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Old January 8th, 2020, 09:47 AM   #1
MartyMaucher
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1756-IF8 AI card drops current loop for a second

Troubleshooting an issue in which a 1756-IF8 AI card is usually working fine reading a ~12mA current loop from a device. When firing a relay from an adjacent relay output card, the current loop drops to 0 mA for about a second.

This seems to be caused by something in the IF8 card as the current loop does not change when bypassing the card/PLC. Got another card on order to test that theory, but does that problem sound familiar to anyone?

The IF8 card is configured as 8 single-ended 0-20mA inputs. Thanks for any info.

Last edited by MartyMaucher; January 8th, 2020 at 10:37 AM.
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Old January 8th, 2020, 12:20 PM   #2
GaryS
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I would look at the loading when the relay pulls in
both cards may have a common power supply that is adequate.
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Old January 8th, 2020, 12:49 PM   #3
Mickey
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Quote:
Originally Posted by GaryS View Post
I would look at the loading when the relay pulls in
both cards may have a common power supply that is adequate.
If so, is there surge suppression on the relay coil?

I think it's always a good idea to have different power supplies for analog and digital input/outputs.
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Old January 8th, 2020, 01:00 PM   #4
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Quote:
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If so, is there surge suppression on the relay coil?

I think it's always a good idea to have different power supplies for analog and digital input/outputs.
if the power in pulled down with the coil energized a surge suppressor will not help they will only absorb the collapsing field surge when the coils is turned off

I fully agree that a separate poser supply for outputs is preferred for outputs
relays with a large load should have an Interpol relay to control them
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Old January 8th, 2020, 01:11 PM   #5
Mickey
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On second thought I should not have quoted your post. My comment was more about using the same power supply, if that is what the OP is doing.
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Old January 8th, 2020, 02:52 PM   #6
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Update: The device sending the 4-20mA signal is a new model ~$20K gas analyzer. After grounding the signal shielding wires, the current loop doesn't drop to 0 as often when activating the relay outputs (to valves).

I think the problem is in the device being sensitive to some electrical noise and it stops sending the signal for about a second and a half. One of those intermittent problems, but I'm leaning towards the PLC not being the cause.

I will look into more surge suppression on the relays, valves, etc. Thanks.
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Old January 8th, 2020, 03:29 PM   #7
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Make sure that you only ground the shield at one point only usually at the source.
if you use more then one ground point you create ground loop currents that can interfere with the signal.
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