Minimum ohms on 0-10v input?

RobertB · Oct 17, 2017

Morning, all...
I searched other threads, couldn't find an answer to this one.
I have a Chinese sensor to connect to a 0-10v input. It only has 672 ohms of resistance max. It doesn't suggest adding a resistor.
1762-IN012C-EN-P MicroLogix 1762-IF4 manual says..... 0-10v inputs have 200K ohms of impedence already.
So.... no worries???

AustralIan · Oct 17, 2017

I think they mean minimum when they say maximum.

I stuff that up all the time, because decreasing resistance increases power...

Mickey · Oct 17, 2017

Can you tell us the model number/catalog number of your sensor?

Manual link for it?

RobertB · Oct 17, 2017

Sorry,
The sensor has been sitting on the shelf here in the plant and was used. No paperwork, and most the numbers are wore off. I can make out the, "0-10vdc output" on it.... and of course.... made in China.
That is why I am suspicious. I normally don't ohm out analog sensors unless there is a problem with them.
1762-IF4s aren't cheap. And if my hunch is correct... I won't get a second chance.

keithkyll · Oct 17, 2017

I doubt that your -IF4 is at risk.
Assume the sensor has 24DC input power on brown and blue leads, and a 0-10V output referenced to the blue lead.
Hang a 10K or so resistor across the 0-10V output, and read it with your Fluke. If all is okay, then connect to the -IF4.

danw · Oct 17, 2017

You can connect the 0-10V output to the AI with its 200Kohm input resistance without a problem.

The ratio of 672/200,000 = 0.00336 = 0.33% load on the voltage output, which is not significant for a 0.5% accurate instrument.

Adding a resistor in series would prevent dropping all the transmitter's output across the input.
Adding a resistor in parallel would lower in the AI's effective input resistance, increasing the load and increasing the error.

Just connect the transmitter and run it.

RobertB · Oct 17, 2017

Not a bad safety measure. And I have a 10k pot laying around.
In my mind, once 10vdc arrives at the terminals... it is 10vdc like any other 10vdc. If the input is rated for 10vdc, it should be up to the card to handle any excessive current that could result.

danw · Oct 17, 2017

A 10k resistor across the transmitter output will load it by about 6%:
672/10,000 = 6.7%

so a 5V output will read low by 0.34V (4.66V), a 10V output will read low by 0.67V (9.23V)

RobertB · Oct 19, 2017

I notice that a lot of threads here on the site vanish without verification that our suggestions worked.
Well, I did put the sensor directly onto the 0-10v input and it works great!
I now believe that the resistor that is often "requested" in these applications is for.... the sake of the SENSOR, not the analog card.
Thank you, all.

danw · Oct 19, 2017

RobertB said:
I now believe that the resistor that is often "requested" in these applications is for.... the sake of the SENSOR, not the analog card.

Without seeing the references to the requirement for a resistor, I'd venture that such a stated requirement is for a DC current source, like 4-20mA or 0-20mA.

Electronic instrumentation senses voltage. So a voltmeter or an analog input for voltage inherently senses the voltage applied.

But current is measured by creating a voltage drop; an IR drop, current (I) times resistance (R), derived from Ohms Law.

So current signals always need a dropping or shunt resistor at an analog input. Sometimes the dropping resistor is built-in to the analog input, sometimes it is switched in mechanically or electronically, sometimes it must be installed externally to the screw/cage terminals.

RobertB · Oct 19, 2017

I agree. Nice to hear from others that have studied and REMEMBER ohms law!
Obviously, the 200Kohms built into the IF4 terminals compensates for any low impedance sensors.
Without ANY resistance, this circuit could have pulled 10 amps. I=E/R, 10=10/0 ohms. 10 amps is probably a 100x more than enough to smoke that analog input!
Best wishes...

GaryS · Oct 19, 2017

The way I read it is the max burden on the sensor is 672 ohms. if you do the math it would mean the sensor can supply about 20ma and still maintain the 10 volts out
A lower ohms would mean it would have to have to supply more current to give you the 10 volts
less ohms = more load
the 200k impedance of an analog input will be no problem if the wires were open (not connected) the sensor would still produce the 0-10 v

keithkyll · Oct 19, 2017

Where did you get the 672 Ohms figure? Is that a spec on the sensor, or did you get that by measuring Ohms across the output pins with your Fluke?

RobertB · Oct 19, 2017

At the sensor's maximum range, it actually had 670 ohms. Not the 672 listed on the sensor. But my Fluke could be off a tad. Or the 672 ohm on the sensor is a general output range tested at the sensor factory.
In any case... I'm happy and so is my set up.

GaryS · Oct 19, 2017

why are you trying to measure the ohms on the output of a sensor
if it's powered the output voltage will influence the reading and if it's not powered you will be reading the resistance on the solid state output device either way the reading will be unreliable at best.
if the data sheet for the device states 672 ohms max load, then the 672 ohms is the lowest value of resistance for the input that you can use. I think it's a translation problem from china to English nontechnical people writhing and translating the documents.
if you use less then the 672 ohms then the sensor will not be able to deliver the 10 at full range
there in no value to measuring the ohms on an output t will not yield any useful information. Post a picture of the sensor label it will help us to give you a better answer

Minimum ohms on 0-10v input?

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