AC Voltage and Current Measurement Circuit Design with Differential Amplifiers - Page 1
Forum section: electronics / beginners

author: Shubham
author_rank: Contributor
timestamp: June 02, 2023, 08:09:09 pm
content: I am trying to finalize the AC voltage and current measurement circuit using a differential amplifier. I have designed two circuits for each. In the high side current measurement circuit 2 and voltage measurement circuit 2, I have connected multiple resistors in series to the inputs of the op-amp. However, in the high side current measurement circuit 1 and voltage measurement circuit 1, I have connected a single resistor to the inputs of the op-amp. I am planning to use 0603 package resistors. As I know that op-amps have very high input impedance, I used a single input resistor in circuit 1. I am confused about which circuit to follow for voltage and current measurement, respectively.

author: barshatriplee
author_rank: Regular Contributor
timestamp: June 03, 2023, 07:57:38 am
content: Have you simulated these two circuits? How were the simulation results? Are they showing expected results?

author: Shubham
author_rank: Contributor
timestamp: June 03, 2023, 08:23:26 am
content: yes they are showing expected output.

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2023, 01:21:52 pm
content: The circuits are the same, except for using multiple input resistors in series vs just one. It might increase the accuracy of the circuit a bit, since the errors in the resistor values will cancel to some degree. It might make more sense to use two feedback resistors and two input resistors, rather than four input resistors.Since it's single supply,it might be better to AC couple the inputs. Refer to the bottom-right circuit, attached below.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 03, 2023, 01:50:34 pm
content: SMT resistors have a very low breakdown value. A 0603 may be only be rated for 100 volts ( quoting from memory, check the specs yourself).That is the reason that for powerline sensing, one sees multiple resistors in series, for safety purposes, as shown in your second circuit.Then you distribute the required resistance value across all the series resistors.SMT resistors are very inexpensive.

author: Shubham
author_rank: Contributor
timestamp: June 03, 2023, 02:00:49 pm
content: Thanks for sharing it, As I am going to deal with the mains 220Vac. I am talking about safety, having multiple resistors in series to the input of the opamp or a single resistor to the input of the opamp safe? Also, having input resistors value larger (as gain of opamp can be maintained by varying the feedback resistor value according to Rf/Ri)?

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2023, 02:29:32 pm
content: What gain do you require?This isn't a beginner friendly project.,Current can be measured with a Hall effect sensor and voltage with a special voltage measurement transformer.If it's not practical to use transformers, use the AC coupled circuit with Y1 capacitors on the inputs. High value input resistors, say 4M7 with 2.2nF capacitors should provide sufficient separation to be safe.https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/

author: Shubham
author_rank: Contributor
timestamp: June 03, 2023, 03:03:44 pm
content: gain 1.2 for high side current measurement, gain 0.003 for voltage measurement. I want to measure current using very low-value shunt resistor as it is highly cost-effective and accurate.

author: jonpaul
author_rank: Super Contributor
timestamp: June 03, 2023, 03:12:17 pm
content: Suggest you DO NOT use this on any dangerous voltage eg 120/240V mains, or power amp output.Stick to low voltages.Jon

author: Zero999
author_rank: Super Contributor
timestamp: June 04, 2023, 09:26:15 pm
content: Quote from: Shubham on June 03, 2023, 03:03:44 pmIt can be done, but is not a beginner friendly project.What's the current range?As I said, use Y1 capacitors and high value resistors, with the differential amplifier circuit I showed in my previous post. If you don't understand this, then sorry, you lack the knowledge to build this safely.