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ohms law?


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KPierson 
Platinum - Posts: 3,527
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Joined: April 14, 2005
Location: Ohio, United States
Posted: July 21, 2008 at 12:59 PM / IP Logged  

If you have a simple LED circuit connected directly to a car electrical system you can use Ohm's Law to determine the proper size of the resistor needed to control the current to the LED.

Say you have a forward current of 25mA for the LED.  We want to designe for a worst case voltage scenario so lets say our battery voltage is 15vdc.  Resistors restrict the amount of current that flows in the circuit so we can use Ohm's Law to determine what size resistor will net us 25mA.

V=IR

R=V/I
R=15V/0.025A

R=600 ohms

You can see in this circuit that it is a simple circuit with basically only a resistive device.  Once you start throwing inductive/capacitive/discreet/ect. components in to the circuit Ohm's Law may or may not hold "true" as it will depend on what exactly you are trying to do.  Many discreet components, for example a basic transistor, have amplification values that need to be factored in.  If you use Ohm's Law to determine the base current going to a 3904 NPN transistor you may find that you have 20mA of current.  However, coming out of the collector you will have a total current capacity of 600mA or so (based on a beta of 30).  Since discreet components, such as transistors and diodes, don't have a resistance Ohm's Law doesn't account for them.

Kevin Pierson
klctexas 
Copper - Posts: 111
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Joined: March 10, 2008
Location: Texas, United States
Posted: July 21, 2008 at 2:26 PM / IP Logged  
Kevin, thank you. This is exactly what I was looking for.
Soldier: This is the worst part. The calm before the battle.
Fry: And then the battle is not so bad?
Soldier: Oh, right. I forgot about the battle.
Mad Scientists 
Silver - Posts: 380
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Joined: February 07, 2004
Location: United States
Posted: July 23, 2008 at 2:51 PM / IP Logged  

Kevin,

You forgot to allow for Vf of the LED.. If we assume the LED has a foward voltage of 2.4v, to calculate resistor size you first take your 15v, subtract the 2.4v for a remainder of 12.6v. The you use the LED current spec and divide your 12.6 by the 0.025a to come up with 504 ohms. You can also calculate the power rating required by taking 12.6v and multiplying it by 0.025 to come up with 0.315 watts, so use a 1/2 watt resistor around 500 ohms.

Jim

KPierson 
Platinum - Posts: 3,527
Platinum spaceThis member consistently provides reliable informationspace
Joined: April 14, 2005
Location: Ohio, United States
Posted: July 23, 2008 at 6:16 PM / IP Logged  

The forward voltage and power calculations don't pertain to Ohm's Law.  He wanted it simple, so he got simple!  :)

Also, by not accounting for the forward voltage you'll get a small buffer in the max current available to the LED.

Kevin Pierson
klctexas 
Copper - Posts: 111
Copper spacespace
Joined: March 10, 2008
Location: Texas, United States
Posted: July 24, 2008 at 8:46 AM / IP Logged  
^^Yes, I was kinda looking for something simple. Just trying to work out the formulas. ohms law? - Page 2 - Last Post -- posted image.
Soldier: This is the worst part. The calm before the battle.
Fry: And then the battle is not so bad?
Soldier: Oh, right. I forgot about the battle.
Page of 2

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