Resistor advice

[john84]

Hey everyone,

I'm new here and wanted some advice on my LED circuit.

I've got a 12v power supply and an array of 3 LED's in a series with a 22ohm 1/2w resistor. Can anyone tell me how much voltage and current each LED will end up receiving with this circuit design?

Thanks

 

[Resqueline]

I seriously doubt you'll find psychics here so good that they can tell you what color (and hence what voltage) your LED's have. The current will depend on the forementioned.

 

[john84]

My LED's are rated at 3.2v - 3.5vMAX and upto 100mA max. I've put a 22ohm 1/2 watt resistor in the series circuit of 3 LED's and just want help to find it if that is the correct resistor.

From what i have calculated, I should end up getting 3.4v and 86mA per LED. But i am not too sure and dont want to risk blowing the LED's. I have done a test run of the circuit for over 2hours and the LED's all light up nicely and are still working as normal. There also seems to be no heat at all on the resistor or the LED's (wondering if this is normal)

 

[john84]

the way i calculated the use of a 22ohm resistor for the LEDs is as follows:

3 LED's each at 3.4v will require 10.2v
12v - 10.2v = 1.8v

1.8v / 0.086mA = 20.93ohms rounded to 22ohm resistor.

Can anyone tell me if my calculations is correct for my leds?

like i said, i've powered the circuit on for around 2hrs without any problem and there seems to be no heat on the resistor or leds at all. So, i'm not sure if something is wrong or not.

 

[(*steve*)]

One option would be to measure the voltage across the resistor and the battery and use these to determine what the other parameters are.

If they don't align fairly closely with your calculations then your assumption(s) were wrong.

 

[Resqueline]

Your calculations are correct, but notice that there's not much voltage left for the resistor so only small changes in the voltages will make a big difference in the current.
Each LED will only dissipate 0.3W which will hardly be noticable individually. If it's not noticable even in a cluster it could indicate the current is lower than calculated.
To get a more predictable (constant) current it's not recommendable to use more than two white (or blue) LED's in series on 12V.