Load driving capability

[Charbel]

Dear,

i want to understand something...
i hae a specification here that says that pressure transmitter loop powered, 4 to 20mA HART output, load driving capability of 600 ohm at 24 VDC.

what is the load driving capability?

 

[Tom Jenkins]

That is more often referred to as max load, or max resistance, or max burden. It simply means that with the 24 VDC power supplied the transmitter will enable it to send a 4-20 mA signal through a circuit with a total resistance of 600 Ohms. That allows, for example, putting a chart recorder with 250 Ohms resistance and a PLC with 250 Ohms resistance on the input channel in the loop and the transmitter will be able to drive the signal through the loop.

If you exceed the max load for the transmitter your output signal will typically drop to zero mA.

 

[Charbel]

Sensor Imipedance and effect on Input module

Dear Tom,

thank you! i was looking at the endress&hausser Promag 53 datasheet, please check below:

[FONT=E+HWeidemannCom-BookBd][FONT=E+HWeidemannCom-BookBd]

"Promag 53​

[/FONT][/FONT][FONT=E+HWeidemannCom-Book][FONT=E+HWeidemannCom-Book]

Current output:
active/passive selectable, galvanically isolated, time constant selectable (0.01...100 s),
full scale value selectable, temperature coefficient: typically 0.005% o.r./°C (o.r. = of reading),
resolution: 0.5​

[/FONT]​

[/FONT]μ[FONT=E+HWeidemannCom-Book][FONT=E+HWeidemannCom-Book]A
• active: 0/4…20 mA, R[/FONT][/FONT][FONT=E+HWeidemannCom-Book][FONT=E+HWeidemannCom-Book]L [/FONT][/FONT][FONT=E+HWeidemannCom-Book][FONT=E+HWeidemannCom-Book]< 700 [/FONT][/FONT]Ω [FONT=E+HWeidemannCom-Book][FONT=E+HWeidemannCom-Book](HART: R[/FONT][/FONT][FONT=E+HWeidemannCom-Book][FONT=E+HWeidemannCom-Book]L [/FONT][/FONT]≥ [FONT=E+HWeidemannCom-Book][FONT=E+HWeidemannCom-Book]250 [/FONT][/FONT]Ω[FONT=E+HWeidemannCom-Book][FONT=E+HWeidemannCom-Book])​

• passive: 4…20 mA, operating voltage V[/FONT][/FONT][FONT=E+HWeidemannCom-Book][FONT=E+HWeidemannCom-Book]S [/FONT][/FONT][FONT=E+HWeidemannCom-Book][FONT=E+HWeidemannCom-Book]18...30 V DC, R[/FONT][/FONT][FONT=E+HWeidemannCom-Book][FONT=E+HWeidemannCom-Book]i [/FONT][/FONT]≤ [FONT=E+HWeidemannCom-Book][FONT=E+HWeidemannCom-Book]150 [/FONT][/FONT]Ω"

I assume active is field powered and passive loop powered.

What is Ri? RL is the load resistance as you explained before.
and why RL for HART should be larger than 250 ohm?

what are the restriction on the input module resistance?

thanks!

Charbel

 

[Tom Jenkins]

Hart requires a resistance > 250 Ohm in order to have enough voltage to make the communicaitons signal readable by Hart devices. Remember, Hart is a high frequency communications signal superimposed on top of the 4-20 mA.


I am not sure about the Ri. I could speculate, but i suggest you contact your source for E & H or their tech support.