A black box has three electrical connections labelled A, B, and C, arranged
in a triangle as shown below.
The box contains three components: a resistor, a small capacitor and a
diode. You know that one component is connected between each pair of
terminals, but you cannot see exactly how they are arranged. You make
the following observations with a 9 V battery connected in series with an
ammeter:
- When the battery is connected with + to A and − to B a current of
3 mA flows- When the battery is connected with + to B and − to C a very large
current flows- When the battery is connected with + to C and − to A no current is
measured- When the battery is connected with − to B and + to C no current is
measuredOn the diagram sketch the arrangement of the three components in
the box indicating the terminals clearly.
I have had a lot of trouble with this particular problem. The only thing I can confidently say is that the diode is blocking the current in the 2 cases, but what bothers me is that in those 2 cases the currents flow in opposite directions (counter-clockwise in the third, clockwise in the fourth), which confuses me.
The first 2 cases probably mean that the diode blocks current in the clockwise direction, because otherwise you wouldn't have a current at all, but that still leaves me with the issue of the third case, where the current flows in the counter-clockwise direction, but apparently still gets blocked.
Can anyone help me with this problem?
Best Answer
Process of elimination/deduction
Always start with what you know.
(i) There are three components in the circuit - resistor, diode & capacitor.
(ii) There are three terminals (A,B,C)
(iii) There is only one component connected between each terminal.
There are only two possible unique configurations of this circuit.
(i) Either the anode of the diode is connected to the end of the resistor (and the cathode to the end of the capacitor)
(ii) Or the Cathode of the diode is connected to the end of the resistor (and the anode to the end of the capacitor)
Between any TWO TERMINALS you will have a circuit consisting of the 'component' which is connected to the other two component connected in series with each other and parallel to the component.
By using DC to test the components you (should) know that;
(i) the resistor will conduct the same amount of current in both directions AND the capacitor in the parallel path (once charged) will prevent current flowing through the diode regardless of polarity
(ii) the diode will appear as a short circuit in one direction only AND the (charged) capacitor will prevent current flowing in the RC parallel circuit.
(iii) Applying the test across the capacitor only the parallel circuit (resistor/diode) will conduct (- in one direction it will give the same (or similar) reading as the resistor, in the opposite direction it will show an open circuit (no current).)
Its now down to you to work out which terminal is A, which B and which C and which way around is the diode connected. For extra credit you could also calculate the value of the resistor in ohms.