Linear current sense amplifier

Discussion in 'The Projects Forum' started by aboico, Oct 11, 2012.

1. aboico Thread Starter New Member

Jul 25, 2012
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0
hi, i am looking for a linear current sensor.
i need a the supply to be 5V
the current will be to 0-4A
any suggestions?

2. shortbus AAC Fanatic!

Sep 30, 2009
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3. wayneh Expert

Sep 9, 2010
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Basically, you need to have a low ohms shunt for your current sense resistor. Low ohms because, starting with only 5V, you don't want too much of a drop across the resistor itself. Suppose you want no more than 200mV drop at 4A. That means a 0.05Ω resistor. If yo can live with 400mV, then you could choose a 0.1Ω resistor.

Then you need an op-amp that can sense from zero to the peak voltage, and a gain chosen to keep the full-scale voltage output - at the peak current - below the positive rail voltage.

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4. Dollarday Active Member

Jan 25, 2012
32
1
I'm assuming that when you say a linear current sensor, you mean a sensor that has linear output, not a sensor that is physically from the company "linear"...

If you want to use current sensor chip I recommend looking for a high-side current sensor. (Senses current between the supply and the load)

You can use a current sensing chip with a set or programmable gain.
(For your application a gain of 10-->50 should be fine)

Here is a design example using a MAX4376 current sensing chip with a gain of 50: (Use the datasheet as reference)
http://datasheets.maximintegrated.com/en/ds/MAX4376-MAX4378.pdf
Look at the typical operating circuit at the bottom of page 9.
You can use the MAX4376F with a gain of 50 and a sense resistor with a value of 0.2ohm.
This circuit will give an output of 1V for each amp flowing through the sense resistor.
(Example: 1A = 1V, 2A= 2V, 4A = 4V)
Look at table 1 on page 8 for other gain or sense resistor values.

It works by amplifying the current flowing through the sense resitor.
There are many chips that work on the same principle as the max 4376. (Like the INA197 from texas instruments) Just make sure they can all work from 5V.

PS: A normal amplifier will only work if it has a common mode rejection range that includes ground and whose inputs are buffered, like an instrumentation AMP

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5. crutschow Expert

Mar 14, 2008
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If you can place the shunt resistor in the ground leg of the supply then you can use a standard op amp to amplify the shunt voltage. If you place the regulator ground at the power supply output, then the shunt voltage drop will not affect the regulated voltage.

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6. bountyhunter Well-Known Member

Sep 7, 2009
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Use a precision op amp and suitable gauge/length of manganin wire for the shunt.

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