Shorting an analog output on purpose? Options

[Bob O]

I am going to assume the AO is 4-20 mA and I know the hardware is a Flex 1794-OE4.

Has anyone ever been asked to short the analog out [Example terminals A-0 to A-1] of the card under a certain set of safety conditions or any condition?

The terminals would be shorted using a set of contacts from a relay.




Thanks

[Mickey]

I am going to assume the AO is 4-20 mA and I know the hardware is a Flex 1794-OE4.

Has anyone ever been asked to short the analog out [Example terminals A-0 to A-1] of the card under a certain set of safety conditions or any condition?

The terminals would be shorted using a set of contacts from a relay.




Thanks

I have seen "open" it but not short it. Why short it? For what purpose?

[Bob O]

Mickey,
I am told they want to do this to force the signal to 0 and in turn force the valve to the closed position.

[Alaric]

I saw it once on a vacuum furnace - the over-temperature alarm relay shorted the 4-20mA command coming from the temperature controller. If you have a form C relay then you could use one contact to short across the input of the field device and the other contact to open the 4-20mA loop, that way you don't have to worry about the load on the analog output.

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[Alaric]

Depending on the input impedance of the the valve and the output impedance of your PLC AO you could throw in a small resistor as long as the overall impedance stayed lower than the max impedance of the AO. In theory the 4-20mA AO should be able to deal with a short but I wouldn't bank on all manufacturers building their product that way. The command shorting relay on the vacuum furnace I mentioned was shorting across a very large rack mount Honeywell controller with a voracious power appetite and a power supply to match.

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[Bob O]

Alaric,

Here is what they are proposing.

Attached File(s)

 Analog_Output.bmp ( 62.87K ) Number of downloads: 36

 

[Mickey]

Mickey,
I am told they want to do this to force the signal to 0 and in turn force the valve to the closed position.

Yes, that's the reason I have done this. But Alaric post number 4 show a good method.
I didn't short the field device though. Just open the circuit.

[Alaric]

If that is what the customer is proposing then give them that. Their design. Their responsibility. As I said, I've seen it before. As a precaution however I would configure the output for 0-20mA and program it to output 0 mA when ever CR06330 is off and operate in the 4-mA range whenever CR06330 is on.

[Bob O]

Mickey and Alaric,

Thank you for your help.

Bob