Hi all,

I am working on a project. It is a control board for automation purposes. Loop current is 4-20 mA, there are 2 zenre diodes in my circuit with 4 volts difference (assume 10 and 14 volts). This board contains HART communication infrastructure. For HART signal I need to have about 450 ohm of resistance in the loop but maximum allowable voltage drop is 4 volts so maximum resistance is 4V/20mA=200 ohm. (with 450 ohm resistor it will exceed 4 volt gap)
So the main question is how to have dc resistance about 150 ohms but ac resistance about 450 ohms.

P.S.1 HART communication frequency is 1200 and 2200 HZ.
P.S.2 I cant use inductance and capacitance because they have effect on filtering.

Best Regards

 

hi Ata,
Welcome to AAC.
The HART standard states that the loop load resistance is 250R.?
Could you post a circuit showing the inter wiring connections, zener's, load etc, it would be easier to follow.
E

 

hi Ata,
Welcome to AAC.
The HART standard states that the loop load resistance is 250R.?
Could you post a circuit showing the inter wiring connections, zener's, load etc, it would be easier to follow.
E

250 is for the loop but this resistance is inside my board.
you don't think of it as a resistor it could be anything (transistor or a combination or ...) the only thing we need here is to havea less than 3.7 voltage drop and ac resistance in order of 450 ohms.
I put schematic in the comment to give you more insight.

 

Be careful not to fulfill your desires.
See