Rustic Engineering

My Mind working on…

How much current (part 1)?

with 7 comments

Someone send to me a question, about how much current my OLED board consumes at all. Well, after measure with a multimeter the answer is: 660μA (aprox.) with display all ON and contrast at 0xFF (max). But this question make me thinking about how measure that current myself, without a multimeter.

When I’m in graduation (some good years ago) I made a little circuit to measure how much current my power supply project output. The circuit is based on “high-side current-sense” methodology. See my hand-drawn circuit above:

 

HIgh-side current-sense

High-side current-sense

 

This circuit is a classic high-side current-sense, where the voltage drop across Rs resistor is isolated by a operational amplifier (op amp) in a differential configuration with unitary gain. The many implementations of technic were based on discrete components or semidiscrete circuitry. In their simplest form, such high-side monitors require a precision op amp and a handful of high precision resistors.

The resistor value should be low (like mΩ scale), to minimize power losses, but don’t be too low, because stability problems. And don’t forget about power dissipation across the resistor.

One common approach for high-side measurements has been the use of the classic differential amplifier, which is employed as a gain amplifier.

So, after reading a lot of theory, I’ve going to search my integrated differential amplifier.

Integrated Differential Amplifier

I should admit: Maxim is always my source of good op-amp IC, so I’m was doing some search and found a good and little evil: MAX4372. Some goods about this guy:

  • Tiny SOT23-5 package;
  • Low cost;
  • 3 gain options (20V/V, 50V/V and 100V/V);
  • +2.7V to 28V range of operation;
  • Consumes only 30μA;
  • 0.18% full-scale accuracy with 100mV Vsense input (This is equivalent to only 0.18mV input offset voltage);

 

MAX4372 typical use

MAX4372 typical use

 

The MAX4372, in a tiny SOT23-5 package is a very good device to make a current sense device. You can set the full-scale current reading by choosing the device:

  • MAX4372T: 20V/V Gain;
  • MAX4372F: 50V/V Gain;
  • MAX4372H: 100V/V Gain;

What means 20V/V Gain? If you have 1A current flow through a 100mΩ Rs resistor, you gave only 0.1V drop. But with 20V/V, you multiply this and obtain 2V. Obviously? The design goal with Gain is thinking in terms of full scale design. If your A/D converter uses 3.3V was reference, you can consider this to check what the max current value you can measure with determined gain value.

A test circuit

I’ve designed a test circuit to test my idea. Basically is a MAX4372T with a 100mΩ Rs. I’ve mounted it in universal board, and my idea is connect it to my USB Low Pin Kit:

 

MAX4372 basic circuit

 

 

Test board

 

To test them, I’ve used a PIC18LF2520 and a great character LCD from Electronic Assembly. See my test circuit working:

 

First test

 

 

Looks great!

 

The pictures show that the global idea works great. Now I need some work to improve stability and other features.

Next steps…

The next step is create a USB device for read, store and show the current measurements:

 

USB Power Monitor

USB Power Monitor

 

Written by forrequi

October 17, 2010 at 03:02

7 Responses

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  1. Hello
    I have used the same IC MAX4372F in my schematic of buck converter. AS i can see you have directly connected the IC across a sense resistor.I had connected a RC network in parallel with the inductor and its DCR and then connected the MAX4372F across that capacitor.I found that the output of the MAX4372 kept on changing depending on the resistor i connected at the output. Is this a wrong configuration ?

    Vinayak

    March 8, 2013 at 00:50

    • Hi, can you post your circuit? The Datasheet list the valid designs too. Thanks for interest.

      forrequi

      March 8, 2013 at 12:29

      • This are my circuits
        one without the MAX4372F and the other with it
        buckmax
        MAXbuck

        Vinayak

        March 8, 2013 at 13:29

      • I see your circuits and that’s a wrong way to use the IC (MAX4372). The IC operation it’s based on measure the differential voltage between RS+ and RS- terminals. That’s difference will be multiplied by a constant, to easy scaling and reading. And it’s only works if you use a Shunt Resistor, because its very low value and very low voltage drop.

        When connected to a RLC circuit, like you did, the IC get a variable differential voltage and the output will reflect this.

        forrequi

        March 8, 2013 at 20:28

      • Thanks very much for your insight. Otherwise i would have been stuck figuring that only.
        Can you suggest me a way to use the voltage across the capacitor and get the current information from it as it necessary for my circuit operation

        Vinayak

        March 9, 2013 at 01:32

  2. upload
    I tried as per the diagram shown. I have now used a sense resistor
    As you have experience using the IC please help me out
    According to calculations the resistor drops 10m*12A=120mV
    So it gets amplified by gain of 50 to become 6V
    But i dont get that in pspice
    I tried to change supply from 2.7 to 3,5,8 and found that output changes with supply too
    What is going wrong ?

    Vinayak

    March 11, 2013 at 03:53

    • Well, I don’t know details about your model, but you can check some things easy:

      1) Check if the pspice MAX4372 model is working according the datasheet with a more simple circuit. A single DC power and a constant current load.
      2) Check if the gain changes the output according to IC type, in a simple circuit.
      3) Check if you are using a switch frequency above the limits of the IC. The MAX4372 operates at small frequencies, and you gain is frequency dependent as its showed at page 5 in datasheet (small-signal gain vs frequency).

      forrequi

      March 11, 2013 at 12:58


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