Isolated mA current sensor - Page 1
Forum section: electronics / projects

author: kiran_shrestha
author_rank: Regular Contributor
timestamp: April 10, 2014, 06:49:05 am
content: I am working on a project Isolated mA range current sensing. Beside hall sensors what are the other isolated methods for the current sensing. If i use Rsense (shunt resistance ) method how can i isolate it . And i was wondering how our digital multimeter measures current. They have a sense resistor but with out common ground how ??

author: pinkysbrein
author_rank: Contributor
timestamp: April 10, 2014, 07:13:43 am
content: The multimeter floats (the common connection is a virtual ground for the circuit, and the other end of the shunt has a voltage difference with it which it measures) and has optical isolation to our eyeballs.If you use a shunt you need something like an isolation amplifier, you also need a battery or transformer to power it.

author: lewis
author_rank: Frequent Contributor
timestamp: April 10, 2014, 07:28:02 am
content: A cheap and simple way to do isolated current sensing is to use a shunt + op-amp + microcontroller to convert the current to a serial stream. If you need greater accuracy, use an external ADC. Throw an opto-isolator on the end and voila! Isolated output. Use an isolated DC-DC converter to power the MCU and op-amp, or you might be able to power it off the signal you're measuring.This technique can also be used to measure the AC mains, both voltage and current simultaneously, but make sure to use a DC-DC converter with reinforced isolation.I've used this technique many times, it can be more accurate than an Allegro hall sensor for example as these tend to suffer from offset and drift at low currents.

author: kiran_shrestha
author_rank: Regular Contributor
timestamp: April 10, 2014, 08:48:04 am
content: Thanks for the infos i'll try it and be back with results

author: tszaboo
author_rank: Super Contributor
timestamp: April 10, 2014, 10:14:21 am
content: Since no real requirements were given, I would say I would use the ADM3260 for isolated power and I2C and ADS1110 with the gain of 8, and the appropriate shunt. This would be a really low part count reliable solution.

author: dannyf
author_rank: Super Contributor
timestamp: April 10, 2014, 11:07:27 am
content: Or a high linearity opto-coupler and call it a day.

author: kiran_shrestha
author_rank: Regular Contributor
timestamp: April 10, 2014, 11:15:34 am
content: Thought of using ADM3260 but couldnt find in digikey.Thanks any way.

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 10, 2014, 11:18:15 am
content: Try an IL300 linear optocoupler. It's good enough for most applications.

author: David Hess
author_rank: Super Contributor
timestamp: April 10, 2014, 11:00:54 pm
content: Quote from: pinkysbrein on April 10, 2014, 07:13:43 amBench multimeters often (always?) include internal isolation power supplies and optical or magnetic isolation between the signal conditioning/analog to digital conversion and the control/display circuitry so they can support typically at least 1000 volts between the measurement common and chassis ground.

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 11, 2014, 12:37:34 am
content: is this AC or DC you wish to measure the current of? If AC, current transformer!

author: ajb
author_rank: Super Contributor
timestamp: April 11, 2014, 01:26:27 am
content: There are magnetoresistive current sensors that are internally isolated and work from DC to 100kHz or more, but I'm not sure if any exist for the mA range. Lowest nominal current I've seen that you can actually buy is 5A, with total accuracy spec'ed about 1.5% (75mA).If by isolation you mean you just need to sense something that's not referenced to ground, a differential amplifier with a high common mode voltage may be a solution.As for how your multimeter works, lone end of the internal current shunt is referenced to the meter's ground--or possibly another point, but the important thing is that it's referenced somewhere--so the meter just has to measure the voltage at the other end and then use Ohm's law to figure current. Note that you don't need a commonground, necessarily, you just need both ends of the current shunt and some way of measuring the voltage of one relative to the other. If you have a differential amplifier, it doesn't matter what the voltage at each input is relative to ground; as long as they're within the amplifier's common mode range, they both wind up referenced indirectly to ground via the amplifier's input circuitry and then compared to one another to produce the output.

author: BravoV
author_rank: Super Contributor
timestamp: April 11, 2014, 01:30:09 am
content: Quote from: pinkysbrein on April 10, 2014, 07:13:43 amDunno why, I find this explanation is truthful and yet so funny.

author: han
author_rank: Frequent Contributor
timestamp: April 11, 2014, 01:39:43 am
content: maybe it's fit (maybe not)ISO-A4-P3-O4 (isolated converter 4-20mA to 0-5V) for my last projecthttp://www.szsunyuan.com/

author: kiran_shrestha
author_rank: Regular Contributor
timestamp: April 11, 2014, 05:09:11 am
content: Quote from: peter.mitchell on April 11, 2014, 12:37:34 amI am measuring DC current.

author: macboy
author_rank: Super Contributor
timestamp: April 11, 2014, 01:49:00 pm
content: Quote from: David Hess on April 10, 2014, 11:00:54 pmCareful! In my experience they definitely do not "typically" support 1000 V between ground and input common/Lo.I have three different types of bench meters on my bench (Keithley 2001 & 199, Thurlby 1905A), and they all specify 500 V (peak) between ground and input Lo.

author: tszaboo
author_rank: Super Contributor
timestamp: April 11, 2014, 05:06:56 pm
content: Quote from: kiran_shrestha on April 10, 2014, 11:15:34 amYou are right, looks like a vaporware. Luckily isolated power <1W is not so hard.

author: hli
author_rank: Frequent Contributor
timestamp: April 11, 2014, 07:18:56 pm
content: Though its rather expensice, you can go with a ADE7912. Its a 2-Channel-ADC (one for voltage, one for current), and is completely isolated for data and power.

author: David Hess
author_rank: Super Contributor
timestamp: April 11, 2014, 08:15:34 pm
content: Quote from: kiran_shrestha on April 10, 2014, 06:49:05 amIf the circuit itself can provide power, then a current shunt can be used and only the output signal needs to be isolated like with the linear optocoupler mentioned. Another common way is to do the analog to digital conversion and isolate the digital side. In the past, a current to frequency converter might be used as an analog to digital converter which is how most of my bench multimeters work.If the circuit cannot provide power on the shunt side, then a floating power supply can and all of the above methods can still be done. Most (all?) AC powered multimeters work this way. I think this is the most straightforward way.A hall effect sensor can make an isolated DC current measurement. I have a Fluke current probe which work this way but accuracy and frequency response are relatively poor. There are some inexpensive integrated solutions for this but offhand I do not know if any have the sensitivity you are looking for. High bandwidth current probes for oscilloscopes that operate down to DC use a hall effect sensor as part of a balance circuit with a counter current driving the current sense transformer so the sensor's gain change versus temperature and frequency response are irrelevant.QuoteMultimeters use a current shunt. Battery powered meters are automatically floating devices so they can make this measurement directly but line operated meters have one or more galvanically isolated power supplies so the signal conditioning circuits are floating with respect to ground.

author: nctnico
author_rank: Super Contributor
timestamp: April 11, 2014, 11:46:58 pm
content: Quote from: NANDBlog on April 10, 2014, 10:14:21 amI'd follow this approach as well. No messing with analog feedback loops through optocouplers. If the ADM3260 is not available a seperate DC-DC converter and an I2C isolator are also a good choice.