Measuring the current consumption change of a device. - Page 1
Forum section: electronics / beginners

author: masoudium
author_rank: Contributor
timestamp: February 15, 2016, 07:56:42 am
content: Hi,I'm trying to measure the current change of a 24V DC powered device in the range of some mili-amps, and send the measured value to a PC.I have designed a board with an Atmega8 and a 2 Ohm shunt resistor placed in series configuration in the power line of the device. I used a differential amplifier circuit to amplify the voltage across the shunt resistor to measure this voltage with uC ADC.( The schematics is attached)It seems that this Idea is not working, as it reads a constant value across the shunt resistor even when there is no current passing through it!What am I doing wrong?

author: han
author_rank: Frequent Contributor
timestamp: February 15, 2016, 08:17:19 am
content: 1. Is the load isolated or share common ground?2. Maybe the Op amp need negative supply?

author: bktemp
author_rank: Super Contributor
timestamp: February 15, 2016, 08:22:08 am
content: The problem is the positive common mode voltage:The LM324 has a supply voltage of +5V but the shunt is connected to +24V. That does not work.Better use an IC spefically designed for highside current sensing.

author: masoudium
author_rank: Contributor
timestamp: February 15, 2016, 08:35:03 am
content: Quote from: han on February 15, 2016, 08:17:19 amHi,thanks1.No it is not isolated.2. LM324 is a single supply op-amp.

author: masoudium
author_rank: Contributor
timestamp: February 15, 2016, 08:37:47 am
content: Quote from: bktemp on February 15, 2016, 08:22:08 amHi BktempThanks, Can you please refer me to a good IC for this application. I need to measure the current in milli-amps resolution. The current won't go higher than 1 Amp.

author: han
author_rank: Frequent Contributor
timestamp: February 15, 2016, 08:45:54 am
content: Its not so precision, but maybe can be usedhttp://www.aliexpress.com/item/1-Pcs-New-5A-ACS712-Range-Current-Sensor-Module-Electronic-Parts-Hall-Effect-Detector/32387779613.html?spm=2114.10010108.100010.8.lBYGxH

author: masoudium
author_rank: Contributor
timestamp: February 15, 2016, 08:54:18 am
content: Quote from: han on February 15, 2016, 08:45:54 amThanks han,It is not appropriate for what I'm trying to do, But I think I can use this chip on other projects.Thanks

author: Kilrah
author_rank: Unknown
timestamp: February 15, 2016, 09:21:28 am
content: Your description lacks details... For example could you move sensing to the low side? That would help.

author: masoudium
author_rank: Contributor
timestamp: February 15, 2016, 09:25:15 am
content: Quote from: Kilrah on February 15, 2016, 09:21:28 amHi KilrahSorry what do you mean by "moving sensing to the low side"?

author: Kilrah
author_rank: Unknown
timestamp: February 15, 2016, 09:35:25 am
content: http://electronicdesign.com/power/what-s-difference-between-high-side-and-low-side-current-sensingThis from the article is the issue you are running into.QuoteAlso you say the Hall effect current sensor is not appropriate but you don't say why. Hard to advise you that way.

author: masoudium
author_rank: Contributor
timestamp: February 15, 2016, 09:59:55 am
content: Quote from: Kilrah on February 15, 2016, 09:35:25 amOh thanks that really helped.I need to test this method.

author: masoudium
author_rank: Contributor
timestamp: February 15, 2016, 12:40:38 pm
content: Quote from: Kilrah on February 15, 2016, 09:35:25 amI didn't say that Hall effect current sensor is not appropriate for my job! I just checked that sensor manual and the precision was not what I needed in my project.I need to measure current change in milli-amps precision if you know any hall-effect sensor that has this precision I would thank you if you let me know about it.

author: Rick Law
author_rank: Super Contributor
timestamp: February 16, 2016, 08:43:37 am
content: The Adafruit INA219 is a quick and painless solution. It is also a 12bit ADC giving 4x the accuracy of the 10 bit ADC.

author: masoudium
author_rank: Contributor
timestamp: February 16, 2016, 09:08:10 am
content: Quote from: Rick Law on February 16, 2016, 08:43:37 amWow. Thanks so much. This is exactly what i was looking for!