0-10V dc analogue signal standards

k steel

Member
Join Date
Dec 2003
Posts
5
I`m just getting into PLC`s and one of the questions that I`ve been asked about but cannot find much information on is 0-10V dc analogue signal standards.
Could anyone please let me know what the basics of this are please ?
 
Standards we hope!

Your best source of information of a historical nature is probably the ISA. Most of us who have practiced this craft have our own version of historical events and therefore some of that is not particularly accurate, however here is my thought on your question.

Voltage and current relationships for instrument control loops evolved like a lot of other things did in that world.
Early on, standards were set primarily by the leading manufacturer.
Foxboro was one of those companies. They used 10-50ma currents and 1-5 volt signals in their equipment for obvious reasons. A 100ohm resistor would drop 1-5volts with the 10-50ma current flowing through it. Nice even numbers don't you think? But then came the 4-20ma folks and the resulting 1-5V was accomplished by using a 250ohm resistor. Now where does that leave 0-10V? Probably used with 0-20ma current loops. 500ohms will give you 10Volts drop for the 10ma and thus history was made. The need for a "live" zero is probably the driving force behind the 10-50 and 4-20 current loops. There may be some who can give you a much clearer picture of the history but roughly that is my take on it. One last thing, the input to instruments are much the same in that they most all use a voltage to interpret the loop data. The resistors quoted above are either added externally or are already in the device and can be switched off and on when needed. Maybe that will help you some but for more precise info, the ISA is the one source you can count on.
 
Randy has explained the when and why, but not the how.

Essentially, with any analog signal a transducer is used to convert a physical measurement into an electirial signal that correlates with the measured phenomenon. This signal is usually then amplified or conditioned to a higher range so that the stronger signal is proportional to the physical measurement and can be transmitted some distance. Inside the PLC this electrical signal is converted to digital data, say for example 0-4095, that is also proportional to the measured physical phenomenon.

For example, a pressure may be measured by a strain guage with an ouput of 20 mV at 100 psig. This is amplified to a signal that is 0 VDC at 0 psig and 10 VDC at 100 psig. This is input to a PLC with a 12 bit analog input, where 0 in the input's data register = 0 psig, and 4095 in the input's data register indicates 100 psig.
 
Thanks to both Randylud and Tom for your rapid replies.
They both helped me with what I was after in differnet ways. Please keep an eye out for my next problem as I`m sure to have another one.- KS
 

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