Analog Loops

Question 1

What are the 2 reasons for a shunt diode located in current based transmitters?

Answer 1

1) Prevent the device from damage if connected to a reverse polarity voltage supply
2) Provide a test point for determining the current

Question 2

What problem does high resistance cause?

Answer 2

High receiver resistance makes loops susceptible to noise

Question 3

T/F The current in a loop with a 2-wire transmitter is generated by the transmitter

Answer 3

False, only 4-wire transmitters generate their own power

Question 4

What happens when loop resistance exceeds the allowed maximum?

Answer 4

The voltage that is available for the transmitter drops below the minimum specified voltage required for operation

Question 5

What are the 4 components of loop resistance?

Answer 5

1) 250 Ohm resistors for voltage conversion
2) Wire resistance
3) Resistance at connections
4) Intrinsic safety barriers

Question 6

What are the 3 basic functions of a loop calibrator?

Answer 6

1) Source current to unpowered devices
2) Simulate power
3) Measure current

Question 7

What must be done prior to calibrating or troubleshooting a control loop?

Answer 7

1) Take precautions to prevent injury or damage
2) Let plant operators know that work is being done

Question 8

What happens if a positive wire from an input card shorts to ground?

Answer 8

A fuse is blown due to high current

Question 9

What is a characteristic of current loops?

Answer 9

They have high impedance and prevent noise