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Old July 13th, 2011, 04:57 AM   #1
flyers
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Analog signal with different source of supply

Hi,

We have a few units of modulating valve with 4 terminals;
1. 24V AC/DC +
2. 24V AC/DC -
3. Signal IN DC 2 to 10V
4. Signal OUT DC 2 to 10V

Signal IN = Analog signal from PLC to control the valve
Signal OUT = Analog signal to PLC as valve position feedback.

Can I tap supply the voltage (terminal 1 & 2) from another a Power Supply Panel and the Signal IN & OUT (terminal 3 & 4) wired to my PLC panel.

Power Supply Panel is located another level from the PLC Panel.

By doing this connection, the common of the analog voltage is different, is that possible?

Thanks in advance.
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Old July 13th, 2011, 08:56 AM   #2
Tom Jenkins
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The 24 VDC terminals in most valve systems are provided to power discrete signals, like limit switches, Local/Remote switches, etc. You can use them for anything, including loop power, but you need to make sure your load doesn't exceed the supply limits.

Now then, if you are using a voltage signal for control and feedback you don't want to power them at all. Most voltage devices are self-powered. In other words, the PLC output will provide the power needed for the output signal, and the valve will provide the power for the feedback signal.

I'm a little suspicious about the 2-10 VDC value. This sounds like you are using a 4-20 mA signal and running it through a 500 Ohm resistor to create the voltage signal. If that's the case you really have a current loop, and the wiring may be a little different. Have yo looked at teh valve manual? Another option is toal to your valve supplier - they should be able to help with details.
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Old July 15th, 2011, 06:10 AM   #3
flyers
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Quote:
Originally Posted by Tom Jenkins View Post
The 24 VDC terminals in most valve systems are provided to power discrete signals, like limit switches, Local/Remote switches, etc. You can use them for anything, including loop power, but you need to make sure your load doesn't exceed the supply limits.

Now then, if you are using a voltage signal for control and feedback you don't want to power them at all. Most voltage devices are self-powered. In other words, the PLC output will provide the power needed for the output signal, and the valve will provide the power for the feedback signal.

I'm a little suspicious about the 2-10 VDC value. This sounds like you are using a 4-20 mA signal and running it through a 500 Ohm resistor to create the voltage signal. If that's the case you really have a current loop, and the wiring may be a little different. Have yo looked at teh valve manual? Another option is toal to your valve supplier - they should be able to help with details.
Tom,

Thanks for your reply, based on the drawing extract from the product catalog, the 24V AC/DC should be for its operation (supply from panel).

The client request us to lay a 2 core cable (for this valve power - terminal 1 & 2) from another DB panel. While the signal (Terminal 3 & 5) will lay a 1 pair screen cable to our PLC.

By doing this, they can categorize all the instruments power from DB panel and signals are to/from PLC panel.
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File Type: jpg LMU24SR.JPG (23.4 KB, 63 views)
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Old July 15th, 2011, 08:10 AM   #4
Tom Jenkins
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I'm not sure I understand completely from just this diagram. It is uncommon to have one pair of terminals that can be either command or feedback. It appears to me that these terminals are only a feedback signal indicating actual position. The reference to "master-slave control signal" I interpret to mean that you can use this output from this valve as the command signal to another valve so that both track together. I don't think you will be happy of you tie your PLC output to these terminals.

If I'm right then terminals 1 and 2 are powering the feedback circuitry and can be tied to any power source.

I encourage you to contact the valve supplier and pose the question to him. He should eitehr be knowledgeable or have good access to someone who is knowledgeable. There are peices to this puzzle missing, and you shouldn't rely on answers from those of us that don't know the whole picture.
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Old July 15th, 2011, 09:14 AM   #5
danw
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This is an odd one. The concerns are certainly legitimate, having 'common' between the power supply and the
PLC AI at different potentials.

The control signal input has to be from terminals 3 (+) to 1 (-), using the power supply return as a common.
24Vac on a common for DC control ? Hmmmmm. 24Vac is very common for HVAC, rare in process.

If the negative return of the power supply, terminal 1 (-), is at a different 'ground' potential than the negative of the
AO on the PLC supplying the signal, then there'll be a common mode issue.

The use of the output voltage U at terminals 5 (+) to 1 (-) assumes that the devices it is driving are at the same
power supply return voltage.

I'm speculating that the triangle warning for 'safety isolated transformer' is probably an attempt to 'float' the
actuator negative terminal and but letting it get pulled to the ground reference of the AO (rather than a having it
connected to a hard ground at different potential.)

Please let us know what you find.
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Old July 15th, 2011, 09:31 AM   #6
nonuke
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I think you should run a wire from the 0V at the PLC to pin 2 of your valve. This way the ref. is the same (just in case!!)
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Old July 15th, 2011, 03:53 PM   #7
shooter
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nonuke is not correct it should be pin 1
make sure the transformer or power supply is not used for anything else as this will disrupt the signal.
you can use the psu from the PLC as inside the unit is a rectifier (siemens valves)
you can use either 4/20 mA output with a resistor but the output Feedback) is only in volts
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Old July 18th, 2011, 03:57 AM   #8
nonuke
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Thanks for the correction Shooter, but I was looking at the text from Flyers, not at the picture.

"We have a few units of modulating valve with 4 terminals;
1. 24V AC/DC +
2. 24V AC/DC -
3. Signal IN DC 2 to 10V
4. Signal OUT DC 2 to 10V"
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