analog sensor load

[sye15929]

hi!
would like to get inputs on how to determine what load an analog sensor output can power. take for example a 4 to 20 ma output from the sensor to an analog card which reads only V (so we put a 500 ohm resistor across the input card). what are the parameters should i look for? anybody got a good source on all these type of calculations example 2 or 3 wire sensors. thanks in advance.

 

[IRstuff]

Have you considered reading the sensor's datasheet or owner's manual?

Or even doing a search on this site, since this topic has come up repeatedly?

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[danw2]

Output capability varies from device to device. The spec is usually published, like IRstuff says, in the spec sheet.

For 2 wire loop powered (passive) analog outputs there's usually a graph, sometime called a 'load line' of resistance vs voltage because higher wiring resistances can be overcome by increasing the power supply voltage:

Active (powered) analog outputs usually just have a statement that the analog output can drive X ohms.

The loop's resistance is the total of the dropping resistor on the analog input(s), wire resistance and the intrinsic safety barrier resistance (if used).

The ISA 50 standard, now the ANSI/ISA-50.00.01-1975 (R2002) [Formerly ANSI/ISA-50.1-1975 (R1992)], entitled Compatibility of Analog Signals for Electronic Industrial Process Instruments says that 4-20mA outputs should be able to drive a minimum of 600 ohms, but there are numerous signal conditioners that top out at 500 ohms.

In the process world, most AI's are 250 ohms or less, so for short to medium runs of wire, a 24Vdc power supply is adequate, as a rule of thumb. Long runs of wire, 10Vdc inputs that use 500ohm input resistors, or loops with more than one receiver AI warrant a closer look at whether the AO can push the current through the loop.

As an aside, all current analog inputs read 'volts', but some bury the dropping resistor internally so it isn't obvious.

 

[sye15929]

hi danw...
thanks for the reply. attached is a

 

[sye15929]

hi danw...thanks for the reply. i hv an input card but i do not know the ai impedance. how can i find out? measuring across the ai gives me reading in terms of Mohms (without my 500ohm resistor in parallel to the ai).

also i have a sensor which states that the MAX loop resistance is 500 ohms (controller input resistor + wire resistor). does it mean i cannot use it? i am relatively positive its ok to use but this statement and my ai resistance which i measured throws me off.

also can a 3 wire sensor be used in 2 wire mode if the sensor output impedance is large enough to power the controller input and the wiring resistance? i am told it is ok.

ps - how do i paste something in this post?

 

[danw2]

>i hv an input card but i do not know the ai impedance. how can i find out? measuring across the ai gives me reading in terms of Mohms (without my 500ohm resistor in parallel to the ai).

You find out by the way you've done it, you measure the resistance.
The input resistance of a Voltage input is a very high resistance, on the order of megohms. Your analog input is a voltage input so it's got a high input resistance.

Its input resistance is so high that the field device cannot drive its milliamp current through the megohm resistance (or so little of the current that it isn't a viable 'signal').

Instead you let the current from the field device flow through the 500 ohm resistor you added. The voltage drop across the 500 ohm resistor is 'seen' by the analog input (a voltage input). There's some minimal amount of error because of resistances in parallel (500 ohms in parallel with X megohm), but not enough to worry about.

>also i have a sensor which states that the MAX loop resistance is 500 ohms (controller input resistor + wire resistor). does it mean i cannot use it?

On the contrary, you're using it exactly as intended.

>i am relatively positive its ok to use but this statement and my ai resistance which i measured throws me off.

Everything makes sense.

>also can a 3 wire sensor be used in 2 wire mode if the sensor output impedance is large enough to power the controller input and the wiring resistance? i am told it is ok.

I haven't a clue, but if a 3 wire device could be used as a 2 wire device, why doesn't the manufacturer present it as such?
As an old guy, I am wary of shortcuts that make maintenance over time a nightmare.

>ps - how do i paste something in this post?

Images must first be posted on a file sharing site. I use

http://www.tinypic.com.

You need to copy/save the URL provided (I use the 2nd one down labeled "IMG Code for Forums & Message Boards") and have access to it.

To insert an image, you click on the button with greenish circle with blue background (that says Picture [Ctrl+P] when you put your cursor over it]) and paste the image URL into the pop-up box.