I just wanted to check before I wire it up, I have a 24v 2-wire 5-20ma pressure sensor.
Do I just connect this through the shield as part of the loop with the power supply (and disconnect jumpers W1-W4) - I'm guessing there's no need to use a resistor to measure the current drop across as required without a shield?
Thanks.
Is this the correct way:
https://www.dropbox.com/s/j4sz4pop5uk4fiz/5-20ma.pdf
Or is 24v too high to use?
I'm puzzled which Olimex product you mean!
Avoid voltages outside what is intended...
John
4-20mA is easy but remember it has to go into an ADC input.
You NEED to use a sense resistor and if you check the sensor it should work with anything from 8 to 32V in most cases but +24V is a common supply for them.
Connect the +24V to the + on the sensor. The GND of the 24V supply connects to the GND on the Olimex board (which I assume is +5V powered) DO NOT CONNECT to any ADC input as shown your drawing.
Connect a 0.1% precision resistor between GND and the ADC input. Connect the - of the sensor to the ADC input and the resistor is now acting as a current shunt.
The resistor value will dictate the voltage range input to the ADC. With 100 ohms this will be 0.4 to 2.00 volts. A 200R resistor will give 0.8 to 4.00 volts.
Don't worry about the fact that the sensor uses +24V for as long as you have the resistor in the circuit, the ADC will never see the high voltage.
If you need a drawing for this to make sense let me know and I'll post one. It's late just now so will have to wait for next time :)