Fluke 87 low mA reading - Page 1
Forum section: electronics / repair

author: catcow1234
author_rank: Newbie
timestamp: May 19, 2016, 07:00:18 pm
content: Hi,I have a Fluke 87 (original model) which has served me well. It's been well looked after and is only used on the bench.Last week I noticed that the mA reading was incorrect. For example, a 1.5v battery and 10R resistor gives the reading of only 120mA but if I put the meter in the 10A range than the reading is 150mA so the fault only seems to effect the mA setting.I've double checked with a Fluke 79 III and that gives the correct current reading (150) in the mA setting so I know it's the 87 at fault.I have checked the fuses which are fine, and cleaned the upper (metal) and lower (carbon) parts of the switch & changed the battery with no luck.I do have the service manual but I can't see any way of calibrating the current range.Can anybody offer any help / ideas?Thanks

author: Cloud
author_rank: Regular Contributor
timestamp: May 19, 2016, 07:36:03 pm
content: Maybe it is the resistance of the shunt

author: catcow1234
author_rank: Newbie
timestamp: May 19, 2016, 07:42:48 pm
content: Wouldn't that effect the 10A reading too?

author: retrolefty
author_rank: Super Contributor
timestamp: May 19, 2016, 07:56:02 pm
content: Burden voltage value of the MA range?

author: MK14
author_rank: Super Contributor
timestamp: May 19, 2016, 07:56:39 pm
content: Quote from: catcow1234 on May 19, 2016, 07:42:48 pmI think there is a good chance your meter is working just fine.On the milliamps setting, it has a resistance (shunt) of about 2 ohms. So 1.5V and 10+2=12 Ohms, can be about 120 milliamps.The 10 Amp range has a very low resistance (shunt), so will give a current much closer to the 150mA.Other meters can have different resistances (shunts), so can't necessarily be used as a comparison.Really you should be using a constant (and ideally calibrated or known/accurate) current source to check it properly. This "rough" way with the battery is just confusing. The battery has an internal resistance as well, which varies.

author: JFJ
author_rank: Regular Contributor
timestamp: May 19, 2016, 08:15:11 pm
content: Quote from: catcow1234 on May 19, 2016, 07:00:18 pmOn the 400 mA range, the burden voltage of a Fluke 87 is, typically, 1.6 mV/mA. On the 10A range, it's only 0.03 mV/mA. That equates to shunt resistances of 1.6R and 0.03R, for those respective ranges. Adding 1.6R to your 10R resistor would reduce the current to about 129mA, but the effect of adding 0.03R would be far less significant.Your Fluke 79 III must have a much smaller burden voltage.

author: MK14
author_rank: Super Contributor
timestamp: May 19, 2016, 08:24:29 pm
content: Quote from: JFJ on May 19, 2016, 08:15:11 pmThis link seems to say it is 1.8mV/mAhttps://physics.ucsd.edu/neurophysics/Manuals/Fluke/87______umeng0800Then remembering to add the test probes resistance as well, it is probably about 2 ohms plus battery internal resistance.

author: catcow1234
author_rank: Newbie
timestamp: May 19, 2016, 08:25:56 pm
content: Thanks for the replies everyone.It looks like the burden voltage could be the issue. I think I will try to get hold of Dave's uCurrent.Thanks again

author: JFJ
author_rank: Regular Contributor
timestamp: May 19, 2016, 09:51:52 pm
content: Quote from: MK14 on May 19, 2016, 08:24:29 pmI found the 1.6 mA/mV in a Fluke 87 service manual on my laptop (perhaps it's for a different revision).

author: ModemHead
author_rank: Frequent Contributor
timestamp: May 19, 2016, 10:23:40 pm
content: If you want to compare current ranges on two meters, just put them in series. The current through both will be exactly the same.

author: MK14
author_rank: Super Contributor
timestamp: May 19, 2016, 10:26:39 pm
content: Quote from: JFJ on May 19, 2016, 09:51:52 pmI agree, it could be different revisions or something.The following link, seems to be a fluke document, nicely discussing burden voltage.http://www.fluke.com/fluke/uses/comunidad/fluke-news-plus/articlecategories/electrical/burdenvoltage

author: retiredcaps
author_rank: Super Contributor
timestamp: May 19, 2016, 10:38:22 pm
content: W2AEW's channel talks about burden voltage with the Fluke 87 and 79 III ...

author: retiredcaps
author_rank: Super Contributor
timestamp: May 19, 2016, 10:45:29 pm
content: Quote from: catcow1234 on May 19, 2016, 07:00:18 pmThe Fluke 79 III has 40mA and A input jacks. If you are measuring more than 40mA, you should be using the A jack.

author: retiredcaps
author_rank: Super Contributor
timestamp: May 19, 2016, 10:55:43 pm
content: Martin's video on burden voltage ...

author: VK5RC
author_rank: Unknown
timestamp: May 19, 2016, 11:28:37 pm
content: You could make a simple constant current source with a LM317 and resistor, that should remove the burden voltage effect.

author: MK14
author_rank: Super Contributor
timestamp: May 20, 2016, 12:15:37 am
content: Quote from: VK5RC on May 19, 2016, 11:28:37 pmhttp://www.bristolwatch.com/ccs/LM317.htm

author: catcow1234
author_rank: Newbie
timestamp: May 20, 2016, 07:06:54 am
content: @retiredcaps, Great Youtube link & you are correct, I was using the A input on the 79 III so the shunt would have been a different value to the 87 on mA so not a good comparison.Thanks again everyone for all the help and advice. I'll make the LM317 circuit later and give it a test.

author: catcow1234
author_rank: Newbie
timestamp: May 22, 2016, 12:56:09 pm
content: I made the constant current circuit using the LM317 (thanks MK14) and the meter reads fine so it looks like I got caught out by burden voltage after allI compared it to a cheap Tenma meter and allowing for tolerances they both read fine (see attached pic).Thanks again everbody for your help on this.