Siemens S7-1200 0-20MA analog output issue help!

[Armament]

We were tasked with repairing units that were supposedly started and commissioned but by the looks of it went horribly wrong. Long story short we have six wall mount air handlers that were built by "technicool".

The issue!

All digital inputs, outputs and analog inputs function. There is one 4-20ma output to a board that fires an scr for first stage heat. Programming has been confirmed by multiple programmers to be correct. No matter what the program is sending as an output, when the load is applied to the scr card, I'm reading about 42-43ma.

-The main plc and expansion card is grounded to the ac ground source

-The 120v line side is grounded in power supply
-The 24dc return on power supply is grounded.

There are two analog outputs and the common return. Output (channel?) 0 is not used and output 1 feeds the scr card and the return comes back to M.

We simulated 4-20 directly to the card and it works perfectly. As soon as it is hooked to the output it's 40-44ma and does not differ with the commanded output.

Any ideas what's going on here? Myself and the PLC technician are moving in day four with these things and we've just about exhausted every thing we could think of.





*the output shown to RDA does not terminate anywhere.

[clogic]

We were tasked with repairing units that were supposedly started and commissioned but by the looks of it went horribly wrong. Long story short we have six wall mount air handlers that were built by "technicool".

The issue!

All digital inputs, outputs and analog inputs function. There is one 4-20ma output to a board that fires an scr for first stage heat. Programming has been confirmed by multiple programmers to be correct. No matter what the program is sending as an output, when the load is applied to the scr card, I'm reading about 42-43ma.

-The main plc and expansion card is grounded to the ac ground source

-The 120v line side is grounded in power supply
-The 24dc return on power supply is grounded.

There are two analog outputs and the common return. Output (channel?) 0 is not used and output 1 feeds the scr card and the return comes back to M.

We simulated 4-20 directly to the card and it works perfectly. As soon as it is hooked to the output it's 40-44ma and does not differ with the commanded output.

Any ideas what's going on here? Myself and the PLC technician are moving in day four with these things and we've just about exhausted every thing we could think of.





*the output shown to RDA does not terminate anywhere.

The use of an isolated 4-20 MA test signal is most interesting and appears to prove a signal isolation issue.

We have used an Analog Isolation Module for just this scenario. The inputs and outputs are optically isolated.

Available from many sources AIM1

Cut Sheet:


http://e-controls.net/media/pdf/greystone/aim1_spec.pdf

[orion242]

Ditto. The SCR input likely does not care for the grounded reference. Willing to bet you need to isolate it.

[Armament]

The problem is the card is not outputting anything. For the Siemens analog outputs do they generate their own 0-20 am signal and you do not require an external power supply like the inputs? I have my doubts. At this point our only last standing theory is all of these scr cards have been replaced due to the fact there was 120v incorrectly wired to the cards. It blew the chips off all of the scr cards. I believe it backed into the Plc and caused line voltage damage to the outputs. We are currently going to experiment with an add on ao board to see if this is the case. Hopefully the add on board is isolated from the base module outputs and doesn't share any circuitry. With the wired detached from the base module you cannot read any current or voltage coming from the outputs. At first I thought this may have been a characteristic of Siemens where if the module see's an OL value it doesn't output any current.

Finding information on how the hardware behaves on these plc's has been challenging. For the past few years all we've dealt with is Allen Bradley and we became pretty familiar with them. I'm almost positive the outputs are of the sourcing type and not to merely regulate the current.

[craftyringwiseveteran]

Disconnect the AO and slap a 499 Ohm resistor across the output. If you can't get the voltage reading across the output to go from 2-10 VDC you know the issue is at the controller end. Otherwise, when in doubt, isolate!

[Armament]

Disconnect the AO and slap a 499 Ohm resistor across the output. If you can't get the voltage reading across the output to go from 2-10 VDC you know the issue is at the controller end. Otherwise, when in doubt, isolate!

We did a lot of experiential with 250 and 500 ohm resistors. Cards should be in today. I'll know more when we try