Analog Input With Potentiometer HELP!!

[zmanvortex]

I am still going to use the 24vdc.

By using a 1k pot and 1.1k resistor in series with the wiper and analog input the linearity is good enough for the application.

Basically the pot isn't going to be used for absolute time adjustment but rather for a relative adjustment to allow the operator to tweak the operation.

Being the programmer that I am I wrote a utility in C# to allow me to test the linearity and different pot, supply voltage, and resistor values.

I have included a screenshot and if anyone would like to have it I can zip it as an attachment. .NET Framework 1.1 must be installed on the users computer for it to work.

 

[Tom Jenkins]

You know, just because you have three leads on a pot doesn't mean you have to use them. Use a 5 kOhm pot with the wiper and one lead in series with a 1 kOhm resistor in series with the analog input (125 Ohm) in series with the 24 VDC power supply. Leave one side of the pot floating in space.

Pot @ Min => 1,125 Ohm @ 24 VDC = 21.33 mA (Specified max at the input is 30 mA so no problem.)

Pot @ Max => 6,125 Ohm @ 24 VDC = 3.9 mA

If you are going cheap this is close enough. If you don't want to accept "close enough" then you should probably use a signal conditioner.

 

[keithkyll]

Leave one side of the pot floating in space.

Convention is to tie the 'free' leg to the wiper. That way, when the pot is intermittent, it will go to 5K instead of open circuit.

If you can handle a minimum voltage of 1.2 volts, consider an LM317 Adjustable 3 Terminal Regulator. Look at the datasheet for application info.
With the LM317, you can create a 1.2 to 5 volt regulated supply.
This will eliminate problems with power supply noise (if it's not dedicated) and linearity. You will also have thermal stability. You get a lot for a buck or so.

 

[rickboy25]

4114 Universal signal conditioner from PRelectronics-$174. Takes a pot input and provides 4-20mA or 0-20mA output very easily with a detachable mini LCD screen used to set the parameters. I'm not sure what state your in, but I believe they are direct in most states.

I think its,
www.prelectronics.com

 

[zmanvortex]

Thanks Rick.

I think its,
www.prelectronics.com

I noticed your signature, are you sure you "think its" or do you "know its".

 

[rickboy25]

Sorry bout that. We just made a modification on our address from www.pr-electronics.com to just www.prelectronics.com and I did have problems with both for a couple of days. Didn't mean to appear slow or confuse anyone.

 

[OkiePC]

You know, just because you have three leads on a pot doesn't mean you have to use them. Use a 5 kOhm pot with the wiper and one lead in series with a 1 kOhm resistor in series with the analog input (125 Ohm) in series with the 24 VDC power supply. Leave one side of the pot floating in space.

That's what we do...soemtimes we use a second 2kohm pot (just 2 legs) and adjust it to get the range we need from the operator control. I just turn the operator control to the max value and trim it down with the panel mounted one until I get the value in the PLC I want. It's faster and easier than trying to find input card specs and doing the math.

 

[scottmurphy]

F0-04AD-1, so it makes it tough to use the supplied 24vdc.
We are looking at replacing an external adjustable timer with a potentiometer and analog card, but it would not be a viable option if our electricians had to do a bunch of solid state tricks to get it to work.

I may be missing something here, but isn't what you are doing by 'making' your own signal conditioner, exactly the 'solid state' tricks you are referring to?

 

[zmanvortex]

Let me re-phrase that.

As few solid state tricks as possible

Basically all we did is add a 1.1k resistor to the wiper circuit to get it to work. It is R1 in my image above. I would not consider this to be a bunch of solid state tricks.

 

[panic mode]

this was out there for a while now:

http://forums.mrplc.com/index.php?showtopic=1128

 

[bradal]

I am really amazed that when given a cheap, easy to use alternative, especially when you mention that electricians on site are going to be wiring it up, you do not just get a signal conditioner. They work, reliable, and if they do fail you get another one, you dont have to worry about getting a 1023 ohm resistor, or a carbon mega-sorting doomahickey, that ONLY you know about. I guess if your looking for job security, or is it job security when everything electronic you do is too confusing for everyone else to figure out. HAHAH, anyways my $.02

 

[zmanvortex]

anyways my $.02

I could almost buy that 1.1k resistor for $.02.
I don't think I could buy that signal conditioner for $.02.
We are an OEM in a tight market so every $.02 counts.

I am baffled as to where you get the job security bit.
If I wanted job security I would get a breadboard and make my own signal conditioner (if I knew how).

Just my $.02.