Relays in Current Loops


Thread Starter


I am working on a poor man's redundant PLC design. My last issue is to isolate the analog outputs from each other (PLC A & PLC B). Lowest cost is to insert pairs of relay contacts to interrupt the loop not being used. Would this method be reliable? AO signals are VFD speed commands from a CompactLogix PLC.

1) I once ran 2 wire loop powered transmitters' loop current through a relay. Any momentary open circuit would trip the transmitter into its initialization routine when DC power came back, even milliseconds later. The initialization routine left the loop without a PV or a false 4.0mA PV until the field instrument woke up and got its first cup of coffee, which was on the order of tens of seconds after the switch over.

It warranted experimentation with a make-before-break relay and the actual brand/model in use. It was non-contact level (either radar or ultrasonic, can't remember).

2) I can't recall having run a powered AO through a relay, but the momentary loss of signal could trigger loss-of-input-failsafe operation and invoke a subsequent initialization routine.

What does the VFD do on temporary loss of its input signal ? Will it "hold last" until a signal is re-established?

Bob Peterson

I have used relay contacts in analog output circuits just to make sure the analog output turned off completely. Usually I would do something like this on the steam valve where I wanted to make absolutely sure that the steam valve was closed.

I have never tried it but back in the dark ages of plc when analog inputs cost $500 a point , I know that it was fairly common to use Reed relays to Multiplex analog inputs. Usually they would use voltage inputs so there was no interruption of current to the transmitters.
An ABB ACS-550 for example, can be set not to trip on loss of reference input. Set that way, the momentary 0ma signal would still be interpreted in accordance with normal AI scaling. However, the decel ramp is probably 5 seconds or so, so you probably wouldn't even notice a bump in motor speed, plus you can set a filter period on the AI anyway (which I typically set to 0.5 - 1.0 sec), so the 20ms or whatever of 0ma would never even be interpreted as a zero speed reference, even momentarily, if you set that filter. Your mileage may vary depending on how your drive handles these issues.

BUT...what would concern me is what happens to the AO if it's working and you open the circuit? It might be damaging, especially if you cut over frequently. So I'd find out what the drive input impedance is, and put a zener diode on the AO's between the PLC and relay at a couple volts higher than the normal VFD AI voltage drop. That way, the AO current has somewhere to go when you open the normal current path through the VFD AI.
Interesting point about open circuiting the AO, Steve.

In 35 years of dealing with single loopers/PLC/PAC AO's or panel meter re-transmit AO's I can't recall an AO that was ever damaged by "open circuiting" it.

The firmware driving the AO, however, is very likely to complain and squawk loudly and throw out flags and error messages that the AO is dysfunctional. In order to quiet that kind of noise, switching the output through a fixed dropping resistor works great. The resistor doesn't even have to be 'precision' resistor, just something like a cheapie 5% 100 ohm 1/2W, something to close the circuit path to keep the firmware happy and content.

Thanks for all the help

VFD AI filter is 1-120s

VFD loss of input is none (last input hold)

I was really wondering if there would be noise on the loop due to the relay contacts, but I see there are other considerations. If the backup PLC is ever activated, it will be because none of the 4 VFDs are running so switchover is not an issue.

Since I don't like inserting circuit elements in the normal circuit an alternative is to run each loop (4-plca, 4-plcb) through signal isolators (AutoDirect FC-33) and using the relays to interrupt the isolators 24vdc power supply.

Thanks to all.
Pepperl Fuchs and Phoenix contact manufactures switching devices specifically for this sort of application. I have used it once for a similar type of application as yours. The one I used was of pepperl Fuchs. You can look at their product catalog as I don't remember the product number /model number which I used.
Consider using a relay contact in series with each output (2 separate relays or a single relay form C). Across each contact to common negative have a 6 Volt Zenner diode. When the contact is closed the Voltage will be 1 - 5 Volts (less than what it takes to make the zenner conduct, When the contact is open the Voltage will rise to 6V and the zenner will conduct the analog output current to common negative maintaining the loop current ready for next time it's required.

I did something similar with a compact PLC when I ran out of analog inputs, I had something like a dozen wells to monitor, each loop was maintained by a diode. A discrete output module sequentially sampled the current to a single analog input by dropping the Voltage to less than the diode forward bias, it could easily multiplex 10 readings a second though in practice I had it set to 1 per second as the wells only changed slowly (tidal).