I need to build an interface to measure an extremely low resistance device. The simple way would be just to make a voltage divider and measure the result. I have done that before.
In this case, I need to be extremely careful how much current I draw across the junction. I am not sure how much I can draw at this time, but I know it is not much.
My digital volt meter can measure the junction just fine.
The range I need to read is something like 0 to 50 ohms where the range I am looking for is about 1.5 ohm. Thats way down there.
If I put my device in series with a 10 ohm resistor, for example, and put the network across 5 volts I could have good range but I would be drawing about .5 amp across the junction and surely blow it up.
Alternatively I could use a much smaller voltage and then use an op amp to bring the reading up to the 0-5V scale the Arduino analog input would like.
I have seen a few sensors available that are 0 - 500 ohm that give a 0-5V output. One of those may work but I will be working down in the mud with a 1 ohm junction.
I would appreciate any advice as I start this project. Is anyone aware of a component such as the 0 - 500 ohm interface I see on the Internet?
Is there something that would be even better when reading low resistance like 2 ohms?
And why don't you change the reference to "internal" or use the "aref" pin and change the reference to external ). Read a good tutorial about "aref" in arduino.
Greetings.
Thanks everyone for the link and the good idea about measuring for a short period of time.
I am doing a project where I want to measure the resistance of a pyrotechnic device without setting it off. I can do some experiments with a circuit like this and see what happens.
There is little technical data about the device I want to measure so I will need to do many tests.
I have not worked with my Arduino for a couple of years -- So it will take a little while to get up and running again. This is my first dialog on the new forum format. Sorry it took me a while to respond.
I think that you will definitely need to use some external hardware so you can use a low constant current, hence a low voltage to measure, then amplify the signal to measure. But it should only need a couple of op-amps, or even just one. A particularly useful device for this sort of application is the CA3140, which can operate on 5V with an input voltage range down to below zero volts. Using a pulsed high current would give more volts but in your application could be hazardous! Take care...
A very simple non-inverting amplifier like this will allow you to measure the resitance as the voltage developed across a divider circuit.
Then you can make the measurement at a much lower (so safe) current.
I'd suggest an MCP6002 and R2=100k, R1=1k to give a gain of 100; then if Rs = 5k1 you will get a test current of 1mA and a 50E resistance will give 5V to the analog input, while a resistance of 1.5E will give 150mV - a reading of around 30 on the adc.
An digital INA226 current sensor breakout board could be a better option.
Simply remove the built-in 0.1ohm shunt resistor, connect one input to ground, and the other input with a suitable value pull up resistor to 5volt. The 16-bit INA226 has a full scale of 82mV, so only a low pull up current is needed.
The 12-bit INA219 can also be uses, but with one input connected to 5volt and a suitable pull down resistor.
Leo..
The other thing to think about is lead resistance - so you ought to look at supplying the current / voltage though two wires and measuring the volt drop with another two ( like you would with a PRT ). … ( or maybe one - thinking about ground issues)
Using the internal voltage reference (~1.1volts ) you can resolve to 1mV , so you can do some sums based on that ( eg 100mv drop on 1ohm, is 100mA , so you can resolve to 0.1ohm ).
You also need to think about range of measurement( eg that 100mA through 50ohm is 5v , too high for using the internal ref ) and do the sums fit accuracy/range compromise .
Needs a precision current source !
Op amp boosting the signal helps … getting complex !
