Single Rail op-amp to detect current

[JazzRei]

Hi,

I need help with my design of single rail op-amp to detect current.
With input range of 1V to 12V into the op-amp to detect current not exceeding 3V.
As I know that I require higher voltage than 12V to power up the op-amp if the input to the op-amp is 12V. How am I able to operate the op-amp?

 

[FvM]

current not exceeding 3V

??? Please Clarify.

 

[JazzRei]

Hi sorry.. I mean I m designing current not exceeding 3A...

 

[JazzRei]

Hi,

Using an op-amp, with the above diagram, is it possible to turn the op-amp on and measure the current which Vcc must be greater than Vin? How do I make Vcc greater than Vin if my maximum Vin is 12V else my op-amp cant operate. I need a single rail op-amp since I got a ositive output. Can anyone give me advice on that?

Thanks.

 

[btbass]

You can use a potential divider to tailer the input voltage to any level you like!

Attached is a typical current sensing circuit.
Adjust R5 and R4 to get your input range.
Adjust R2 and R3 to adjust op amp gain. Gain = (R2 + R3) / R3.

 

[JazzRei]

Hi btbass,

U measure the current at the input? What is the output for? What is the R26 and R1?

 

[FvM]

You have already posted the solution in your previous thread. I don't understand, why you are starting a new discussion instead of simply calculating the given circuit and see, how it performs. There is no better one.

https://www.edaboard.com/threads/198299/#post834164

 

[JazzRei]

Hi FvM,

The circuit can't work. If the power supply is 12V, then how much power I need to power up the op-amp? It must be greater than 12V. I can't use external power supply. How do I power my op-amp then?
It will not work if I use 5V to power op-amp and with power supply of 12V.

 

[btbass]

R26 is the current sense resistor. R1 is a filter for the output voltage which goes to your A/D input, so you can measure the output voltage which is related to current.
This is low side current measuring, if you want Hi side, use a ZXCT1081 IC.

 

[FvM]

The circuit can't work.

Of course it can.

If the power supply is 12V, then how much power I need to power up the op-amp?

You didn't manage to make a clear statement about your intended power supply. In this thread, you even didn't say, if you intend a high side measurement. Actually, the input voltage can be higher than the OP supply, because the voltage divider reduces the voltage at the OP input .

It will not work if I use 5V to power op-amp and with power supply of 12V.

With a respective resistor ratio, it's possible, but not reasonable, because the measurement gets inaccurate. Why can't you supply an OP with the DC input voltage?
Using a special high side sense amplifier is the opther option.