Specific question for DMM measuring current, introducing voltage drop - Page 1
Forum section: electronics / beginners

author: StevenMarshall
author_rank: Newbie
timestamp: March 20, 2025, 11:32:10 pm
content: I may be overthinking this, but I wanted to ask just to be sure. While determining the correct resistor value to use in series with the LED backlight of an LCD graphics display, I found in the display specs that the typical operating current is 80mA, with a maximum limit of 100mA.To measure the current, I connected a Digital Multimeter (DMM) in series and, after a few attempts, found that a 52-ohm resistor provided a reading of 79mA, which seemed in the ballpark. So far so good!But then I remembered that every DMM introduces some series resistance. And sure enought, when I removed the DMM and connected the LED backlight directly, I noticed a significant increase in brightness. This made me worried - in a sence I worry the current is no longer 79mA because I removed (serial resistor) DMM.So how on earth do I measure the correct value of the current?Should I measure the resistance of the DMM and simply solve an equation for this circuit to determine correct value of the current?

author: bdunham7
author_rank: Super Contributor
timestamp: March 20, 2025, 11:42:48 pm
content: Quote from: StevenMarshall on March 20, 2025, 11:32:10 pmThe LED current will be quite non-linear as you change resistors, since LEDs are not resistors. Note that your DMM is actually telling you the correct current, the error results because the presence of the DMM in the circuit actually changes the current. You could approach this a few different ways.One way would be to measure the resistance of your DMM current range, select resistors as you did until you get the desired current and then substitute in a resistor with a value equal to the sum of your test resistor and your DMM resistance. You can then verify the result by measuring the voltage across that resistor and dividing by its resistance to get the current as-installed.Another would be to not use the current range of the DMM at all and just try resistors, starting with higher values and working down, and determining the current by measuring and dividing in the same way as I explained above.If you want to get rough preliminary estimates of current without extra calculations you can try switching to a higher current range. The 10A range of most DMMs will have a burden resistance of much less than 1 ohm, but depending on how accurate it is and how many digits it has, you can probably measure to the nearest 1-10mA.

author: David Hess
author_rank: Super Contributor
timestamp: March 21, 2025, 01:08:35 am
content: Quote from: StevenMarshall on March 20, 2025, 11:32:10 pmAfter you install the resistor, measure the voltage across it, and then calculate the current from that.

author: Kasper
author_rank: Frequent Contributor
timestamp: March 21, 2025, 02:19:18 am
content: Quote from: David Hess on March 21, 2025, 01:08:35 amBeat me to it.This is what I'd do.

author: TimFox
author_rank: Super Contributor
timestamp: March 21, 2025, 02:56:25 am
content: It's called the "burden voltage" of the current meter, and should be specified in the datasheet.

author: Peabody
author_rank: Super Contributor
timestamp: March 21, 2025, 03:04:06 am
content: Quote from: David Hess on March 21, 2025, 01:08:35 am+1. Or you can insert a very low value resistor in the backlight line, and measure the voltage across it. One ohm is a mathematically convenient value.

author: StevenMarshall
author_rank: Newbie
timestamp: March 21, 2025, 08:18:34 am
content: Some great ideas posted above! Thank you!The truth is, I've learned all that in highschool but had other priorities back then... like fooling around with the classmates.