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op-amp

Discussion in 'General Electronics Chat' started by sravan kumar padala, Aug 9, 2012.

1. sravan kumar padala Thread Starter New Member

Jun 3, 2012
11
0
hi...
I want to convert current to voltage for which I went on with the opamp circuit but what the problem with it is its only working for 0.1A input and just require a I-V converter with input current range of .001A to 4A.

please do suggest the circuit.

thank you in advance.

2. WBahn Moderator

Mar 31, 2012
24,562
7,700
I'm having some problems understanding your English (and rest assured, your English is far, far better than my ability to communicate in your native tongue -- unless it happens to be English!).

So I don't know what you mean by "for which I went on with the opamp circuit..." What opamp circuit?

If all you are trying to due is measure the current in a circuit, then you can just measure the voltage across a small-valued resistor put in series with the circuit. But if you want to measure over a range more than three orders of magnitude, then you have some potential challenges. So consider the following:

Is switching in different resistors an option depending on whether you are seeing a current of 2mA versus 2A?

How much voltage drop can you tolerate across the current sensing resistor?

How are you going to measure the result?

3. sravan kumar padala Thread Starter New Member

Jun 3, 2012
11
0
sir,
I've used the below circuit for conversion of current to voltage and this has included an op-amp so I've mentioned that its and opamp circuit. but the problem with this is I can only give an input of 0.1A and by varying the resistor I get proportionate voltage.

and I want to convert the current to voltage but in your previous description it is assumed in reverse manner.

thank you 4. WBahn Moderator

Mar 31, 2012
24,562
7,700
I can't access the image. Don't put images where people would have to sign-in to view them. Instead, upload your image to the AAC server. You can do this by choosing to "Go Advanced" (or by selecting "Post Reply") and then choosing to "Manage Attachments".

If you have a circuit that you want to conver the current to a voltage, what happens if you run that current through a resistor? A voltage appears across the resistor that is proportional to the current flowing in it. The problem is that this voltage interferes with the normal operation of the circuit, possibly too much. If so, then you need to find a way make the measurement without introducing significant interfering voltages. There are several opamp circuits that can do this, but most approaches are only going to be intended to work with a fairly small range of currents. Hence the questions I asked.

5. maanga New Member

Aug 20, 2012
12
3
Hello Sravan,
You will need 3 resistors (R1+R2+R3) if you want to measure three ranges.
Connect the three resistors in series, lowest value resistor (R1) first and in that order.
The other end of the lowest value resistor (R1) will be your common point (-ve) of measurement.
Connect your opamp to the extreme of the resistor network (Common point and floating end of R3). Use two 100 k series resistors with opamp +/- inputs. Connect (protect) two 3.3 volts zeners back to back across the +/- inputs of opamp. This will prevent volage exceeding 3.3 volts and damage the IC
Use a three position switch to select one of the three resistor joints.
Measured current to flow through common point, through resistors to switch and the +ve measurement probe.
Since you want to measure 1 volts across each range,
Now say you want to measure 1A, 0.1 A and 0.01 A. which amount to 1 ohm, 10 ohm and 100 ohms respectively.
the total resistance of R1 +R2+R3 equals 100 ohms.
R1 and R2 equals 10 ohms
R1 equal 1 ohm
So R2 equals 9 ohms and R3 equals 90 ohms.
Opamp input impedance should exceed 10 Mega Ohms.
I hope this is clear to you, I currently do not have means to draw or scan.
regards,
Maanga