# Homework Help: Black box guessing

1. Oct 6, 2009

### prescott2006

if a black box was given,with two wire pull out from the box.no other additional info was given,then how will you determine the impedance in the circuit?what is the resistance?if given that is a passive circuit,what will the circuit look like?

2. Oct 6, 2009

### mgb_phys

What would happen if you connected it in series/parallel with a know resistor?

3. Oct 6, 2009

### prescott2006

the impedance will rise/fall?i am newbie in electronic,not too sure what you want to hint.

4. Oct 6, 2009

### vk6kro

"Passive" doesn't mean it can't contain power sources. Just no transistors, opamps or other ICs.

You would measure the voltage out without a load and the current out into a short circuit.

You may not be able to work out exactly what is in the box, but you would be able to predict how it would behave under different load conditions.

Imagine a circuit where you had a battery in series with a resistor inside your box.
If you did the above tests, what would you be able to work out about the battery and the resistor?
Remember that if there is no current flowing in a resistor, there is no voltage across it.

5. Oct 6, 2009

### prescott2006

could we probably know the circuit configuration in the box by guessing?given the circuit inside is passive circuit.

6. Oct 7, 2009

### vk6kro

Unless the circuit happened to be a battery in series with a resistor, you could not guess what it really was.

But it doesn't matter. If you know what it is equivalent to, you can predict what would happen with different load resistors. That is all you need to know.

For example, if you had two equal resistors in series across a battery, and took the output from across one of them, the open circuit voltage would be half the battery voltage, but you couldn't tell there was a higher voltage battery present without taking the screws out of the box.