A fresher's question

There is an 4-20mA analog signal from a temperature transmitter connected to a PLC analog input. If I'd like to connect the same signal to a panel meter, is it possible to do this by paralleling the connection from the PLC?

No, you have to series it.
You also have to check the loop impedance, a lot of 4-20ma signals only operate up to 500ohms. You will have to check the appropriate manuals for this.

Doug

We put several devices in series on a 4-20 mA loop all the time, but besides the two big items Doug metioned, there are a couple of minor considerations. First, some panel meters and some PLC inputs have one side of the 4-20 mA input tied to power supply common. If that is the case the second device in the loop won't read the signal. Second, loop powered indicators get their power by running the current through a diode to get 3 Volts or so power. The literature may say something like "150 Ohm impedence at 20 mA" which is true. But at 4 mA that is the equivalent of a 750 Ohm impedence.

You should generally make sure you use an externally powered meter with an isolated input or else use an indicator with a re-transmit capability. These are common and fairly inexpensive.

If the sensor is a two-wire device, the limitation is not the sensor but rather the power supply voltage. Check your sensor data sheet as it should list a formula to calculate the required power supply voltage.

BTW, a panelmeter designed to read current will typically have very low impedance and will probably cause an insignificant amount of load to the current loop when connected in series, but it must be isolated as Tom indicated above. If it is an analog panelmeter with just two connections, you should be fine, but if it is a digital panelmeter, you might have to provide isolation.

The other option is to use a panelmeter that reads voltage, and connect this in parallel across the current sense resistor of your analog input channel.

Typically this will be a 250 ohm resistor to generate a 1 to 5 vdc signal. This implies that your panelmeter has some 'smarts' so that it can be offset and scaled to read your engineering units, unless you just want to read the raw signal for diagnostic purposes.

O.K., Everyone but lucy gets an A- .

TO: The Instructor

I had posted a response to get the person to think but I guess it came across as being nasty so it got pulled. The whole question boils down to ohms law and that's what I had questioned back. If they were able to answer my question then they already had the answer if not then maybe a refresher course?


Drewcrew6

You could also use an loop isolator. See the following link for an example:

Weidmuller Isolator

If you use an isolator then you can remove the panel meter without affecting the signal to the PLC. I.e. perform maintenance.