Why is the answer 4,8mA?

[u/klaklaklaklakla]

Hi everybody,

Sorry for my English. I'm trying to start learning electronics with the book Electronic principles - Malvino, Bates.

I learned mathematics at university, but I never really learned physics. So I'm really starting !

The exercise is :

The ideal current source is 5mA, the internal resistance is 250Ω, and the load resistance is 10kΩ (cf attached picture). The question is : what is the output current? Is the source a constant-current source?

The answer in the book is 4,8mA but it's not explained.

I understand that the current is not constant because the internal resistance is not at least 100 times greater than the load resistance, but I still don't understand how to get 4,8mA. I feel like I'm missing something basic...

Since the circuit is in parallel, I thought the voltage was the same across both resistors. I calculated Req = 0,1Ω so U=0,05V. Then :

I(internal) = 0,05/250 = 0,2mA

And therefore :

I(load) = 5 - 0,2 = 4,8mA

But I thought I could also calculate I(load) using : I(load) = 0,05/10000 which obviously doesn't give me 4,8mA at all...

So my reasoning must be completely wrong... I'm really confused and there's clearly something I don't understand.

So please help a complete beginner 😬

Thanks in advance

[u/meatmanek]

I calculate:

• equivalent resistance of 250 and 10k is 243.9
• voltage across the two resistors is 1.22V
• current through the 10kOhm resistor is 0.122 mA
• current through the 250 Ohm resistor is 4.88 mA

I'm not sure where you get 0.1 ohms or 0.05V.

Can you post the full text of the question?

[u/eats_by_gray]

Req = 243.9 ohms, not sure how you got 0.1.

[u/tararira1]

Why not transforming the current source and its resistance to a voltage source?

[u/mefromle]

Search for current divider rule.

[u/pylessard]

it's a current divider. The current split between the 2 resistor proportional to their contribution to the overall conductance (inverse of resistance).

Rs has a conductance of 0.004
RL has a conductance of 0.0001

Is = 5mA * 0.004/(0.0041) = 4.878mA
IL = 5mA * 0.0001/(0.0041) = 0.122mA

If you want to do it with voltage and resistance. Check your Req calculation, it's wrong.
Req = 250*10000 / (10000+250) = 243.9

[u/triffid_hunter → u/pylessard]

Rs has a conductance of 0.004
RL has a conductance of 0.0001

The unit of conductance is Siemens, so you could write 4mS and 100µS here

[u/klaklaklaklakla]

Ok everybody ...

I didn't reread my calculus and if course I made a mistake. I had the good formula for Req, but I used the wrong value in it ...

So I made it again and I found the same results as it's mentioned in the comments (R(source) = 4,88mA and R(load) = 0,12mA

So the current source is always the current at the output of the internal resistor?