AI problem with Siemens - Drives me mad!

Hi everybody,
I have a strange problem with Siemens EM 231 - 0hcc22 - 0xa0 s7-200s analog input module.

I need to measure 4 analog values. Two of them are pressure and the other two ara temperature.

I have no problem with tempertatures.

The pressure transmitters are siemens 0-10bar 4-20mA. I have to measure the pressure about 1.5 bar and the measurement has to be sensitive. So i measure pressure in the same pipe and take the average.

The problem is when i connect any of the transmitters to AIW2 ,
AIW2 takes 32760 value and in addition to this, other three analog inputs read 4000 more. I will give values definitely to be more clear. transmitters are named trans1 and trans2 , connector of AIW0 is A , connector of AIW2 is B.

1)A is connected to trans1 OR trans2, B is disconnected
AIW0=10560
AIW2=0
AIW4=12150
AIW6=17340

2)A is disconnected , B is connected to trans1 OR trans2
AIW0=4000 ???
AIW2=32760 ??
AIW4=16250 = 12150 + 4100 ?
AIW6=20400 = 17340 + 4060 ?

3)A and B are both disconnected
AIW0=0
AIW2=0
AIW4=12100
AIW6=17200

4)A and B both are connected to any of transmitters
AIW0=14500 = 10560 + 3940
AIW2=32760
AIW4=16000 = 12100 + 3900
AIW6=21100 = 17200 + 3900

As you see, there is no immediate value for AIW2. it is 0 when disconnected and 32760 when connected.
And more strange thing is that , when AIW2 is connected and reading 32760 other three analog inputs read approximetly 4000 more.

i had given 24V DC to the second input directly to make the machine work as a temporary solution and it was remained so for 2 days. i guess i may have damaged the input myself, because 2 days ago i have changed the analog module and transmitters. after changing them , i read normal values from both AIW0 and AIW2.

A stranger thing is that i have changed the old module becasue of the same AIW2 problem.

Analog module is supplied with 24v dc from the smps in the panel and their 0's are same.

Strange problem as you see.. any ideas?

Hi emokar.

had given 24V DC to the second input directly to make the machine work as a temporary solution and it was remained so for 2 days. i guess i may have damaged the input myself, because 2 days ago i have changed the analog module and transmitters. after changing them , i read normal values from both AIW0 and AIW2.

Click to expand...

You mean that you have put 24V directly on the 4-20mA input, with no transmitter in between. If that is so, then the analog input may have been damaged. Many analog current inputs cannot survive 24V directly for any longer period. It depends on the size of the meassuring shunt. I dont know if this applies to the analog card you are using, but if you say that you exchanged the card and the problem was resolved by that, then it is a strong indicator that this was indeed the problem.

if the input is damaged because of 24v applied , does this explain why other inputs are 4000 (4000/32760) more than normal?

Well. If the circuit board has become physically burned, then all kinds of strange behavior can happen.

I agree with JesperM, the input is v. likely damaged due the excessive current. for the future: if you don't want to use a analogue input 4-20mA, you should terminate it with a resistor. siemens recommends a value from 1.5 k to 3.3 k. if you don't do that you may have the wrong measurements on the other channels as described.

I was worried that i would damage the input by applyind 24vdc directly but i had no choice beacause stop of that test machine would cost more than an analog module. i wish i had a resistance with me

but i still couldn't understand the behavior of module. why does it add 4000 to all inputs?

I dont know the S7-200 that you are using, but for S7-300 the input resistance for current signals are typically 25 ohm.
That means that at 24V, the resistor has to disipate 23W. Most resistors that are used in electronics these days are 0.5W or less.
It says for the abovementioned analog input that 40mA (=0.04W) is the "destruction limit".
If you open up that analog card, you will find that the resistor and the surrounding area has been totally burned.

The surprising thing is not the funny values when you connect the damaged input, but that the other inputs continue to work if you do not connect the damaged input.

Hi emokar

The resistors in the EM231 and EM235 are 250ohm which with a 24V supply would give about 2.5 watts and which probably has blown the resistor. First thing is to replace the analogue module. The next stage is to track the problem. I also use the EM231 for measuring two pressure and two temperature, my problem is that machines are spread around the world and so it is not possible for me to visit them to trace problems. The guys doing the checking for me are usually mechanical fitters, nice guys, but fault tracing is not their thing. So I keep it simple, first identify one transmitter that you can confirm is working, in your case I would probably choose one of the temperature transmitters. Power down, mark all cables and then disconnect all but the known transmitter and in your case connect it to AIW6, check that it reads OK. Power down and move the connection to AIW4 and so on until you have confirmed all inputs give a similar reading and so are working. Now connect the tranmitter back to its normal connection. Next step is to connect the suspect pressure transmitter into AIW2 and see if AIW6 varies. If no problem appears then proceded to connect the other inputs one at a time. If connecting the suspect transmitter does give a problem you have two possible sources, one is the transmitter the second is a ground loop. If it is easy then first replace the tranmitter. If the error persists you need to add 4-20mA signal isolators to the pressure transmitters to correct the problem.

Regards

Bryan

thank you for your attention.

i wonder if i would damage the input by applying 24vdc if it was a 0-10V voltage input..

Hi

I think these voltage ranges are getting confused. (or is it me getting confused by the voltage ranges?)

An analogue input expecting to see a signal ranged between 0V and 10V is just that. The maximum, give or take whatever the manufacturer's tolerance may be, is 10V. Say +10% or 20% tolerance would enable the input to accept up to ~12V without damage.

Now the device which controls what that voltage to be measured is, the transmitter, will require to be powered in order for it to operate. It has its own circuitry etc, and the most common voltage used for powering field sensors and transmitters is 24V.

This does not mean you can connect a circuit with a 24V potential across the terminals of your analogue input. You'll probably exceed the normal operating voltage (10V) by about 140%. Not many manufacturers build in tolerances of +/-140% on components!

Ken

Hi emokar

Maximum voltage on the EM231 is 30V d.c. so you would be OK at 24V. When you add the link to make it into a current input though you connect that voltage across a 250 ohm resistor. One other thing you can't have 4-20mA and 0-10V on the same module. It won't cause damage but the voltage input range will be limited to 0-5V.

Hi Ken, you posted whilst I was writing. The vaules given above are from the Siemens manual.

Regards

Bryan