Looking for current sensing range 1-2mA up to 2A... analog or digital output - Page 1
Forum section: electronics / beginners

author: doublec4
author_rank: Regular Contributor
timestamp: September 22, 2020, 02:32:02 am
content: Hi all,I have a project with an IOT type controller that has an I/O expansion module with analog and digital inputs.The device is controlling 2 powered RFID tags at 12V and they draw roughly 1-2mA.The device is also controlling 2 pairs of LED lights that draw about 1.2A peak (they are flashing).I am looking to perform current sensing to detect an "error" state where the RFID tags or LED lights may not be functioning properly so the system can report the error.I have found a few current sensing devices that interface with Arduino through I2C but my controller uses regular analog or digital I/O interface. Is there a sensor that you know of that outputs an analog range or even simply a high low depending on the amount of current passing through? I am wondering what is the best (simplest?) way to achieve this.Also, I am a little unclear on if I should be looking for high or low side current sensing for a project like mine?Thanks!

author: Electro Fan
author_rank: Super Contributor
timestamp: September 22, 2020, 03:06:03 am
content: maybe something like this?https://www.adafruit.com/product/4226?gclid=EAIaIQobChMIvvWEz-P76wIViIbACh0EAwh9EAQYASABEgI14_D_BwEor this?https://www.adafruit.com/product/1164

author: WattsThat
author_rank: Frequent Contributor
timestamp: September 22, 2020, 03:29:39 am
content: QuoteThat is entirely up to you since it is dependent upon where you insert the current sensing device.Taking a step back and thinking about the intent and not actual question...I don’t see how sensing the supply current of an RFID tag is a good indicator of an error condition since that current is unlikely to be a static level. Have you actually measured the current over the full range of operating conditions?IMO I think you’re approaching the point of significantly diminishing returns for trying to add smarts to something that doesn’t need it. Sensing led current to know if they’re working? Power supplies are more likely to fail than led’s. Result is the same, no light.

author: doublec4
author_rank: Regular Contributor
timestamp: September 22, 2020, 06:18:28 am
content: ElectroFan, Thanks! I had previously found the first one you linked to and the problem was I2C output as my controller doesn't seem to be capable of interfacing with that.But the second one with regular analog out sounds like it would work!

author: doublec4
author_rank: Regular Contributor
timestamp: September 22, 2020, 06:27:32 am
content: Quote from: WattsThat on September 22, 2020, 03:29:39 amThanks, the part I was unclear about is the effect or limitations (if any) of high side or low side current sensing for this sort of application.Using a multimeter I've measured the current draw on the RFID tag as it is powered with a 12V supply... I am still learning how these work so I'm not entirely sure during what conditions this would change?My AC to DC supply has a fault light and output for a power fail condition as it also has a battery back up. So if the incoming power fails or something happens to the supply, it fails to the battery power supply and an indicator is triggered at the control panel.The reason I wanted to see if the LEDs would fail to turn on (however unlikely) is because at the control panel if someone pushes the buttons to turn the LEDS on and they cannot see the LED lights from the control panel, they have no indication that the LEDs are not working. It sounds like several layers of redundancy but the lights are to warn others that someone is working in the area. I'm just putting together a rough proof of concept to show the idea. When it comes to safety, everyone loves several layers of protection.

author: Zero999
author_rank: Super Contributor
timestamp: September 22, 2020, 10:16:02 am
content: Why not use something to detect the light from the LEDs, to check they're working, such as a phototransistor?

author: doublec4
author_rank: Regular Contributor
timestamp: September 23, 2020, 05:20:48 am
content: Quote from: Zero999 on September 22, 2020, 10:16:02 amInteresting idea for the LEDs, could work. For the RFID tags I would still need to figure something out. Going to dive into some TI references and see if I can find anything that will work for me

author: Zero999
author_rank: Super Contributor
timestamp: September 23, 2020, 09:29:51 am
content: Quote from: doublec4 on September 23, 2020, 05:20:48 amI doubt measuring the current drawn by the RFID tags will be a good enough indictaion of their functionality. Can't you simply read them, every now and then, to check they're working?

author: doublec4
author_rank: Regular Contributor
timestamp: September 24, 2020, 06:58:55 am
content: Quote from: Zero999 on September 23, 2020, 09:29:51 amThis idea was also on the table, but manually checking the tag isn't viable due to its location. A piece of equipment with a tag reader is what is actually reading the tag as it passes over and enters the area where personnel could potentially be working. I threw around the idea of putting a tag reader beside the tag to ensure it is working but I thought current sensing would be perhaps a simpler and lower cost solution.For the sake of learning, I've been reading into current sense amplifiers (lots of good info from TI) but for the life of me I can't really find a "beginner" level introduction into designing a simple current sensing amplifier circuit with analog voltage output (0-5V).If anyone has any walkthrough examples on how to design a circuit like that and select the current sense amplifier that would be very helpful. Regardless of if it works for this application, I'd love to know how to make one!

author: doublec4
author_rank: Regular Contributor
timestamp: September 25, 2020, 02:19:47 am
content: For anyone who stumbles across this in the future, I found this video to be very helpful and really helped me understand what is happening in more detail:It gives a nice clear explanation with calculation examples for the Rshunt portion of my problem