# How to measure current at the input if smps

#### mohan.n2293

Joined Jan 23, 2017
60
Hi all,
This is Mohan,

Actually i tried to measure input current of smps(3 phase input), to make smps automation circuit,we have used a 1 ohm resistor to the the series of all the three phases of smps ,normally when i measure in multimeter i saw 40mA at the smps input when i measure(voltage) across the 1 ohm resistor i got 0.04v so when i use formula I=V/R,im getting 40mA but i need to interface it with micro controller .

Can anyone please suggest me a AC amplifier circuit ,with isolation .
Please find the image to know my measurement place

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#### LesJones

Joined Jan 8, 2017
4,226
Normally you would use current transformers but if you are only measuring 40 mA then I have never seen any for such small currents. Why are you using a three phase supply to such a tiny power supply ?

Les.

#### mohan.n2293

Joined Jan 23, 2017
60
Hi less out smps spec is input=600v max , 450v noaml and output is5v,2A,we will use this for our agri products .
since the input is 450~600v the input will be in milliamps.

#### R!f@@

Joined Apr 2, 2009
9,919
Why the heck does one require a 3 phase input for a 5V, 2A PSU ????

#12

#### mohan.n2293

Joined Jan 23, 2017
60
Our product is agriculture based ,we will diploy this product in fields ,normally in our India 440v 3phass supply will be Thier in the fields to run motors.
in fields ,we cant get 1phase or 2 phase supply,only 3phase will be avilable

#### LesJones

Joined Jan 8, 2017
4,226
For such a small load you can just use one of the three phases. Either one phase to neutral (250 V) or between two phases (450 V) The very slight inbalance of the phases does not matter..

Les.

#### mohan.n2293

Joined Jan 23, 2017
60
Thats the issue here we will not get neutral in fields only 3 phases are available without neutral.
our smps also working without neutral.

#### R!f@@

Joined Apr 2, 2009
9,919
Use two phase only.
You do not need 3 phases for that.

#### crutschow

Joined Mar 14, 2008
34,834
For such high voltages you might consider a current transformer, but of an unconventional configuration
Most current transformers are designed for a high to low current step-down ratio but, for low current, the opposite could be done.
You could use a standard transformer with the primary in series with the line and the secondary giving an amplified current output, which then gives a voltage across the burden resistor.
For example, a 120V to 28V transformer at a primary current of 40mA would have a secondary current of 120/28 * 40mA = 171mA.
A burden resistor of 10Ω across the secondary would then give a voltage of 1.71V.

One critical factor is that the transformer would have to tolerate the 600V between primary and secondary.
The type of transformer wound on a plastic split bobbin with the primary and secondary completely isolated should work.

#### ian field

Joined Oct 27, 2012
6,536
Our product is agriculture based ,we will diploy this product in fields ,normally in our India 440v 3phass supply will be Thier in the fields to run motors.
in fields ,we cant get 1phase or 2 phase supply,only 3phase will be avilable
At 40mA - you're probably not going to unbalance the 3-ph supply to any degree of consequence.....................

#### mohan.n2293

Joined Jan 23, 2017
60
if a smps designed to work in 3 phase can also work in 2 phase right ,
so for that 2 phase i need to test the input current how much it is taking

#### R!f@@

Joined Apr 2, 2009
9,919
if a smps designed to work in 3 phase can also work in 2 phase right ,
Depends on the design. Most of the time it would...but we need to see the diagram to be sure.

so for that 2 phase i need to test the input current how much it is taking
Why do you need to measure that current ?

#### LesJones

Joined Jan 8, 2017
4,226
I think if I had to measure such low currents (As I think it would be difficult to get suitable current transformers.) I would have a separate circuit for each phase powered via a suitably rated isolation transformer. The voltage from the sense resistor would be amplified using an op amp then rectified with an active rectifier which was connected to an ADC input of a small microcontroller which would convert it to digital format. This would then be fed to an opto isolator (Of suitable isolation rating.) the output of the opto isolators could be at ground potential. Each of these three units would be housed in a double screened box. The inner screen would be connected to one end of the sense resistor. The outer screen (More a safety enclosure than a screen.) would be connected to ground. Is this for testing power supplies that you manufacture or for some strange reason do you need to know how much current they consume in your application ?

Les.

#### mohan.n2293

Joined Jan 23, 2017
60
This project is to test smps efficiency by collection info about input current ,voltage .and compare them with output voltage and current

#### R!f@@

Joined Apr 2, 2009
9,919
This project is to test smps efficiency by collection info about input current ,voltage .and compare them with output voltage and current
Strange you need 400V phase to phase to do that or 3 phase for that matter.

Would you be doing this in an agricultural field ?
If so, why ?
You could this at home using a single phase supply