# Calculation current trough resistors **URL:** \ **Category:** LEDs and Multiplexing\ **Created:** [November 24, 2024, 1:18pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299 "2024-11-24T13:18:24Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![danyCro](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/danycro/32/1126465_2.png) [@danyCro](https://forum.arduino.cc/u/danyCro)\ **Post date:** [November 24, 2024, 1:18pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/1 "2024-11-24T13:18:24Z")
Hi, can someone explain to me why my calculate are not the same in practice? I bought a bunch of 1/4 resistors and found LED from USB Card Reader. It is red color in transparent glass. Let say that LED is Vl=1.60 & I= 0.02 mA. I tested on two different batteries and two multimeters. My Goal would be **20 mA** trough resistors. Example 1) Vs=9.10v Vl=1.60 I= 0.02 A R=372 (I used R400) LED shine too bright, and it says that the current is **over 200 mA**. How is that possible? Do I mistake somewhere? Example 2 Vs=2.70v (2x AAA) Vl=1.60 I= 0.02 A R=55 (used R400,again) LED shine to bright, and its says that the current is **116 mA**. That I use R55, will be worse. Example 3 Vs=2.70v Vl=1.60 I= 0.02 A R=55 in calculation. Finally, this time I found that 2K resistor gives me a **goal** of **19.5 mA**!! At same time LED voltage shows 1.62v, which corespond to mA. For 9v battery it will be worse. So what happened between **R55** and **R2000**?? No too many videos who calculate resistors and then test current in practice. ![20241124_132929](https://europe1.discourse-cdn.com/arduino/original/4X/5/e/1/5e1883883e90a0670b5302d9eea5f5b560d622ce.jpeg) ![20241124_132915](https://europe1.discourse-cdn.com/arduino/original/4X/7/4/7/7475c248c3fbf34c9afce7b8e3a774a14e3aef9e.jpeg) ![20241124_132855](https://europe1.discourse-cdn.com/arduino/original/4X/e/9/3/e932f676399a2d0b09b0826ec86c0ea9e13c2cb1.jpeg) ---
**Author:** ![jim-p](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/jim-p/32/922840_2.png) [@jim-p](https://forum.arduino.cc/u/jim-p)\ **Post date:** [November 24, 2024, 1:32pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/2 "2024-11-24T13:32:35Z")
> [@danyCro](#): > > (I used R400 400 ohm thru hole resistors are quite unusual. However a R400 SMD = 0.4 ohms ---
**Author:** ![sterretje](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/sterretje/32/391294_2.png) [@sterretje](https://forum.arduino.cc/u/sterretje)\ **Post date:** [November 24, 2024, 1:49pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/3 "2024-11-24T13:49:56Z")
Meter connected wrong? To measure current, you need to connect red wire form the LED to the A of the meter and set you meter to one of the current ranges. ---
**Author:** ![danyCro](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/danycro/32/1126465_2.png) [@danyCro](https://forum.arduino.cc/u/danyCro)\ **Post date:** [November 24, 2024, 2:28pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/4 "2024-11-24T14:28:30Z")
![20241124_152250](https://europe1.discourse-cdn.com/arduino/original/4X/4/7/7/4773434e991b9af29c5be455bca98d49ea520bdf.jpeg) ![20241124_152449](https://europe1.discourse-cdn.com/arduino/original/4X/c/0/0/c001c7707c6d5ad810af165e1b20bd5942187306.jpeg) Resistors are like it must be,I think 🙄 400 ohm (2x200 in my case). ---
**Author:** ![Dave\_Lowther](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/dave_lowther/32/1092885_2.png) [@Dave\_Lowther](https://forum.arduino.cc/u/Dave_Lowther)\ **Post date:** [November 24, 2024, 2:32pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/5 "2024-11-24T14:32:11Z")
> [@danyCro](#): > > Let say that LED is Vl=1.60 & I= 0.02 mA. In all the places where you say 0.02mA, I think you mean 0.02A or 20mA, not 0.02mA. ---
**Author:** ![jim-p](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/jim-p/32/922840_2.png) [@jim-p](https://forum.arduino.cc/u/jim-p)\ **Post date:** [November 24, 2024, 2:41pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/6 "2024-11-24T14:41:59Z")
See @sterretje comment about using the meter correctly ---
**Author:** ![danyCro](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/danycro/32/1126465_2.png) [@danyCro](https://forum.arduino.cc/u/danyCro)\ **Post date:** [November 24, 2024, 2:49pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/7 "2024-11-24T14:49:28Z")
Yes. I using it right. See position at 5A and 200mA, om 9v battery ![20241124_154457](https://europe1.discourse-cdn.com/arduino/original/4X/9/2/d/92d307cb4c578889b85dde808072942878f58930.jpeg) ![20241124_154649](https://europe1.discourse-cdn.com/arduino/original/4X/b/6/a/b6a17ab847a5408232edcf922031e9e29c9f7f18.jpeg) And 3v bellow pictures. ![20241124_153857](https://europe1.discourse-cdn.com/arduino/original/4X/9/3/a/93ae020990264e8dcdfd123a0bf897b8e58aa070.jpeg) ![20241124_153626](https://europe1.discourse-cdn.com/arduino/original/4X/8/0/1/801ea1661d7a3175cdf2e8cf2c7cbc6230055f52.jpeg) ---
**Author:** ![danyCro](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/danycro/32/1126465_2.png) [@danyCro](https://forum.arduino.cc/u/danyCro)\ **Post date:** [November 24, 2024, 2:54pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/8 "2024-11-24T14:54:51Z")
> [@Dave\_Lowther](#): > > In all the places where you say 0.02mA, I think you mean 0.02A or 20mA, not 0.02mA Yes, sorry, you are right. Like normal LED, 20 mA. If calculate from behind. If I got with 2000 ohms 19 amps, that means that source voltage must be over 30 volts. 🙂 What is impossible. ---
**Author:** ![david\_2018](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/david_2018/32/1013044_2.png) [@david\_2018](https://forum.arduino.cc/u/david_2018)\ **Post date:** [November 24, 2024, 3:01pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/9 "2024-11-24T15:01:38Z")
Can you show a view of the entire breadboard? The top two pictures don't make sense. 0.01 on the 5A scale would be 10mA (or somewhere from 0.006A to 0.014A to allow for rounding). That should not be off scale on the 200mA scale. The internal meter connections for the 5A scale is a heavy ware between the 5A and GND terminals, with the meter measuring the voltage drop across that wire, so it seems unlikely that would add enough resistance to lower the current. ---
**Author:** ![danyCro](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/danycro/32/1126465_2.png) [@danyCro](https://forum.arduino.cc/u/danyCro)\ **Post date:** [November 24, 2024, 3:24pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/10 "2024-11-24T15:24:57Z")
![20241124_161024](https://europe1.discourse-cdn.com/arduino/original/4X/b/2/a/b2a317563337b085cff6a176626462e888b84b64.jpeg) ![20241124_161037](https://europe1.discourse-cdn.com/arduino/original/4X/0/f/6/0f66518fb215fac0a5f1b6fca50cfdcbf5636ee9.jpeg) ![20241124_161113](https://europe1.discourse-cdn.com/arduino/original/4X/2/0/f/20fe9c79f00e1ca1dbdd7a8b9e33235d1bad9a86.jpeg) On red multimeter, inside board, where 5A jack is, was disconnected. Don't know why. I solder them to make a bridge (because I thought that not show A messaure good). But I can try test with black multimeter which have 10A. ---
**Author:** ![PaulRB](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/paulrb/32/216330_2.png) [@PaulRB](https://forum.arduino.cc/u/PaulRB)\ **Post date:** [November 24, 2024, 3:27pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/11 "2024-11-24T15:27:57Z")
> [@david\_2018](#): > > Can you show a view of the entire breadboard? @danyCro I see you are still carefully keeping half the breadboard out of the picture. Have you replaced the batteries inside the multimeters? 80~130mA would burn out a small led in seconds. ---
**Author:** ![jim-p](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/jim-p/32/922840_2.png) [@jim-p](https://forum.arduino.cc/u/jim-p)\ **Post date:** [November 24, 2024, 3:36pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/12 "2024-11-24T15:36:41Z")
I think there are shorts in your breadboard. Can't think of any other logical explanation ---
**Author:** ![anon85221860](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@anon85221860](https://forum.arduino.cc/u/anon85221860)\ **Post date:** [November 24, 2024, 3:51pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/13 "2024-11-24T15:51:57Z")
Some ammeters aren't so good. V1 + V2 + V3 = V\_S V1 / R1 = V2 / R2 = series current, circuit current ![image](https://europe1.discourse-cdn.com/arduino/original/4X/7/f/d/7fd95c62bad2dcfd3a31e18172f1685ad5096c41.jpeg) ---
**Author:** ![EmilyJane](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/emilyjane/32/821537_2.png) [@EmilyJane](https://forum.arduino.cc/u/EmilyJane)\ **Post date:** [November 24, 2024, 3:55pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/14 "2024-11-24T15:55:53Z")
> [@PaulRB](#): > > carefully keeping half the breadboard out of the picture. Yes, @danyCro post a photo showing the entire breadboard. ---
**Author:** ![danyCro](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/danycro/32/1126465_2.png) [@danyCro](https://forum.arduino.cc/u/danyCro)\ **Post date:** [November 24, 2024, 4:05pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/15 "2024-11-24T16:05:57Z")
It is over 200 mA on 9 v battery 🙂 But only for take pictures. Other half? Hmmm 🤔 That my other project (which @PaulRB know). But don't worry, I disconnect all from x4 +/- rows, to not disturb of this test (after he asking for show it). ---
**Author:** ![kmin](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/kmin/32/1147370_2.png) [@kmin](https://forum.arduino.cc/u/kmin)\ **Post date:** [November 24, 2024, 4:24pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/16 "2024-11-24T16:24:29Z")
Make it simple. Just wire that led with single 470ohm resistor to 9V battery. For expected 1.8V forward voltage it should draw ~15mA. And shine bright. Then measure the current. If it doesn't match at all you have some problem with your multimeter. ---
**Author:** ![build\_1971](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/build_1971/32/689930_2.png) [@build\_1971](https://forum.arduino.cc/u/build_1971)\ **Post date:** [November 24, 2024, 4:53pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/17 "2024-11-24T16:53:46Z")
Something is hiding there! Or you really shoukd worry... 2 broken multimeters... ---
**Author:** ![danyCro](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/danycro/32/1126465_2.png) [@danyCro](https://forum.arduino.cc/u/danyCro)\ **Post date:** [November 24, 2024, 5:07pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/18 "2024-11-24T17:07:12Z")
Ok. I showing a full board (before I start soldering). Now with other multimeter. This one show 83 mA on 3v (40 mA lower then red one - 127 mA). But still over my calculation. Maybe is just a LED? If is from USB then maybe is little different. Do you guys get good current result in practice after calculation? > [@build\_1971](#): > > 2 broken multimeters. Maybe. They are cheapest ones. Red have normal big fuse, but black don't have fuse. Or is it some small chip. But voltage show correct. ![Screenshot_20241124_175503_Video Player](https://europe1.discourse-cdn.com/arduino/original/4X/a/8/4/a84a945b891b335b14e5f5b985f578ed5f938c19.jpeg) ![Screenshot_20241124_175531_Video Player](https://europe1.discourse-cdn.com/arduino/original/4X/0/f/3/0f3704ccad9c9869b33f28f421b8f1b154f77b5f.jpeg) Why multimeter? LED shine to high on that two resistor. I see with eyes. On 2000 ohm glow like normal. EDIT: I was used now IR LED from my camera. Resistors 2x R200 really help to decrease light, but multimeter show still 120 mA. ---
**Author:** ![david\_2018](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/david_2018/32/1013044_2.png) [@david\_2018](https://forum.arduino.cc/u/david_2018)\ **Post date:** [November 24, 2024, 5:12pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/19 "2024-11-24T17:12:25Z")
What does the black meter show when you measure the resistance of the resistor? ---
**Author:** ![PaulRB](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/paulrb/32/216330_2.png) [@PaulRB](https://forum.arduino.cc/u/PaulRB)\ **Post date:** [November 24, 2024, 5:15pm UTC](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299/20 "2024-11-24T17:15:45Z")
> [@danyCro](#): > > It is over 200 mA on 9 v battery 🙂 But only for take pictures. That is about 10x the max current of a small led. It would burn out while you take the picture. So I don't think that current reading can be true. Did you replace the multimeter batteries with fresh? [Next page](https://forum.arduino.cc/t/calculation-current-trough-resistors/1325299.md?page=2)