mA to Voltage Analog Input

I have a micrologix 1100 that has a 0-10V analog input.
I have a flowmeter that provides a 4-20mA (0-400gpm) signal.
My micrologix is controlling a pump. When I turn the pump off after x amount of seconds i am looking to see 0gpm, If I dont I sound an alarm and prevent the pump from starting back up.
I realize that I can use a 500ohm resistor across the In0(-) and In0(+)to get 2-10Volts.

My question is this the best practice or is it better to have an actual analog input that senses mA?

If you have a choice, use an analog input. Rarely is life so nice.

To be or not to be....

Seems to me if all you need to see is 0gpm......a 500ohm resister is a nice inexpensive option.

Well, yes, if you can find a resistor with a value that creates exactly 0 gpm. Most likely it will be off a little bit. So then your program will have to be set up to allow for an error tolernace. That may create other problems and so it goes....

Indeed...The amount of precision required pretty much decides it. From the original description I may have mistakenly assumed(doh) that an extreme level of precision was not required. I did purchase some high precision resistors in the past online that provided pretty good resolution for this very thing...Close enough for gov't work...as they say.

My question is this the best practice...?

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Would using a resistor to create a voltage be the best practice IF a 4-20 mA analog input is available? For me, the answer is no, but everyone has his opinion.

I think $20 for a good precision resistor with right wattage rating may be a good investment. If it works and everybody is happy leave it alone. If it sorta works then you have something to base a decision to really do it right and go 4 20 all the way.

Dan Bentler

All instrumentation inputs are voltage inputs.

A 4-20mA analog input always has a precision resistor across the current input field terminals somewhere, even if the resistor is not out in the open, or exposed. Because the amplifier circuits work on voltage, not current. The voltage drop across the resistor is the 'input' to the instrumentation amplifier.

The only difference between using 500 ohms with a 4-20mA current signal connected to a voltage input is the impedance error due to parallel resistances on the input. But look at that error for a typical 1Mohm input resistance for a voltage input:
500 ohms / 1,000,000 ohms = 0.05%, which is negilible, particularly in this app.

Precision 250 ohm resistors are very common in the instrumentation field; putting two 250 ohm resistors in series for 500 ohms would work OK. I buy mine for $1.00 each from the distributor.

I'd use 500 ohms in heartbeat.

danw said:

All instrumentation inputs are voltage inputs.

...I'd use 500 ohms in heartbeat.

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I Agree.

If I'm starting from scratch and I have to buy analog cards and have the choice I go with 4-20 mA inputs. It's cleaner and customers seem to expect it that way.

If the PLC has built in voltage inputs and I don't need extra analog I/O for any reason, I have no qualms about using the resistor technique. I don't even use extra special resistors - just the tight tolerance ones from DigiKey or whatever. You have to do a little math for scaling anyway, so using a simulator to get actual data values at 4 and 20 mA and putting them into the calculations is no problem and eliminates accuracy concerns.

to gabsdad:
is a regular practice to place a resistance en the current loop to generate a voltage signal. you did ok.

...is a regular practice to place a resistance en the current loop to generate a voltage signal. You did ok.

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"Regular" does not always mean it is the BEST practice. Often, technicians do a thing because it is the EASIEST practice. Besides, Gabsdad never said he has done this practice. He was asking if he SHOULD do it.

I hope my doctor does not do what is the EASIEST treatment for my medical problems....

Tom Jenkins said:

If I'm starting from scratch and I have to buy analog cards and have the choice I go with 4-20 mA inputs. It's cleaner and customers seem to expect it that way.

If the PLC has built in voltage inputs and I don't need extra analog I/O for any reason, I have no qualms about using the resistor technique. I don't even use extra special resistors - just the tight tolerance ones from DigiKey or whatever. You have to do a little math for scaling anyway, so using a simulator to get actual data values at 4 and 20 mA and putting them into the calculations is no problem and eliminates accuracy concerns.

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Hit the nail on the head. Where I work, spending a few dollars on a resistor as opposed to buying additional equipment, when either will do the job effectively.....is considered best practice.

to lancie1:

it may not be the best practice. but..

which are the problem that such connection could lead to?

"best", for me is a balance between quality, simplicity, price, quick response to the requeriment, etc, etc..

only gabsdad knows if this easy/low price solution fits "best" to his project.

only gabsdad knows if this easy/low price solution fits "best" to his project.

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This is very true. I wonder though why he was asking us for our opinions....