Diode circuit problem

1. The problem statement, all variables and given/known data
A moving coil ammeter is to be adapted to detect small alternating currents. Which of the diagrams shows how a diode could be connected in order to make the conversion ?

(Options shown in attachment)

(Ans: (3))

2. Relevant equations

3. The attempt at a solution
I am not sure if I understand the problem. What does it mean by "detect small currents"? Also, it doesn’t state if the diode are considered to be ideal or not? But I think I am supposed to consider them as ideal.

There are my thoughts:

Circuit (1): The diode is non-conducting if + at the upper terminal and – at the lower terminal, so all the current pass through the ammeter. If the signs are switched, no current passes through the ammeter as the diode is now conducting.

Circuit (2): Opposite to the functioning of circuit (1).

Circuit (3): If + at the upper terminal and – at the lower terminal, the diode is conducting and ammeter records a reading. If the signs are switched, the circuit gets open and no current through ammeter.

Circuit (4): Opposite to the functioning of circuit (3).

But I am not sure which option to select as answer. :confused:

Any help is appreciated. Thanks!

Attached Images
File Type: png diode circuits.png (13.4 KB)

http://ift.tt/1h1pTu4

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