Homework problem

js74

Joined Oct 14, 2010
8
I have been having trouble trying to solve this homework problem. I am to design (on paper) a simple circuit using these materials (don't necessarily need to use all of them):
-as many as 6 one volt batteries
-a 25 ohm resistor
-a 100 ohm resistor
-a 3300 ohm resistor
-a LED

The LED needs at least two volts and 10 mA to light up, and anything over 25mW of power will burn it out.

The closest I got was just using two volts of power and 125 ohms of resistance, which game me 16mA of current but my power was 32 mW and everything else I tried either gave me too much power with the small resistors or not enough current with the 3300 ohm resistor.

Any hints/tips/ideas are appreciated.

js74

Joined Oct 14, 2010
8
I tried using two batteries (two volts) and the 100 ohm and 25 ohm resistor. This game me 16mA of current but 32mW of power which is slightly too high. I tried other combinations but found that if I used the 3300 ohm resistor I'd never have enough current and without it I always have too much power.

Last edited:

blueroomelectronics

Joined Jul 22, 2007
1,757

bertus

Joined Apr 5, 2008
20,566
Hello,

Can you show us how you calculated the 16 mA?
Of wich component may the power not be larger as 25 mW?

Bertus

js74

Joined Oct 14, 2010
8
Hello,

Can you show us how you calculated the 16 mA?
Of wich component may the power not be larger as 25 mW?

Bertus
The LED can't have more than 25 mW of power running through it.

I did I=V/R

v=2 R=125, 2/125=.016 A or 16 mA.

then for power P=V^2/R so 4/125 which equals .032W or 32 mW

bertus

Joined Apr 5, 2008
20,566
Hello,

What is the voltage drop of the LED (Vled) ?

Iled = (Vbatteries - Vled) / Resistor_in_series_with_led

Bertus

js74

Joined Oct 14, 2010
8
Hello,

What is the voltage drop of the LED (Vled) ?

Iled = (Vbatteries - Vled) / Resistor_in_series_with_led

Bertus
I am not sure. If I could build the circuit I could measure the voltage across the LED but I don't have the gear to build it and am not sure how to calculate it.

bertus

Joined Apr 5, 2008
20,566
Hello,

The LED needs at least two volts and 10 mA to light up, and anything over 25mW of power will burn it out.
Vled will be 2 Volts at 10 mA.

LEDs, 555s, Flashers, and Light Chasers

Can you now complete the task?

Bertus

js74

Joined Oct 14, 2010
8
Hello,

Vled will be 2 Volts at 10 mA.

LEDs, 555s, Flashers, and Light Chasers
Can you now complete the task?

Bertus
Right, but doesn't that make the power going through the LED over 25mW?

P=V^2/R
P=2^2/125
p=4/125
P=.032 W or 32 mW

bertus

Joined Apr 5, 2008
20,566

djsfantasi

Joined Apr 11, 2010
6,499
I can't wait to see the answer! I think I have it, but my answer is an example of a basic electronics confusion that I have.

js74

Joined Oct 14, 2010
8
I'm still confused though. The problem said there needs to be 2V to light the LED as well. This means I need more voltage than 2V then because there won't be 2V going into the LED, correct?

Georacer

Joined Nov 25, 2009
5,182
Yes, your circuit will have a slight voltage drop, that will lower the battery voltage before it reaches the LED.

js74

Joined Oct 14, 2010
8
Okay so I guess my main question is how do I increase the voltage and current without increasing the power through the LED? When I calculate it using 3300 ohm resistor my current gets way too low (even if I use the 6 Volts i am allotted) so I am assuming the 3300 ohm resistor cannot be used.

I=V/R
I=6/3300
I= 1.8 mA

Georacer

Joined Nov 25, 2009
5,182
Did you read bertus's link? What did you understand or didn't understand from it. I has the form of the circuit you need to make right in the beginning,

js74

Joined Oct 14, 2010
8
I read the link and I got that the LED needs 2 volts to turn on and then runs on current. I'm just not understanding how to apply it I guess. I've been trying to hammer it out but still struggling. I've been using V= IR to calculate voltage across the resistor and thus the voltage drops but I haven't been able to get it to work.

Georacer

Joined Nov 25, 2009
5,182
You know that your LED needs 2V to run. You also know that a current I will go through it, which will be in the area of 10mA. You know that a resistor will be placed in series with it.

The resistor will have the same current as the LED. Try combinations of source voltages and resistances to get the best result. Remember that the voltage that is applied on the resistor is Vsource-Vled=Vsource-2.

JoeJester

Joined Apr 26, 2005
4,259
Are you sure the 3300 ohm resistor isn't a mistype?