ladder logic help

[jbarkei]

Hi, I have to write some logic for a 4-20mA output room temp transmitter. The span is -40 - 140F. How do I write this. I am not sure how to do the negative. Please Help.
Thanks

 

[strantor]

I assume you are talking about a PLC and not actual relay logic? If so, what PLC/programming software? Is the sensor already connected to an analog input? what do you want? I.E. do you want an alarm to go off at a set temp?

...wait...Why am I going to the trouble of asking all these questions? you obviously don't care too much or you would have gone to the trouble of including the details in the first place. If you don't care, I don't care. Show me that you care.

 

[KeepItSimpleStupid]

if you wanna learn, start here: https://www.analyzemath.com/line/Tutorials.html

Note the point, slope equation of a line and you have 2 points: (-40,4) and (140,20) or you can reverse them. From there, you can find the slope m. Now find the intercept b.

Now, you can find the equation of y=mx+b

x would be your input and it would be temperature in this case. Y would be the current. If you want the opposite just use (4,-40) and (20,140) for the points.

You did fail to note whether you need to convert 4-20 to temperature or temperature to 4-20?

It really is high school algebra.

 

[Reloadron]

Hi, I have to write some logic for a 4-20mA output room temp transmitter. The span is -40 - 140F. How do I write this. I am not sure how to do the negative. Please Help.
Thanks

I am not sure the term "ladder logic" applies here, I believe what you are after is what KISS was kind enough to link to.

About a year ago I gave away several temperature transmitters that had 4 to 20 mA outputs. That thread can be found here. I assume what you have is the same thing with a different span?

Your span is -40 to 140 Deg. F. so you have a span of 180 degrees F. Your current out is 4 to 20 mA or a span of 16 mA. That gives you about .888 mA per degree F.

So what exactly are you looking to do? I assume you plan to run that 4 to 20 mA into a A/D converter?

Ron