S7-1500 AI RTD input, max supply voltage

Hello guys,

did you ever measure the voltage on RTD PT100 probe connected to the analog input card? Since temperature is measured with three wire RTD Pt100, I wonder what is the current or voltage on this probe during normal operation.

Thank you.

Whe using an RTD what you measure is resistance, some manufacturers apply a small voltage to the RTD and they read current. This way they calculate resistance.

For a 3-wire RTD without an integrated transmitter, the RTD inputs actually measure wire resistance from supply to sensor, as well as the current that flows through the sensor.
To measure the voltage drop there are 2 wires to the one side of the sensor.
Without the wire resistance (or voltage drop) compensation you would get a measuring error.

Because of the extra cost for going from 2 to 3 wires for long cables, and because of better noise resistance, it is more common to use an integrated transmitter that outputs 4-20mA.

widelto said:

some manufacturers apply a small voltage to the RTD and they read current. This way they calculate resistance.

Click to expand...

No, all RTD inputs supply a constant current, usually on the order of 1mA, read the voltage drop created by a constant current, calculate the resistance.

A DMM does the same thing, generates a current, reads the voltage drop, calulates the resistance.

Thank you very much to all, especially danw. I have read on many places that some small (known) current is applied, but couldn't find value. So, approximately voltage on the Pt100 probe is between 100 and 140 mV (for temperature range 0 to 100 deg C).

Now, if I want to check the Pt100 probe with regular digital multimeter (with battery 9V), what is the voltage, what would be the voltage on the multimeters probe?

The reason I ask these questions is I would like to test the insulation of the probe to the ground. Probe cannot be removed from installation place to test it in the calibration oven.
According to this information: https://www.gilsoneng.com/reference/rtdinfo.pdf
I could use low voltage megger (50V to 100 VDC), but wonder if digital multimeter can be sufficient.

Use a DMM on the lowest Ohms setting possible and change scale until you get a reading. Check from one lead to ground and then the other lead to ground. Should be on the order of megohms.

RTD's elements are insulated from the metallic sheath with MgO, which is hygroscopic, it absorbs moisture through the transition, which reduces its resistance.

You can read the resistance of an RTD element with a DMM. I don't know what current is sourced to make the reading (a better grade DMM spec might tell you), but the reading will include the copper lead wire resistance. Or put a 2nd Ohmmeter in series with the RTD when testing with the DMM and see how much current the DMM sources.

I've heard about low voltage insulation testers but have never encountered one.