voltage monitoring

[jimcav]

Hello

I have a micro logix 1000 with analog input, I need to measure a voltage from 0 to 18 but my input is only 0 to 10 how can i scale 0 to 18 for 0 to 10.

thanks

 

[rdrast]

Use a signal Isolator, or a resistor divider network.

 

[timbo_uk]

What about this convertor?

http://www.farnell.com/datasheets/42001.pdf

You MAY be able to use it. Assuming that putting the 10k pot across the input would allow adjustment. Saying that, though, you may be able to do this on your 0-10v input direct to the PLC, depends on output impeadance of the device you are using for the 0-18v (what is it???) and the input impedance of the PLC.

 

[panic mode]

if you ask me, i would just use divider and scale signal in plc.

note that even if you use signal conditioner, you still have to check the specs (range) as 18V may be too much for many of them. if you do like idea of signal conditioner, i would recommend Weidmuller WaveDC Pro which can convert just about anything to anything (any range as long as not exceeding max +/-200V, max +/-100mA). it's also very narrow (slim line), 3-way isolated, uses very little power and can be powered from very wide AC/DC range.
it is also very easy to use because of nice software that makes configuration simple and fast:

 

[panic mode]

you can make voltaqe divider with just about any pair of resistors. if they are same value, you will get 1/2 of input voltage (so 0-18V would read as 0-9V). don't choose too low or too high resistor value. anything in 1..100k should be just fine.


http://en.wikipedia.org/wiki/Potentiometer

 

[TConnolly]

Use this: http://www.dataforth.com/catalog/doc_generator.asp?doc_id=3&attr_index_id=65
It will isolate the input of 0-20V and output a 0-10V signal that is 1/2 of the input voltage.

Now we have to scale it. Here is where the bottom dwelling ML1K brick shows itself to be dumber than a brick - as AB invests engineering resources to redo that which it has already done inorder to dumb down it bricks because "bricks should be stupid."

From the manual we can see that the maximum input voltage is 10.5V and that 10.5V will return 32767, the maximum value an integer can have. We can also see that at 10V the value it returns is 31207. Now the dumbed down ML1K has had the SCP instruction stripped out of it and it doesn't do floating point math. However we do have the SCL instruction and we can scale our voltage into millivots, so that at 18V we will get 18000.

Although the max process variable is 18V, the signal conditoner has a 20V input and scales its output accodingly, so 20V is what we need to use.

To calculate the rate parameter of the SCL intruction we take the engineering units range, 20000 mV and divide it by 31207.

20000/31207 = .64088185. Then we multiply it by 10,000 to get 6408.8185. Rounding, we select the value of 6409 as the rate parameter for the SCL instruction. The offset in this case will be zero.

So programming this rung:
SCL I:0.4 6409 0 N7:0
will scale the analog input 0 and put the millivolt value in N7:0. By using millivolts we can infer the decimal place, thus 17500 is 17.5 volts.

 

[EnBomp]

same problem but with S7-200

hello,
i've got the same pblm in measuring the voltage but i want to take readings for every point in the voltage cycle of the local electric network using the s7-200 & EM 231