Measuring voltage with an Analog current Input (NI PCI-6238)

[karthik9]

is there a way to measure voltage using 6238 . . . .

I have to measure the voltage across a releay . . . abt 12V and 40 ohm.... hence i cannot use any divider ciruits ....

i have to use a voltage buffer .... followed by a voltage to current converter .... is there any other simpler way to determine the voltage

[Henrik_Volkers]

Ohms Law?

[karthik9]

Ok let me explain my problem again .....  not sure but the way the device measure surrent is by finding the voltage across it.....   hence it has a inbuilt resistor.....so my relay coil is a resistor and not a currrent source ...which would cause a voltage drop ..... is there any other way without using buffers and v to i converters

[Henrik_Volkers]

Don't know the possible current ranges...

But if you know the internal current shunt value , one additional series resistor could do the job at the cost of a maybe low input impedance.

Otherwise a voltage buffer and the resistor should do the job. The V->I converter is the shunt and the resistor 😉 

If the OP doesn't drive the current needed, add a transistor .... (or for low cost maybe a simple pull up resistor aiding the OP is sufficient... )

Why not measure the current in the coil? It's the current that makes the magnetic field 🙂 and you can extent the current range with a bypass resistor .. (with apropriate rating 😉 )

[karthik9]

its written 0-20mA    . . . .  i cant measure the current directly as it is in a PCB

[Henrik_Volkers]

and what uncertainty do you need? coarsly 1%? if you add a 2.4k (+ shunt) in series to your 40 Ohm load ?  would add 5mA additional load to the app. 860mA ....

[karthik9]

OK let me try again ... btw sorry for the mistake it 400ohm

NOw the input of 6238 consists of a sense resistor(abt 100 ohm)  followed by an amplifier with some gain Ag .... I=V/R(hence you get the current by measuring voltage)  ,.... the current fed to this shall be from some current source ....   now the problem is i cant take voltage directly from the resistor as it would vary the voltage .....