monitoring 12vdc power supply with plc

Hi Guys,
i have a micrologix 1100 and want to use its analogue input to measure a 12v battery voltage level using resistors. I want to know whats the best way to do this and what should i consider. I have tried using the R=V/I to get the size of the resistor at 20mA but when i tries this the current i got going into the analogue module was about 1mA.Can anyone give me the neates solution.

You forgot that mA input itself has its own resistance.

Whichever model of MicroLogix 1100 you are using, and whether you use expansion I/O or the inbuilt analog inputs, they can be selected to be Voltage input, which have a high impedance, and will not draw any significant current.

The standard (2) analog inputs are 0-10V dc input, so to measure your 12V battery (which I will assume could be higher when charging), I would target your signal range at 0 to 15V dc.

Now all we need to do is "scale" that signal voltage to present 0-10V dc to the voltage analog input.

A simple potential divider consisting of 2 resistors, in a ratio of R:2R is all that is required, see the attached picture. 15V applied to the outer of the series resistors, will see 10V at the midpoint of them.

Resistance values are not really important, but don't take them too low, so as not to draw too much current from the battery. And dont have them too high so that the high impedance of the voltage input becomes a factor.

I would suggest the two resistors are 10K and 22K. Although not exactly R:2R ratio, the measurement can be scaled yet again in the PLC back to the 0-15V signal. Any adjustment needed for accommodating the not quite R:2R ration can be done when re-scaling back to 0-15V measured.

Alternatively, you could just use a 47K 20-turn potentiometer....

Input (0-15V) across the whole pot, output (0-10V) from wiper to ground. Set the pot with a multimeter before connecting to the analog input.

NB. Your battery -ve needs to be connected to the COM terminal of the analog input. If this is not possible - you will need to buy a signal isolator.

I cant see anywhere in the micrologix programming software where i can change the configuration to 0-10 v dc for the analogue input. is that a default setting?

oops - forgot the drawing....

Page 55 of the manual...

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1763-um001_-en-p.pdf

If you are using analog expansion modules, IF2OF2 or IF4 be aware that the default setting for the inputs is Current !!

Ok thanks. so it means i am stuck with using current if am using IF4? Cos like i said can see anywhere in the program where i can change the input to voltage.

To configure a 1762-IF4 to a voltage input in RSLoigx500 see this post:

http://www.plctalk.net/qanda/showpost.php?p=447752&postcount=2

The IF4 will be a little different than the example but you should get the idea.

To configure the IF4 module for voltage input, double-click it in the IO configuration, and then choose the Analog Input Configuration tab.

Its pretty straightforward from there, but I would choose the
Raw/Proportional Data format, as it gives you best resolution. (See first picture)


With an input range on the module of 0 to 10V, Raw/Proportional Data gives you 0 to 32,767 in your input image data.

Use a SCP instruction to scale this back to 0 to 15V, but you'll want better resolution than Integer, so make the destination a Floating Point data-table location. (See second picture)

PS don't forget the switch on the IF4 module itself to set the channel input to Voltage.

eababio said:

I cant see anywhere in the micrologix programming software where i can change the configuration to 0-10 v dc for the analogue input. is that a default setting?

Click to expand...

Yes....
the default configuration data is for Voltage Input.

However, and perversely !!...
The default switch setting on the module is for Current Input.

You have to wonder why ....

Ok thanks. so it means i am stuck with using current if am using IF4?

Click to expand...

I am lost why don't you just use the onboard?

bce123 said:

I am lost why don't you just use the onboard?

Click to expand...

Perhaps his onboard is already used ???

Thanks for the answers. It will save me buying a transducer as i wanted to get accurate results cos my client is particular about that.