Piggybacking 4-20mA Signals

[MisInformed]

If you are just looking at building a new PLC just for information gathering purposes why don't you look at some of the data acquisition software out there? Rockwell has RSView and other manufacturers make some version of this software. We use RSView for our ammonia monitoring system and for maintaining our tank temperatures for our state ag&mrkt requirements.

 

[shoeman]

4-20mA Conditioner

What everyone is talking about will work, and work very well. My concern would be that a failure in the loop, especially in the new data monitor, would keep the machine from running. If it is alright for that to happen, then forget my post. I have done exactly what you are talking about. I have done it in 2 ways.

#1. Run the 4-20mA to a signal conditioner that has 2 outputs.

#2. Feed the signal out of the control system via an output that is tied internally to the input.

Either way, a failure in your new system wouldn't effect the old one. I am used to being in an industry where availability was the driving force for design. Please forgive me if this is too complicated for your issue.

 

[Tom Jenkins]

Shoeman brings up a good point relative to loss of an indicator or PLC input card.

There is another way to accomplish this. You can use a "dropping resistor" on one or both of the devices, and use a voltage input on the PLC or indicator. When the device is removed, if the resistor is still in the circuit you still have continuity.

For example, if you have a 5 VDC input you can use a 250 Ohm resistor across the terminals, and 4-20 mA becomes 1-5 VDC. You loose some resolution on the signal, because most input cards are based on 0 VDC, but that isn't usually a problem. Note that you still have to make sure that the "-" terminal isn't tied to power supply "-".

Another trick I have used is to put a Zener diode across the terminals. Normally the current will go through the device. If the device is removed the current will pass through the diode, again maintaining continuity.

 

[pinworm21]

Have you thought about a data logger ? If all your doing is gathering information then if you have an extra serial port then a data logger might be the answer.


Dave

 

[danw]

Another trick I have used is to put a Zener diode across the terminals. Normally the current will go through the device. If the device is removed the current will pass through the diode, again maintaining continuity.

Tom,

Interesting approach I'd never thought of. Which zener do you typically use for 4-20/1-5?

Dan

 

[Tom Jenkins]

It has been so long since I needed to pull that particular trick out of my hat that I don't have the catalog number at hand. However, as long as the Vz is greater than the max voltage across the input you should be OK. (I think I used about 6 Volts Vz) You need to make sure the wattage is OK as well - for 20 mA @ 5 VDC you probably want at least 1/4 Watt.