input impendance

[jcole]

I have a controller/transmitter that has the following specs for the input signal input:

4-20mAdc, 249 ohms input impedence, or 1-5Vdc

Could you explain to me the 249 ohm ohms input impedance? Why is this info important?

I am new to current loops/analog control. I notice impendance rating for alot of the devices I install but dont really understand the importance of it.

Thanks for replies.

 

[david luchini]

I believe the "input impedance" is used to convert the 4-20ma signal to a voltage signal used by the controller. It's just easier for the controller to "read" voltage than current.

For instance, at 4mA the controller would read 0.995V at the input impedance, and at 20mA the controller would read 4.98V at the input impedance.

 

[eric9822]

I believe David is correct but usually a 250 ohm resistor is spec'd vs. 249 ohm. If the field device transmits 1-5 VDC then no resistor is used. If the field device transmits 4-20 mA then the resistor is connected from the pos to the neg input connections on the controller.

 

[gar]

100518-2247 EST

It is probably better to describe the current loop receiver as having a 249 ohm input resistance instead of impedance. This is because you really want what is essentially a pure resistance at the frequencies of interest. 249 is possibly a more standard and available resistance value than 250. It makes no difference. 220 could be used also. Calibration from current to voltage is taken care of in the receiving instrument.

Higher resistances in the receiver reduce the allowable resistance in the external loop. Suppose you have a regulated voltage source of 24 V for the source voltage in the loop. Then maximum total loop resistance to allow 20 MA is 24/0.02 = 1200 ohms, but you really need some margin. If 250 is used in the receiver, then between the transmitter and wire resistance the maximum is 950 ohms. Safer to limit the real external loop to maybe 700 ohms.

.

 

[jcole]

So basically your saying the info is important to know so I dont put to much resistance is the current loop and not have the ability to reach 20mA. Makes good sense.

Thanks for the replies.

 

[don_resqcapt19]

So basically your saying the info is important to know so I dont put to much resistance is the current loop and not have the ability to reach 20mA. Makes good sense.

Thanks for the replies.

Exactly. Most transmitters will have a spec that gives you the maximum loop resistance. The most common ones at the plant I often work at say the maximum loop resistance is 750Ω. Most of the devices that we connect to the transmitters have a resistance of 250Ω, so we normally will only connect two devices in series with the transmitter.

 

[jcole]

Hello guys.

This is an old post that I have another question on.

I have controller that says "the mA output card supplies up to 4 analog (0-20mA/4-20mA) signals into an impedance of max. 500 ohms. The mA output card cannot be used to provide power to a two wire (loop powered) transmitter."

What does it mean by "into an impedance of max. 500 ohms"?

Why would it be important to know that the output cannot provide power for transmitter? Is it just telling me I need my own voltage source for the loop if I have a looped powered transmitter in the loop?

Thanks for replies.

 

[Christoff84]

Hello guys.

This is an old post that I have another question on.

I have controller that says "the mA output card supplies up to 4 analog (0-20mA/4-20mA) signals into an impedance of max. 500 ohms. The mA output card cannot be used to provide power to a two wire (loop powered) transmitter."

What does it mean by "into an impedance of max. 500 ohms"?

Why would it be important to know that the output cannot provide power for transmitter? Is it just telling me I need my own voltage source for the loop if I have a looped powered transmitter in the loop?

Thanks for replies.

Your output card has become the transmitter, meaning it can only output a 4-20mA signal through a max of 500 Ohms of impedance. If you have multiple receivers in series reading the signal, their total impedance can't be more then 500 Ohms.

With a 2 wire device you would need to supply external power to the loop. The output card would regulate the current in the loop to match whatever the output is set to.

At 500 Ohms the output card is probably only working with 10 Volts max.

 

[steve66]

Hello guys.

This is an old post that I have another question on.

I have controller that says "the mA output card supplies up to 4 analog (0-20mA/4-20mA) signals into an impedance of max. 500 ohms. The mA output card cannot be used to provide power to a two wire (loop powered) transmitter."

What does it mean by "into an impedance of max. 500 ohms"?

Why would it be important to know that the output cannot provide power for transmitter? Is it just telling me I need my own voltage source for the loop if I have a looped powered transmitter in the loop?

Thanks for replies.

Its been a long time since I did 4-20ma controls, but I believe I remember seeing PLC analog outputs that did not provide their own voltage source. (Basically an open collector output.)

You could connect it to a sensor that provided its own power source (a 4 wire device).

Or, you could wire a voltage source in series with the output in series with the sensor. But that would still be a loop powered device.

It seems to be common sense that you couldn't use a loop powered device at 0-20ma, because at 0 ma the output would provide 0 volts at 0 amps and there wouldn't be anything to power the sensor.

Not sure why you couldn't use a series voltage source with 4-20ma.
Steve