# 0-10V Analog Input possible problem



## Rikki (Mar 23, 2007)

Can anyone tell me if anything bad will happen if I put +24vdc into the input of an analog input module setup for 0-10vdc?


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## MDShunk (Jan 7, 2007)

It's not really a general practice, but you can normally put up to 40vdc into a 0-10v analog input with no problem according the manufacturer's literature I've been exposed to. My experience is primarily with Seimens and A-B, so that's where I speak from. The 0-10v is all the software is going to monkey with, but the module itself will take up to 40. 

If, for instance, I needed one or two more discrete inputs, and the only places I had left were some spare spots on an Analog input card, I'd be tempted to use them rather than buy another entire input card. What the hell.


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## miller_elex (Jan 25, 2008)

You could flood the other inputs, making them erroneously appear high.

Like a mixed air temp of 462deg F.


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## MDShunk (Jan 7, 2007)

miller_elex said:


> You could flood the other inputs, making them erroneously appear high.
> 
> Like a mixed air temp of 462deg F.


Okay, I'll bite. How?


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## miller_elex (Jan 25, 2008)

It happened once.

Except it was a 10V signal that got slammed onto a 5V input.


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## Hunter1151 (Nov 4, 2010)

Rikki said:


> Can anyone tell me if anything bad will happen if I put +24vdc into the input of an analog input module setup for 0-10vdc?


Use pull down resistors to change 24VDC to 10VDC, the value I am unsure of at this time, just google it, there are calculators out there to help, if you don't destroy the input, your CPU may throw errors


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## John Valdes (May 17, 2007)

Rikki said:


> Can anyone tell me if anything bad will happen if I put +24vdc into the input of an analog input module setup for 0-10vdc?


Probably not. Did you have an accident happen? It should be okay if you caught it right away.


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## etb (Sep 8, 2010)

MDShunk said:


> It's not really a general practice, but you can normally put up to 40vdc into a 0-10v analog input with no problem according the manufacturer's literature I've been exposed to. My experience is primarily with Seimens and A-B, so that's where I speak from. The 0-10v is all the software is going to monkey with, but the module itself will take up to 40.
> 
> If, for instance, I needed one or two more discrete inputs, and the only places I had left were some spare spots on an Analog input card, I'd be tempted to use them rather than buy another entire input card. What the hell.


Sure, that's the "max continuous"; sometimes they also spec max transient. But I don't see the reason to take the chance. I've used spare AIs as DIs like you suggest, but if you just put in a voltage divider, or a resistor and a zener diode you'll know you're not going to blow anything.



MDShunk said:


> Okay, I'll bite. How?


I'm not sure of the details but on AI modules with dedicated channels internally (as opposed to an analog switch or mux) they are all on one silicon substrate. Most analog circuitry for 0-10 usually runs a supply of 15V; if you exceed this on the input, the input protection will kick in. Although that protection can save the input from damage, the current that the protection is dumping can affect the other channels via the substrate diode. Depending on architecture, it may 'influence' their readings, or make them go all peg high/low. This can happen if you go below the range as well. Eg giving a 0-10 AI a -10 signal.



Hunter1151 said:


> Use pull down resistors to change 24VDC to 10VDC, the value I am unsure of at this time, just google it, there are calculators out there to help, if you don't destroy the input, your CPU may throw errors


You can do that but it might fry the output of whatever is giving you the 24VDC. Instead use a voltage divider. For these numbers and knowing nothing else about your application, use a 3k-ohm and 2k-ohm resistors. That makes your 0-24 map to 0-9.6 and gives you 1.2k-ohm output impedance to AI on PLC (plenty) and 5k-ohm load on your 24V output (which will pull 4.8mA; if too much, just adjust the values, keeping them in proportion).


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## miller_elex (Jan 25, 2008)

I did work for a company that used exclusively shielded cable on all inputs.

Another shop didn't ever use any shielded cable except inside of drives and the RS-485. The trick of course, is to come into the drive at a place of your choosing, not where the only 1/2in KO is.

Siemens wanted shielded FLN (rs-485) cable, and the terminations had three terminals, + shield -. No one can ever make up their mind about the shield terminal, but now I know, you only land that thing at the source. We thought initially that the shield terminal was not bonded to anything, but it is safer to splice it without landing it.

We bonded the shield for the points to the drive, at the drive, not at the controller.




As far as your 24VDC to an input issue, I've seen voltage dividers on older stuff for changing 2-10VDC to 1-5VDC, but never anything involving 24VDC. Sounds like you accidentally flooded an input. Keep your color scheme straight, no two companies have the same color scheme, and I've noticed different men at the same company use different schemes. There's every possible combo out there, and everyone thinks their combo is the only right combo and everyone else has it wrong.


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## McClary’s Electrical (Feb 21, 2009)

miller_elex said:


> I did work for a company that used exclusively shielded cable on all inputs.
> 
> Another shop didn't ever use any shielded cable except inside of drives and the RS-485. The trick of course, is to come into the drive at a place of your choosing, not where the only 1/2in KO is.
> 
> ...


 

Yep, but the shield can be landed on either end, but not both ends.


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## Electric_Light (Apr 6, 2010)

Anything beyond 10v up to the tolerance of input circuit will just register as 100%.


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