AC current measurement, analog front end - Page 1
Forum section: electronics / beginners

author: Muhammad Nasar
author_rank: Contributor
timestamp: June 14, 2013, 02:02:01 am
content: I have come up with the following circuit(attached) for measuring AC current using a 0.22Ohm shunt, first using a voltage follower then next opamp is configured so as to get a programmable gain through switches(Controlled by MCU)+to add dc offset of 2.5V and the last opamp to bring the signal to its original phase as signal was inverted by the programmable gain op-amp stage. The Output will be fed to the ADC of a MCU. The op-amps used are ultra low input offset ones.I want your suggestion for improving the circuit or indicate me something that I missed or have done wrong.

author: Kremmen
author_rank: Super Contributor
timestamp: June 14, 2013, 08:04:06 am
content: A couple of general comments:Current sensing usually occurs at quite low voltage levels because you don't want to a) cause unnecessary losses in the circuit you are sensing and b) to avoid heating the sense resistor itself. Thus the low voltage is susceptible to ground voltage offset in a circuit like yours. The "proper" way is to use a differential amp, measuring the voltages at the terminals of the sense resistor directly, preferably using what is called a Kelvin connection. This will negate errors caused by ground offsets from track resistance and similar sources. Whether this is relevant is pof course dependent on the actual currents and voltages of the application.Secondly, you seem to have several curious resistor values is your circuit. I forget if they are in the E48 or E96 series, but even so, it makes life easier to standardize the values to as low an E series as practical. I very seldom specify anything else than E12 series, preferring to combine resistors in parallel or series to get a desired off value. For volume production this is not an issue of course, but for prototyping it makes life considerably easier when you have all the values in the shelf. For example, i maintain the full E12 from 1 ohm to 1 Mohm in 1/4 W through hole, 1206, 0805 and 0603 sizes. Makes it easy to prototype using 1 size and then move to another for further prototyping or final assy. A case in point, your last inverting amp uses 16k resistors. 15k is in E12 so surely that would be a more sensible choice.Some observations:The sense resistor is almost ~½-1 millionth of the resistance of the second stage input resistance so i don't know if the first voltage follower is actually doing anything useful. The source impedance of the sense circuit should be low enough as it is.But for the AC, i would pick a properly spec'ed dedicated current sense amp with no further consideration. Only in this case it won't work really, because those are as a rule unidirectional. Perhaps you would like to take a look at a component i have used in similar if not identical cases:http://www.ti.com/lit/ds/symlink/ths4131.pdf. Provided your A/D has differential capability, this approach could be worth considering. You could get away with a single amp instead of 3 and also get the ability to sense the current differentially.

author: Psi
author_rank: Super Contributor
timestamp: June 14, 2013, 08:43:07 am
content: We need more info on the AC your measuring, its voltage, nominal current and also frequency. (Thing are different at 50Hz vs 1mhz for a dc/dc psu etc..)

author: Muhammad Nasar
author_rank: Contributor
timestamp: June 14, 2013, 09:42:41 am
content: Quote from: Psi on June 14, 2013, 08:43:07 amI gonna measure load of about 1KW at Vrms=230Volts, and frequency of 50Hz, i.e. 0.22ohm shunt would be in series with the load.

author: Kremmen
author_rank: Super Contributor
timestamp: June 14, 2013, 04:39:41 pm
content: Several things come to mind, not the least a question of safety. This would obviously be a mains circuit so exactly what is the current measuring MCU supposed to do. The circuit you have sketched does not provide galvanic isolation so the entire circuit including the MCU will be at mains potential i.e. life threatening to touch.The next question is, how is the circuit you have shown related to the actual circuit? This cannot be it, because a 1kohm in series with the sense resistor is not gonna pass enough current at 230VAC to produce 1 kW of dissipation to any connected load. Also, the sense resistor is grounded, not a typical configuration although possible (low side measurement, perfectly possible but but seldom drawn like this).Really, think carefully about the safety issue. A non-isolated mains circuit is very difficult and risky to prototype because when measuring things, the slightest slip of hand invites death.

author: Muhammad Nasar
author_rank: Contributor
timestamp: June 14, 2013, 05:41:44 pm
content: Quote from: Kremmen on June 14, 2013, 04:39:41 pmThanks Kremmen for your response and suggestions, the circuit diagram that i have attached is from the simulation software (multisim) as there the 1000ohm resistor is load, value that i used only for simulation purpose and i was changing it quite frequently so as to check simulation results.One thing I also want to mention is that is I live in country Pakistan, where there is very difficult and expensive to buy non-jelly bean parts, previously i considered to have a current transformer and a voltage transducer to get the measurement, but due to there cost i went for the resistive solution, i know that it would be dangerous to work with mains non-isolated system so i will take care in every respect.For the resistor values problem that you mentioned in previous reply, its not a problem because I have E96 resistors with close enough values to match with that of circuit.Thanks again Kremmen.

author: Kremmen
author_rank: Super Contributor
timestamp: June 14, 2013, 06:56:37 pm
content: OK fair enough but still, you should understand that no amount of care in circuit design is enough if the MCU you use to measure the current is supposed to be human-operated. It just cannot be done safely without galvanic isolation.At mains frequency, you don't need any special stuff for a current transformer. You can make one from a regular transformer by rewinding and connecting it appropriately.