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vbug123 (Mechanical) (OP)
14 Jun 08 10:51
Could someone help me with this? I have a pressure transducer that outputs 30 mV max. How can I connect it to an AB PLC input card set up for 0 - 5V?

Thanks in advance.
danw2 (Industrial)
14 Jun 08 18:48
PLC 5v inputs are "high level" inputs designed for devices generally known as pressure transmitters (as opposed to transducers), that have a conditioned signal output.  

In a transmitter, the pressure transducer's low level output is amplified, linearized, characterized, scaled and usually isolated, too; with some form of zero and span adjustment.

Your transducer is raw mV.  At a minimum you need to amplify the signal so that your scale is more than 6 parts in 1000.

If you're asking the question how to do it, you're probably better off buying a pressure transmitter with a 1-5V or 0-5 V output.  Google will give you at least 1,000 hits on pressure transmitter.

Dan
roydm (Industrial)
15 Jun 08 0:33
Vbug123
                You have 3 options.
Purchase a 4 - 20 mA transmitter, thats more common in an industrial situation especially if the transducer is remote from the PLC You would use that with a 4 - 20 mA input card and 24 VDC power supply.

The transducer you have sounds like a strain gauge type, you will likely need a stable voltage to supply it (e.g 10 VDC). A thermocouple input module, will typically have a mV input range that can be used to read your transducer.

Purchase a mV to V transducer, you might be able to find one that will also supply the voltage to excite the transducer, try Googling "Strain Gauge Transducer?

If you need more help give us a few more details on w.hat you have and what you are trying to acheive

Regards
Roy
vbug123 (Mechanical) (OP)
16 Jun 08 14:18
Thanks Dan and Roy for your valuable inputs.  So now I am thinking of two options: either an amplifier to work with this mV output transducer (running with 10 VDC supply)or a volt/mA output transmitter that I can connect directly to the PLC input card.  My only concern with the volt/mA output transmitter is that its highest operating temperature rating is 121 celcius, whereas, I need something that can withstand a temperature of 150 celcius.

Thanks again.
vbug123    
roydm (Industrial)
16 Jun 08 17:10
Vbug123,
        Yes but the transmitter doesn't have to be exposed to the high temperature. It can be either at the PLC or in a local junction box where it's not so hot. The major vendors can supply one to mount in the terminal box attached to RTD
Is your transducer a strain gauge type as I described?
   If so check out the following link.
www.alstron.com.sg/Msystem/pdf/straingauge/m2lcs.pdf
This may be the sort of thing you are after. If you look at the schematic on page 3 you will see how it supplies the bridge circuit from terminals 4/5 Many different manufacturers offer similar products.
It would be best to mount the transmitter close by as this will minamize the error caused by voltage drop in the reference supply.
One advantage of using a 4-20 mA version, you can avoid having to run additional power wiring.
How far is it from transducer to PLC?

Regards
Roy
vbug123 (Mechanical) (OP)
18 Jun 08 2:10
Roy,
Thank you very much for the link.  I will get one.

To answer your questions:
- I am not sure whether it is a strain gauge, but what I know is that it is a pressure transducer that runs on a 10 VDC with an output of 3mV/V.      

- The tranducer is located 10 feet from the I/O rack.

Thank you.
     
roydm (Industrial)
18 Jun 08 6:18
Vbug123,      
                  Just make sure the transmitter you select has the right approval for your location, if that's an issue.
There are hundreds of similar devices out there e.g. Weidmuller STG, Mod-Tronic to name just a couple.
Good Luck
Roy
Kiljoy (Electrical)
19 Jun 08 16:55
You could try a different analog input card.  An AB thermocouple card can read in the mV scale.  We've used it for mV Load Cells.

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