shunt current or voltage sensing ? - Page 1
Forum section: electronics / projects

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: August 30, 2018, 04:18:35 am
content: Hello there,I am trying to sense a shunt (https://www.rc-electronics-usa.com/current-shunt.html) coming from DC rectifier output.keep in mind, Current Rating, for example 100 Amps, Output Voltage which indicates the resistance as the voltage produced for the rated current. V = I × R. For example an output rating of 50 mV at 100 Amps which implies 0.0005 Ohms (0.5 mOhms) resistance.suggeset the protection criteria also, becasue its using in a voltage control circuit.To do so, my plan is to use similar diffrential amplifier circuit as below,1. Does it possible to use current to voltage converter ?2. Does common mode is a problem?3. Ideally what input voltage or current to op amp for such shunt?

author: rx8pilot
author_rank: Super Contributor
timestamp: August 30, 2018, 04:28:13 am
content: Curious question....why not use a current sense amplifier that is optimized for the task and has very good CMRR and common mode range?Example:http://www.ti.com/product/INA240

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: August 30, 2018, 04:39:10 am
content: Quote from: rx8pilot on August 30, 2018, 04:28:13 amThank you for prompt reply.How about, INA133?i have use it for simulation, most funny thing is even you change its input voltage, output is always -11.90v!I have use 10k Load.

author: rx8pilot
author_rank: Super Contributor
timestamp: August 30, 2018, 04:45:15 am
content: Why would you choose that part when a dedicated current sense amplifier solves a number of task-specific challenges and keeps the circuit rather simple?You could roll your own, but I suspect it would be a considerable challenge to match the CMR and CMRR of an integrated solution.

author: digsys
author_rank: Unknown
timestamp: August 30, 2018, 04:45:47 am
content: Yup, there are dozens of current sense amps, and what you chose will depend on critical requirements ie voltage, bidirectional (unipolar w sign o/p ormid-supply w/ ref), fixed gain, linearity, resolution etc etc So start with - 1/ Uni or bi-directional? The INA133 is fine, depending on your specs.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: August 30, 2018, 05:20:25 am
content: QuoteKindly post a suitable circuit to understand how current sense amplifier solves critical problems.QuoteTake a look my first circuit please,There are several fundamental problems with this simple circuit. First, the input impedance seen by V1 and V2 isn't balanced. The input impedance seen by V1 is R1, but the input impedance seen by V2 is R1' + R2'. The configuration can also be quite problematic in terms of CMR, since even a small source impedance imbalance will degrade the workable CMR. This problem can be solved with well-matched open-loop buffers in series with each input (for example, using a precision dual op amp). But, this adds complexity to a simple circuit, and may introduce offset drift and non-linearity.The second problem with this circuit is that the CMR is primarily determined by the resistor ratio matching, not the op amp. The resistor ratios R1/R2 and R1'/R2' must match extremely well to reject common mode noise—at least as well as a typical op amp CMR of ≥100 dB. Note also that the absolute resistor values are relatively unimportant.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: August 30, 2018, 05:38:02 am
content: Quote from: Md Mubdiul Hasan on August 30, 2018, 05:20:25 amThis forum is not a free design house. A current sense amplifier is pretty much a differential amplifier optimized for the characteristics of current sensing - very high common-mode rejection (because your current sense voltage will be small, and you might have it on a high voltage with respect to ground etc). Read the datasheet. It pretty much tells you what you have to do.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: August 30, 2018, 05:44:05 am
content: Quote from: digsys on August 30, 2018, 04:45:47 amThank you for participating in this flow.I am not sure wheather its bidirectional or not.My application seems easy, but little bit confused that what input voltage/current should be fed from shaunt?My co worker said its 1.2 v, comming from external shunt related circuit. Not sure what protection it needs to support.Take a look similar application, where I am using RC component in negative feedback path.this amplifier has a gain of 50, so 5.0V out for 100A through the shunt. R2-R5 represent connection and leadwire resistances, not real resistors.

author: rx8pilot
author_rank: Super Contributor
timestamp: August 30, 2018, 05:50:00 am
content: Quote from: TheUnnamedNewbie on August 30, 2018, 05:38:02 amTo be clear, I have never attempted to re-create a current sense amplifier with discrete parts or generic differential amplifiers. After looking at the specs of what is available for $2 in a tiny package with rather fancy performance.....it is likely beyond my personal skill set to best it. If I could meet or beat the specs, it would be more parts and more money and more size.It could be fun as a skill test, but I am generally in a position where I just need to get projects out the door. They need to be as simple, reliable, small, easy, efficient, and low cost as practical to meet defined performance criteria.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: August 30, 2018, 05:51:53 am
content: QuoteWhat current and voltage limit you can imagine for my posted shunt datasheet ?You have mentioned High voltage, could you clarify a limit?

author: digsys
author_rank: Unknown
timestamp: August 30, 2018, 06:16:05 am
content: My current (excuse the pun) dedicated current sense amp is the MAX9929F. It is bi-directional BUT with a 0 -> V+ op in both directions.Has a simple DIR pin, for reverse flow. If you don't know if there'll be reverse current, then this is perfect. Nothing else to add !! No calcs, nothing.There are many others similar.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: August 30, 2018, 07:59:26 am
content: QuoteYes, I understand. Application is battery charging and discharging.