i have a thorny problem in as much as i am trying to run 2 leds in parallel but the two leds in question have different ratings, one is a bog standard green and the other a high brightness Blue both having different voltage/current ratings, what is the best way to wire these guys up so they both come together when power is applied?



Neil.

 

Use a different resistor in series with each LED.

 

+1 to #12's comment. In other words you'll have to use a combination series/parallel circuit not actually having the LEDs directly in parallel with each other.

What voltage are you using to supply them?

 

one takes 3v the other 1.7 so yes thats a good idea, i can do a different value resistor for each led,

thanks.

Neil.

 

A tutorial I wrote a couple of years ago...

LEDs, 555s, Flashers, and Light Chasers

 

You can either tell us what the supply voltage is or work it out from Bills tutorial.

 

i am having trouble calculating the resistor values, i am using a supply of 9v, one led needs 1.7v at 20mA, the other needs 3.0V at 30mA, i guess i could use 20mA for both though since the blue led is VERY bright, so if i use 20mA for both, how do i calculate the voltage drop across the resistor in each case?

Neil.

 

I just took a gander at that link Bill, very interesting read, i will find time next week to experiment i hope.

thankyou.

Can someone give me the basics to get me started please?

Neil.

 

You have plenty of voltage to run both LEDs in series with one resistor. A 270Ω resistor should put the current around 16-18mA.

 

(9-1.7)/.02 = any resistor with more than 365 ohms (390)
(9-3)/.03 = any resistor with more than 200 ohms (220)

or,

(9-1.7-2.7)/.02 = any resistance more than 230 ohms (240)
to give both of them .0192A
give or take the tolerance of the voltage ratings on the LEDs and the accuracy of the 9 volt supply.

Edited to comply with KJ6 in post#11

 

...(9-1.7-3)/.02 = any resistance more than 215 ohms (220)
to give both of them .01954A...

The Vf of the blue LED will be less than 3 at 20mA, probably by 2 or 3 tenths which makes 220Ω potentially dangerous to the health of the green LED. 240Ω is the lowest I'd risk without a test.

 

(9-1.7-2.7)/.02 = any resistance more than 230 ohms (240)
to give both of them .0192A

doesn't that equate to 37.5mA each which is way too much for the green led?

9v/240R=37.5.

Neil.

 

Not if you put the LEDs in series as described by KJ6 in post # 9

"i guess i could use 20mA for both though since the blue led is VERY bright"

Putting both LEDs in series and allowing 20 ma for both of them allows the use of only 1 resistor.

 

oh yes you are right, i misread the equation. :x


well anyway i got the resistance thing right and the leds are working ok.

thanks.

Neil.