Measuring Current using Resistor

[steve_venkat]

This is my circuit.I'm trying to measure the current by using an resistor(250 ohm) across the negative power supply given to the instrument.
For that instrument 24VDC power supply is used.I connected the positive voltage directly to instrument.Negative supply from the 24VDC SMPS is connected to 1st pin(GND) of IP connector.The 2nd pin of connector is connected to instruments negative side.

It working fine.I can measure the current taken by the instrument.But sometimes it malfunction...It will take 4 to 20mA current.

 

[kellys_eye]

The resistor needs to be in series with the supply to measure current and at 20mA a 250ohm resistor is dropping 5V which my be affecting her circut in use.

 

[Bluejets]

Usual to use a 1 ohm resistor in series with the circuit and measure the voltage drop across it.
By ohms law this voltage drop just happens to be the current reading.
e.g.......... E (measured by meter) = I (0.02A ) * R ( 1.0 ohm) = 0.02V
and for the other end of the range and everything in between... same thing ...E = 0.004 * 1.0 = 0.004V

I think that's what you are asking..??

 

[kellys_eye]

I get the impression the OP used 250Ω to get the particular maximum range of 20mA at 5V for the processor A-D converter......

Using a much lower value resistor and scaling it using an op-amp would have been better but there are also integrated solutions for current measurement these days anyway....