low voltage problem

mcadaj

Member
Join Date
Apr 2020
Location
Texas
Posts
2
I have a analog card 1715-IF16 with six 4-20ma instruments on it.
The card in a single-ended method. The instrument has been service for a while with no issue. One day lost signal and check the output voltage was 4 vdc and 0 ma. Disconnect the voltage goes to 20vdc. The transmitter reads fine when reading from a fluke MM with loop power. Tried another point and would read good for 3 minutes or so, than slowly drains voltage to 4 vdc. Tried a simulator and results the same. The card is remotely wired using a common ground. Must say the other 5 transmitters are reading o.k..
Any advice would be helpful
 
Not that I understand in the slightest, but



does the problem stay with the transmitter or with the channel on the 1715 (i.e. swap two transmitters' connections to the 1715)? Or is changing the channel what you meant by "Tried another point?"
 
Take one of the 5 working ones and put it in the suspect input - if it reads your sensor / wiring is bad, if it doesn't, your input is bad.
 
Just a guess. With the transmitter attached to the Loop Tester you have no earth and so no earth problem. The answer is to replace the sensor. Reading OK for 3 mins then draining away still says replace the sensor. Just be glad that the other 5 points read correctly, we have had problems where one bad sensor causes the rest to have a fit. When diagnosing it is always the last one you test that causes the problem, never the first one!!
 
Firejo
I thought that voltage supply also, but the 24vdc ps is distributed to other FTA's and seen no problems with other instruments.
BryanG
I asked the inst. tech to replace the transmitter even with no issue with the device. Haven't heard back.
janner_10
Can't move other devices to the channel or it will trip equipment. Moved the inst. to other channels with same results
Thanks all for your suggestions, but I'm thinking it has to be a grounding issue.
 
Firejo
I thought that voltage supply also, but the 24vdc ps is distributed to other FTA's and seen no problems with other instruments.
BryanG
I asked the inst. tech to replace the transmitter even with no issue with the device. Haven't heard back.
janner_10
Can't move other devices to the channel or it will trip equipment. Moved the inst. to other channels with same results
Thanks all for your suggestions, but I'm thinking it has to be a grounding issue.

Hello my friend.
I know it's been a long time, but I'm having a similar problem, sometimes some channels fail and I needed to relocate some instruments to other channels. I suspect grounding. Did you solve your problem?
 
What do you mean common ground
I have seen way to many badly wired analog signals
I would like to see exactly how they are wired
Each analog input needs to have it’s own source and return
Some instrument drivers regulate the neg return not the + source
As common return could cause problems
Without having all the details it is hard to say what’s going on
You could be getting some stray voltage in on the common
A leg on the common ground may have just opened
Try this, completely disconnect all the other inputs and see what happens
Also try this disconnect the problem instrument and put a 250 OHM resister across it measure the voltage across the resister 20 ma should read 5 vdc
4 ma should read 1 vdc
 
You can have a common cable, for example 0V DC, that powers different devices such as solenoid valves, actuators. These devices do not care that the voltage drop of that cable varies by 0.2 V when connecting or disconnecting a device.

But if this cable is used as a common analog signal cable, those voltage variations do matter a lot.

As GaryS says, remote sensors must have exclusive cables.
 

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