 04:49:34 pm
content: Quote from: bdunham7 on March 31, 2025, 02:13:22 pmExactly this. Very easy experiment to prove it; run a 40mA current in the 10A jack so you get a reading of 0.04, then switch the range to mA without moving the lead, and measure again. By amazing coincidence, the reading will be 00.4

author: Aldo22
author_rank: Super Contributor
timestamp: March 31, 2025, 05:20:27 pm
content: Quote from: AVGresponding on March 31, 2025, 04:49:34 pmHmm, you're right!Can you explain?

author: bdunham7
author_rank: Super Contributor
timestamp: March 31, 2025, 05:34:54 pm
content: Quote from: Aldo22 on March 31, 2025, 05:20:27 pmIt's going to depend on the exact design of the meter, but the basic idea is that the shunts are in series and the meter reads the voltage between the selected shunt and the COM jack. If you have the lead connected to the 10A jack instead of the correct mA jack, then your current is only going through 1% (or whatever, it can vary) of the shunt but the meter still reads the voltage because it is connected through the bypassed shunt.

author: Aldo22
author_rank: Super Contributor
timestamp: March 31, 2025, 05:46:15 pm
content: Quote from: bdunham7 on March 31, 2025, 05:34:54 pmYes, that seems to be the case with my meter.The µA range shows 2 µA.Thank you.