4-20ma PLC Input Only Reading Lower Half of Range

[GaryS]

where are you getting the 4 volts max from I see nothing there that shows that
the supply for the sensor is 10-30VDC an industry standard
the data sheet says it short circuit protected 10 to 30VDC
the min operational supply voltage is 10VDC
10/380 = 0.026 amp or 26ma
it would be able to drive the load at that
24/380 = .063 amp or 63 ma
24/200 = .12 amp or 120 ma
keep in mind that these are current regulator devices they will try to maintain the current no matter what the supply voltage is as long as it stays with in the rated range 10-30VDC
when they design the input modules they know what current and voltage will be used
all the ones I have seen actually read the voltage drop across the burden resister with a differential opamp
try to hook it up with the grounded leg of the power supply and see what happens
I looked at the sensor data sheets they are showing the input module is connected on the grounded leg of the supply
the ifm module shows connecting the sensor the same way
both the outputs X and Y are shown with the load connected to the -DC supply or Grounded side
output X - burden resistor -DC supply
output Y - burden resistor -DC supply
just make sure that both the ifm input module and the sensor are connected to the same -DC source and it should be bonded to the ground conductor

 

[Aabeck]

If the PLC is reading up to the point where you get 4.5 volts, then stays steady as the voltage stays steady is the PLC input a Voltage Input, expecting 0 to 10 volts?

 

[John Soltesz]

1. The sensor will work only with a max of 200 ohm load.
2. The input impedance is 390 ohms.
3. Check Basic DC Circuits 101.
So for a test put a 500 ohm resistor across the sensor output.
Measure the voltage across the resistor while moving the sensor through its physical range.
The voltage should be 2-10 vdc.
If so then a work around is change the input to 0-10vdc, use a precision 500 ohm resistor across the input and connect the sensor across the resistor. Scale appropriately and off you go to the next incompatible device.

 

[GaryS]

it's clear to me that you are using the ma input
data sheet current input resistor is 380 ohms
voltage input resistor is 50K or 65K depending on the input used
you would never be able to push 12ma through that resistor with a 24 VDC supply
again the problem has to be in the way it is wired
make sure that both the input module and the sensor are connected to the same power supply and recheck the polarity on all connections
the sensor outputs must be connected to the ifm input
internally the inputs connected to the -DCV supply through the 380 ohm resistor the electronics read the voltage drop across the resistor
separate power supplies with the -DCV not connected together will give you all kinds of problems. There has to be a common reference point for everything to work correctly.
double check that the wiring matched the diagrams shown on the data sheets

 

[John Soltesz]

1. The sensor will work only with a max of 200 ohm load.
2. The input impedance is 390 ohms.
3. Check Basic DC Circuits 101.
So for a test put a 500 ohm resistor across the sensor output.
Measure the voltage across the resistor while moving the sensor through its physical range.
The voltage should be 2-10 vdc.
If so then a work around is change the input to 0-10vdc, use a precision 500 ohm resistor across the input and connect the sensor across the resistor. Scale appropriately and off you go to the next incompatible device.

Oh crepe! I got this wrong. Due to the sensor limit you need to use a 200 ohm or less resistor across the 10 volt input. That gives you a max volt to the input of 4 vdc. It still can work but the resolution is a bit less.
Why would the sensor have such a low impedance limit and the input card have such a high impedance limit? Poor combination choice.

 

[mendonsy]

Oh crepe! I got this wrong. Due to the sensor limit you need to use a 200 ohm or less resistor across the 10 volt input. That gives you a max volt to the input of 4 vdc. It still can work but the resolution is a bit less.
Why would the sensor have such a low impedance limit and the input card have such a high impedance limit? Poor combination choice.

I don't think so! The way I read that the minimum resistor value would be 200 ohms. A larger resistance is less load.
Most PLC 4-20ma analog inputs have a 250 ohm resistor so the built in resistor should work OK.

 

[GaryS]

You seem to be making things far more difficult then it needs to be
The manufactures that design and build both the sensors and the I/O modules spend a lot of time testing there equipment before they release it, they know what they are doing.
You are way to concerned with the 280 ohm resistor in the input module. That is really not relevant as long as you have the power supply to drive the load. With the 4 wire sensor you have more power to drive the load. The 10 volts will drive a 500 ohm load with a 280 ohm resister you only need 6 volts to drive 20ma. The higher the resistance the higher voltage needed to drive the 20ma through
Remember ohm's law voltage / amps or voltage / resistance
So as I have said before your problem is in the way you have it wired. You must connect everything exactly as shown in the data sheets. You have proven this in your own testing. You have stated that the output goes through the full range when a resister is connected between the output and the –DCV. That is exactly the way the data sheet shows it connected. Forget that that 280 ohm resistor is there. Its just a current input.
Just make sure that both the sensor and the input module are connected to the same –DCV supply

 

[bernie_carlton]

RETs post #13 (use a signal conditioner - example given) is the only viable solution given the sensor limitations. Ohm's Law isn't just a suggestion.

 

[Bering C Sparky]

Ohm's Law isn't just a suggestion.

Now there is as good a Signature Tag Line as I have ever seen! :site:

 

[Aabeck]

Ohm's Law isn't just a suggestion.

Of all the laws ever written (and you know this if you've ever been in my car) Ohm's Law is generally the only law I obey.

 

[mendonsy]

Of all the laws ever written (and you know this if you've ever been in my car) Ohm's Law is generally the only law I obey.

And probably a couple of Issac Newton's too, like gravity for example.

 

[Aabeck]

There's a couple of railroad crossings around me that allow me to break the law of gravity when I'm going fast enough.

 

[Bering C Sparky]

There's a couple of railroad crossings around me that allow me to break the law of gravity when I'm going fast enough.

Actually that is only kind of half right.

On the up side of the curve I suppose you could say you are breaking this law. (In a loose form of speaking)

On the down side of the curve you are most assuredly reinforcing Newton's theory in a big way.

LOL

BCS

 

[danatomega]

Gravity:

 

[standeven]

Sorry about the late reply, I just got back from holidays. Though watching the thread derail into a discussion on gravity was entertaining

Several of you nailed it: the sensor required a load resistance of less than 200 ohm. With the PLC input resistance being ~380 ohm, the sensor was unable to reach 20 mA.

Our solution for this prototype was to wire in a ~400 ohm resistor in parallel to ground, which brings the load resistance to less than 200 ohm and the sensor can operate over the full range. Resolution, however, is halved.

For future production we'll be switching the sensors to ones that can handle a higher load resistance.

Thanks everyone for the help!