# I have a few question concerning Black Boxes

Next term for Physics, my teacher has said that he has ordered some black boxes and will be putting a diode, capacitor, resistor and a transistor in each. There is 3 people in my class and he will be making each black box different (he'll put each component between a different terminal).

We've been told that over the holidays we need to figure out how we're going to test for each component.

My questions are:

1) What does a black box look like? (For example is it a rectangular box with say 4 inputs on the left side and 4 outputs on the right side? etc)

2) To test for diodes would I attach a crocodile clip to a DC power supply and then to the one of the black box inputs? Would I then attach another crocodile clip from the output that is opposite to the input I just mentioned and then to an ammeter? See if any electricity flows and then swap the circuit around by connecting the output and the power supply and the ammeter to the input and see if energy flows through (only one of these ways would work as electricity only flows one way when a diode is used). I would then test each input/ output to see where the diode is. Is this correct?

3) When finding a resistor does that mean that I would use the above procedure but electricity would flow both ways? And also how to I find the ammount of resistance?

4) My notes on finding a capacitor say.. fading light once power pack is turned off, but how would I set up the circuit to find a capacitor? How do I find the capacitance?

5) I don't really know what to do when it comes to finding a transistor. I imagine that one of the inputs would act as the base and two of the outputs would act as the emmiter and the collector. How would I determine where the transistor was?

I would really appreciate it if you could tell me if i'm heading in the right direction with this, thanks :)

Not sure if your teacher means the same thing but in general a black box just has en input and an output.

Input --> [Black Box] ---> Output.

Your procedure to test for a diode seems ok. However at a great enough voltage current will flow freely through the diode at both direction. Don't know if this is the correct English term but directly translated from my native language it would be 'back voltage'.

For a resistor you check that Volt/Current is constant and this is also the resistance of the resistor.

For a capacitor you can check that the voltage over the black box slowly fades after you have disconnected the voltage supply.

For a transistor I have no idea because I flunked my electronics course lol.

Your procedure to test for a diode seems ok. However at a great enough voltage current will flow freely through the diode at both direction. Don't know if this is the correct English term but directly translated from my native language it would be 'back voltage'.
So to stop the 'back voltage' from happening, would I lower the voltage?

For a resistor you check that Volt/Current is constant and this is also the resistance of the resistor.
Sorry, but what do you mean by Voltage and Current being constant?

Back voltages are usually quite high like 10 V or something. Just use low voltages and you will be fine.

I mean that the voltage divided by the current should be constant. The voltage divided by the current is the resistance and for a resistor this should be constant (for non extreme values of the voltage).

Yeah I understood the bit about voltage divided by current = resistance. For some reason i'm having trouble with the bit about it being constant. Does it mean that it will not vary? Can you please give me an example of a constant resistance and a non constant resistance?

A resistor has constant resistance meaning it does not change much when you change the voltage. This means that the curve on the Voltage-Current diagram is a straight line.

Other components do not have this property.

Oh okay I understand :D Thankyou so much for your help!