# how to measure resistance ?

good day,

could you all kindly assist me in designing a resistance meter using arduino UNO ? or mega ?

i'm trying to measure resistance in a three phase cable.

usualy we do it at work via a regular multi-meter but we have to change the probes position manually to measure resistance :

AB
BC
AC

and the unit is ohms

i have tried searching the internet but, nothing usefull !

Three probes and a box with a three position, two pole switch of some sort.

You could design a voltage divider and use the wire itself as one of the resistors. This would give you a voltage drop, which could be measured on your analog pin. You would want to design the resistor value to give you a good range of voltage drop. What range are you expecting to see in resistance of the wire? The smaller your range, the more accurate you can get a measurement.

The calculation for a voltage divider is Vout=Vin*R2/(R1+R2) and is wired like this

I made a quick online Xcel spreadsheet to demonstrate how it works. Download it to play with the numbers. One problem with the arduino is the ADC does not have a very high resolution, so your accuracy may not be great. Microsoft OneDrive - Access files anywhere. Create docs with free Office Online.

What range of resistance values are you interested in?

D2 -- resistor -- A
A2 -- A
D3 -- resistor -- B
A3 -- B
D4 -- resistor -- C
A4 -- C

AB
D3, A4, and D4 set to input without pullup
Change D2 to an output and set High
Change A3 to an output and set Low

Same thing for BC and CA.

The resistor is selected be low enough to put the voltage across the phase within a useable range, but high enough not to overload the output of the Arduino (absolute max 40mA, better to stay at or below 30mA max).

If you use the internal reference voltage, low voltages may not require amplification.

ironchariot:
The calculation for a voltage divider is Vout=Vin*R2/(R1+R2) and is wired like this

I made a quick online Xcel spreadsheet to demonstrate how it works. Download it to play with the numbers. One problem with the arduino is the ADC does not have a very high resolution, so your accuracy may not be great. Microsoft OneDrive - Access files anywhere. Create docs with free Office Online.

thank you for the responce,

my rage is from 0 to 10 ohms maximum, because we will be measuring resistance in a three phase motor

MarkT:
What range of resistance values are you interested in?

thank you for the responce,

my rage is from 0 to 10 ohms maximum, because we will be measuring resistance in a three phase motor