# How to check Voltage source or current source in black box?

#### wwl17442644

Joined Sep 11, 2019
9
Hi,

There are 2x 2-terminal black boxes, one has a 1V voltage source in series with 1R resistor and another one has 1A current source in parallel with 1R resistor.

How can we know which one is which?

all tools and instruments can be used.

WL.

#### Ian0

Joined Aug 7, 2020
9,446
all tools and instruments can be used.
A screwdriver to take the lid off the black box?

Think Norton-Thévenin equivalents.

#### wwl17442644

Joined Sep 11, 2019
9
A screwdriver to take the lid off the black box?

Think Norton-Thévenin equivalents.
anything else?

#### ericgibbs

Joined Jan 29, 2010
18,630
hi ww,

Hint:
Remember that an ideal Current source has an infinite source impedance and an ideal Voltage source, zero impedance

E

#### RBR1317

Joined Nov 13, 2010
712
If it is a black box, then you can't know what is inside.

"Black box is a technical term for a device, circuit or a system that is observed in terms of its input, output or transfer characteristics without any knowledge of its internal workings. Black-box modelling usually refers to behavioural modelling of such devices, circuits or systems."
V. Ceperic and A. Baric, "A brief overview of the black-box behavioural modelling of electronic circuits for transient simulations," 2012 Proceedings of the 35th International Convention MIPRO, 2012, pp. 72-77.

#### wwl17442644

Joined Sep 11, 2019
9
If it is a black box, then you can't know what is inside.

"Black box is a technical term for a device, circuit or a system that is observed in terms of its input, output or transfer characteristics without any knowledge of its internal workings. Black-box modelling usually refers to behavioural modelling of such devices, circuits or systems."
V. Ceperic and A. Baric, "A brief overview of the black-box behavioural modelling of electronic circuits for transient simulations," 2012 Proceedings of the 35th International Convention MIPRO, 2012, pp. 72-77.
where are you from?

#### wwl17442644

Joined Sep 11, 2019
9
hi ww,

Hint:
Remember that an ideal Current source has an infinite source impedance and an ideal Voltage source, zero impedance

E
Thanks, anything else?

#### ericgibbs

Joined Jan 29, 2010
18,630
hi,
What else do you want.?
Hang another 1R across the pins and measure the voltage.

E

#### wwl17442644

Joined Sep 11, 2019
9
hi,
What else do you want.?
Hang another 1R across the pins and measure the voltage.

E
in both cases the reading would be 0.5V, would they?

#### ericgibbs

Joined Jan 29, 2010
18,630
hi ww,
Come on ww, get with it, read post #4.
Do what I suggested in post #8, what voltage do you measure.?
E

#### Sensacell

Joined Jun 19, 2012
3,396
Measure the temperature of the black boxes.

The one with the current source will be dissipating power, while the voltage source containing box will not. (Assuming an open circuit on the output)

#### wwl17442644

Joined Sep 11, 2019
9
hi ww,
Come on ww, get with it, read post #4.
Do what I suggested in post #8, what voltage do you measure.?
E
View attachment 244791
Hi E,

that R1 for Voltage source should be in series with it. .

thanks
W

#### ericgibbs

Joined Jan 29, 2010
18,630
hi sensa,
As I read the TS description, the 1R's are inside the box together with the 1A or 1V source.?

E

#### wwl17442644

Joined Sep 11, 2019
9
Measure the temperature of the black boxes.

The one with the current source will be dissipating power, while the voltage source containing box will not. (Assuming an open circuit on the output)
Thank you so much. I think that is what I am looking for.

#### Ian0

Joined Aug 7, 2020
9,446
Measure the temperature of the black boxes.

The one with the current source will be dissipating power, while the voltage source containing box will not. (Assuming an open circuit on the output)
I wonder if this was the intended answer? Or was the student supposed to say that they were equivalent and it was impossible to tell them apart (electrically) ?

#### ericgibbs

Joined Jan 29, 2010
18,630
hi ww,
It is always a good idea to post a quick sketch with a question.

E

#### dcbingaman

Joined Jun 30, 2021
1,065
Hi,

There are 2x 2-terminal black boxes, one has a 1V voltage source in series with 1R resistor and another one has 1A current source in parallel with 1R resistor.

How can we know which one is which?

all tools and instruments can be used.

WL.
This is a Norton vs Thevenin equivalence. A voltage source in series with a resistor may be replaced with a current source that produces the same voltage in parallel with the resistor. There is no experiment you can perform, nor anything you can connect that will show any difference. Electrically they act the same.
Consider though the voltage source compared to the current source. Which one is consuming power in the box? The only way to tell would be see if heat is being dissipated out of the box. But if it is a true black box. Then no energy can escape from it or enter it. It is a closed system.

#### wwl17442644

Joined Sep 11, 2019
9
This is a Norton vs Thevenin equivalence. A voltage source in series with a resistor may be replaced with a current source that produces the same voltage in parallel with the resistor. There is no experiment you can perform, nor anything you can connect that will show any difference. Electrically they act the same.
Consider though the voltage source compared to the current source. Which one is consuming power in the box? The only way to tell would be see if heat is being dissipated out of the box. But if it is a true black box. Then no energy can escape from it or enter it. It is a closed system.
Thank you so much man.

#### Ian0

Joined Aug 7, 2020
9,446
This is a Norton vs Thevenin equivalence. A voltage source in series with a resistor may be replaced with a current source that produces the same voltage in parallel with the resistor. There is no experiment you can perform, nor anything you can connect that will show any difference. Electrically they act the same.
Consider though the voltage source compared to the current source. Which one is consuming power in the box? The only way to tell would be see if heat is being dissipated out of the box. But if it is a true black box. Then no energy can escape from it or enter it. It is a closed system.
So the black boxes are not black bodies!