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how to use op-amp to measure current?

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christie

Mon Sep 01 2008, 09:29AM

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I have doing a project that about measure current for a transistor by using op-amp.
I have no ideas on it.
I hope u all can help me.
thanks.

pdi33

Mon Sep 01 2008, 11:16AM

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hi christie,
just convert the current to voltage ny passing it through a low value sensing resistor and amplify that voltage using an op-amp. use a instrumentation amplifier for best results.

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christie

Ajay

Mon Sep 01 2008, 12:43PM

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[topic moved to Electronics Discussion Forum]

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christie

christie

Tue Sep 02 2008, 01:31AM

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I have draw a circuit as below.
I do not know that circuit is correct or not.
Hope u all can help me.
Thanks

pdi33

Tue Sep 02 2008, 02:15AM

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if pin2 of ina118 is the +ve input and pin3 is -ve input then the circuit looks o.k. and the output will swing to the +ve side. else just interchange the pins and check. As i had mentioned earlier in some other thread, the op-amp can swing only to the +ve side and cannot sense the -ve difference as long as the supply voltage to the op-amp is only +5V.
change the Rg resistor according to the gain required.

good luck.

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christie

christie

Tue Sep 02 2008, 03:09AM

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I have simulate the circuit by change the gain. But the output of current very small.
That is around 349.2pA.

pdi33

Tue Sep 02 2008, 05:53AM

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christie,
the output of the op-amp will be in volts not amperes. check the voltage at the output. this would be proportional to the current u want to measure.

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christie

christie

Tue Sep 02 2008, 05:59AM

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if my output voltage is 4.358V, Vcc and Vs is 5V, then how to calculate the current for the output? thanks

pdi33

Tue Sep 02 2008, 07:39AM

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o.k. i think u need to understand the concept of measurement first:
let me explain to u what u actually have to do by taking an example.
say the assignment is to measure/display the current in the range of 0 to 200mA.
the ADC u have decided is say ADC0808 which has 8 bit resolution (0 to 256 counts full scale).
now this means that for 0V the ADC will give a digital output of 0x00 and
when the input is 5V it will give a digital output of 0xff(256).

then understand the following steps:

step1: convert the current to be measured into voltage by passing it through
a sensing resistor.

step2: as the voltage obtained is very small , pass it through a instrumentation amplifier and scale the voltage
level to suit the ADC.
now here u have to do some calculations to find the gain.
calculate the input voltage to the ADC required by it to generate a output of 200 counts:
that would be Vadc = 5 x 200 / 256 = 3.90V.
now this is the expected output of the op-amp amplifier.
calculating the max. input volatge to the amplifier @200mA measuring current,
Vin = 0.2A x 0.1ohms (sense resistance) = 0.02V
So the gain should be;
gain = Vadc/Vin = 3.9 / 0.02 = 195.

step3: now connect the whole setup as follows:

current sensing ckt ----> amplifier ---> ADC ---> uC.

step4: write the code for the uC to interface the ADC and display it as required on LCD/LED display.

now in the above example, let us cross check if the values are correct by taking an intermediate current value say 100mA,
in that case,
Vin = 0.1A x 0.1ohms = 0.01V
Vadc = Vin X gain = 0.01 x 195 = 1.95V
now the digital output of the ADc to the uC would be
= (256 /5V) x 1.95 = 100 counts
which is just what we require.

hope u understood the concept.

* inspired to develop,developing to inspire *

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christie

ExperimenterUK

Tue Sep 02 2008, 12:00PM

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@Christie..
With the resistor values given your transistor should be destroyed
by the large currents.

I'm assuming your project is about measuring gain or current ratios.

Are you happy that you fully understand how transistors work?

Look at a few of the simple electronics books,
look at how they explain transistor gain, and what they measure.

Then copy then

Use a voltmeter to in place of the INA118 to get an idea of what to expect.

Then re-read pdi33's great post above and all should be clear to you

[ Edited Tue Sep 02 2008, 11:43PM ]

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