Different multimeters give different Diode values. Why? - Page 1
Forum section: electronics / testgear

author: iaso
author_rank: Contributor
timestamp: April 03, 2018, 07:13:25 pm
content: So I'm a bit of a newbie and my question might be the most obvious thing ever to the resident greybeards here, but I'm a bit confused.I have 2 multimeters. A Brymen 867 and a Fluke 177.When I test a 1N4148 diode the Brymen gives a value of a 0.569 and the Fluke (using the same Fluke leads) gives a value of 0.625.If this normal? Which multimeter should I trust. Both are brand new (I bough the Brymen and was then gifted the Fluke the same week, great timing) and I am apparently clueless.

author: georges80
author_rank: Frequent Contributor
timestamp: April 03, 2018, 07:19:58 pm
content: The forward voltage of a diode is current dependent (among other things). The meters will apply a current to measure the voltage, each applies a different current...cheers,george.

author: lem_ix
author_rank: Regular Contributor
timestamp: April 03, 2018, 07:24:17 pm
content: There are even nice graphs in the datasheet so you can see what to expect for your meters. Enjoy your

author: iaso
author_rank: Contributor
timestamp: April 03, 2018, 07:31:58 pm
content: Thanks guys that makes sense. I was happily playing around with my Brymen until I see these different values with the fancy Fluke and I was getting worried.

author: Fungus
author_rank: Super Contributor
timestamp: April 03, 2018, 09:10:12 pm
content: Homework exercise: Put another meter in series when you measure the diode, see the current being applied by each of your meters.Look it up in the datasheet, see if it matches the observed voltage drop.

author: cvanc
author_rank: Frequent Contributor
timestamp: April 03, 2018, 09:23:07 pm
content: Quote from: Fungus on April 03, 2018, 09:10:12 pm^^^This! Exactly this. Great learning suggestion! Let us know what your readings are, OK?

author: hugo
author_rank: Regular Contributor
timestamp: April 03, 2018, 10:36:18 pm
content: Quote from: georges80 on April 03, 2018, 07:19:58 pmThe diode testing current for a Fluke 179 multimeter is 1 mA, for instance.

author: iaso
author_rank: Contributor
timestamp: April 04, 2018, 03:29:31 pm
content: If I did my homework correctly the Fluke 177 applies 1mA and the Brymen 867s applies 0.3mA. The data sheet gives a typical of 0.4mA. So I guess that is in limits?

author: Vtile
author_rank: Super Contributor
timestamp: April 04, 2018, 03:41:02 pm
content: I think the homework were after something like image below.

author: Fungus
author_rank: Super Contributor
timestamp: April 04, 2018, 03:42:58 pm
content: Quote from: iaso on April 04, 2018, 03:29:31 pmThat makes no sense. Diodes don't have typical currents.Vtile has the right idea...

author: iaso
author_rank: Contributor
timestamp: April 04, 2018, 05:38:12 pm
content: I think I understand. I went ahead and got another diode. A Diodes Inc 1N4148W-7-F SMD diode I and soldered it to a test board. Values I get from testing are 0.607V @ 0.1mA (Fluke) and 0.551V at 0.4mA (Brymen).Attached a plot. Makes sense..I think

author: PA4TIM
author_rank: Super Contributor
timestamp: April 04, 2018, 07:39:53 pm
content: A diode starts conducting with a few fA. The current through the "resistance" of the diode causes a voltdrop. The higher the current, the higher the voltdrop. But this "resistance" is not a constant like a normal resistance. It follows an exponential curve. So if the meter sources 0,4 mA the voltdrop will be 0,551 for your diode, at 1 mA it is 0,6V. With 10 mA it wil be higher, lets say 0.7V and at 1A it can be over 1 V.At first there is a big increase in voltage with a little increase in current, but the rate that voltage increases, decreases the more the current increases. So lets say it is 1V at 1A, then it can be 1.1V at 2A and 1.15 at 5A and 1.17 at 10A (just made up numbers to show what happens, In real life a 1n4148 reacts different, that will be some smoke at 1A, a tiny bit more smoke at 2 A and no smoke at 10A (because there is nothing left to smoke)