Measuring current - Page 1
Forum section: electronics / general

author: danolteanu
author_rank: Newbie
timestamp: December 08, 2022, 03:40:54 am
content: Hi,I have a question regarding the indication on A and mA scale of multimeters.I have a circuit with a 2.5V DC baterry and a 15.1 ohm series resistance.When I measure the current in this circuit on 10A scale the indication is 0.164A and on 400mA scale the indication is 145mA.This indication is the same an 3 multimeters I have (Fluke 87V, UT71E and DT9207A).I thnik the fact is related to the different shunt resistors of multimeters on scale A and mA but do you think it's right for a multimeter to indicate with so much difference on these scales?thank youBR/Daniel

author: jonpaul
author_rank: Super Contributor
timestamp: December 08, 2022, 03:45:09 am
content: current shunt has resistance depend on range, see specks of the meter.Use Ohms law to predict resultsJon

author: danolteanu
author_rank: Newbie
timestamp: December 08, 2022, 03:49:53 am
content: Hi Jonpaul,I've measured the shunts, and according to the Ohm law with the multimeter in circuit the indications are correct.But I was wondering if all multimeters befave the same?BR/Daniel

author: IanB
author_rank: Super Contributor
timestamp: December 08, 2022, 03:53:22 am
content: Mostly they do all behave the same way. This is not unique to the particular meters you have.

author: ejeffrey
author_rank: Super Contributor
timestamp: December 08, 2022, 04:06:27 am
content: You can watch Dave's many videos on current measurement burden voltage and his microcurrent project which uses a very small shunt resistor to minimize this.There are other techniques that can reduce this further, such as using hall sensors or transimpedance amps, but those have disadvantages that make them not suitable for general purpose multi meters, and you can't eliminate resistance entirely.

author: bdunham7
author_rank: Super Contributor
timestamp: December 08, 2022, 04:46:28 am
content: Quote from: danolteanu on December 08, 2022, 03:49:53 amMost of them do. There are a few exceptions that I know of.Some meters only have one current shunt value and either amplify the shunt voltage or simply omit the lower current ranges. Fluke 8842A meters do this, they only have 200mA and 2000mA ranges with one 01R shunt.Some have transimpedance amplifier setups that reduce the burden voltage using an amplifier with feedback. DMMs with this feature include the EEVBlog 121GW, the Fluke 8808 and the current modules in the old Fluke 8500A/8502A/8505A/8506A. This works readily with DC but is probably a bit harder to get to work with AC current. All of the ones I listed have this worked out AFAIK and will work with AC current.Other than that, the vast majority of meters use switched current shunts that will cause the meters resistance to change with the range.

author: danolteanu
author_rank: Newbie
timestamp: December 09, 2022, 05:59:06 pm
content: Hi,Can somebody who have a UT71E multimeter to measure the shunt on mA scale?Because mine indicates 6ohms and it seems to me too much...ThanksBR/Daniel

author: bdunham7
author_rank: Super Contributor
timestamp: December 09, 2022, 06:18:43 pm
content: Quote from: danolteanu on December 09, 2022, 05:59:06 pmI don't have one, but a 5R shunt would not be an unusual value in a DMM and in series with the fuse and internal wiring, 6R seems reasonable. Is the meter reasonably accurate?

author: danolteanu
author_rank: Newbie
timestamp: December 09, 2022, 06:21:09 pm
content: Hi,Yes the multimeter it's pretty accurate, but I compare with Fluke87V which has 1 ohm on mA shunt.BR/Daniel

author: TimFox
author_rank: Super Contributor
timestamp: December 09, 2022, 06:24:25 pm
content: Note that the burden voltage (and resistance) may be higher than that calculated from the current and voltage full-scale ranges on a voltmeter, especially for high-current and low-voltage ranges using a four-terminal resistor, since the burden includes the fuse and the leads from the resistor to the front panel connectors, but the sensed voltage comes from the other two terminals on the shunt resistor.

author: bdunham7
author_rank: Super Contributor
timestamp: December 09, 2022, 06:26:16 pm
content: Quote from: danolteanu on December 09, 2022, 06:21:09 pmYes, there's quite a bit of variance among makes and models. The Fluke 27, for example, is 5R+ on mA and 500R on µA. 1R is better, but 5R is not unusual.

author: DavidKo
author_rank: Frequent Contributor
timestamp: December 15, 2022, 06:49:36 am
content: It is always the compromise between voltage on the shunt and its value. Higher the voltage, easier to measure. That is also the reason why the devices like the one mentioned inMaxim's AN106exists. To measure mOhms you can than use normal multimeter, but you need high current.