Amped up...

1.5V is half the needed data. How much current does the LED need?

When you know that resistance in ohms is  (12 - 1.5) / current in amps.

If you have a 2ma LED then   10.5 / .002 = 5250 ohms 

Charlie

Depends on forward current desired

LED usually 1.7v forward drop. @15 mA 12v = 820 ohm. Usually 1K ohm resistor used.

Here is a handy calculator

Awesome

Thanks Alan, it said for my 2.4 v LED in need a 560ohm 1/4 watt, and It worked perfect!

Thanks,

Ernest

Thanks

Charlie,

  Thanks for the reply. I needed Resisters for dummies, but they were sold out...lol

Many thanks,

Ernest

Ohms law is easy to

Ohms law is easy to understand if you think of plumbing.

Voltage is like water pressure.

Current is like water flow.

Resistance is how difficult it is for water to pass through a pipe (the smallness of the pipe).

Ohms Law says that current (how many electrons make it past a certain point in a given amount of time) is proportional to voltage (the magnituded of the force pushing on them little electrons) divided by resistance (how difficult it is to do the pushing).

Current (Amps) = Force (Volts) / Resistance (ohms)

Going back to the plumbing analogy

Water flow rate = Water Pressure / Smallness of the Pipe

Which makes sense if you think about it for a few seconds.

Electronics types use the symbol I (captial i) for current (in Amps),  E for voltage (sometimes voltage is called Electroc Motive Force which is where the E comes from) expressed in Volts, and R for resistance expressed in Ohms.  So Ohms law is usually written as

  I = E / R

You wanted to know R so reorder the above equation using simple algebra (divide both sides by I and multiply both sides by R) making it

R = E / I

The voltage (E) in your circuit is the total volts (12) minus the volts that will be across the LED (1.5).

Current is the rated current of the LED (which ranges from 0.015 amps (or 15 milli amps) down to 0.002 amps (or 2 milli amps).

R = (12 - 15.) / 0.015  (or 0.002 depending on your LEDs.

Does this help or just confuse things further?

Charlie

LED current

Your typical 3mm or 5mm single color LED has a current rating of about 20 milliamps  ( 0.020 mA ), 2.1 fV

so, using our handy-dandy formula:

E = IR, or R = E /I

R = (12 V - 2.1 V ) / ( 0.020 mA )

R = ( 9.9 V ) / ( 0.020 mA )

R = 495 Ohms minimum

I use standard size resistors, so either 820 ohms or 1000 (1k) ohms.

Using larger resistance means less current through the LED, so it lasts longer.  Isn't this fun?

(YRMV, but shouldn't)

(P.S. question:  how does power factor fit into the plumbing analog?)

Ohm's Law beautifully presented

@Charlie - good show - I had this in mind to post, but you did an excellent job of presenting it.

@Don - have a slice of pie - P=IE, Power in Watts.

oh, one step more -if you wish

Keeping with the Plumbing analogy... (thanks for a great start, Charlie)

Since you're designing this for LED's  (light-emmitting diode),  think of that Diode as a check valve.   Works one way... stops current the other way- to a point.  Even a small check valve will fail with a fire-hose attached.  There is a Reverse voltage designation to specify the limit of what a diode can handle when improperly connected.

You may see bridge rectifiers (4 diodes symmetrically arraned) to avoid blowing out a diode when used with alternating current.

Historically, early vacuum tubes were known as valves, and they served the functions of diodes and transistors.  1Ns (diodes and 2Ns (transistors, as they Were numbered), act as check valves or throttled valves in a circuit.  These days, there are many specific combinations readily available that replicate many sophisticated activities designed in the old vacuum tube days.  IC's provide many of these in covenient forms.