LEDs and resistors, heat

[joshua17ss2]

I am making a few led strips, they contain 12 leds, in sets of 2, for every 2 leds there is 1 resistors, i needed it to run on 12volts, i did the math and came up with 330ohm 1/4 watt resistors, i got everything soldered together, and it lights up just fine, but last night i noticed after 2 or 3 minutes that the resistors were getting really hot to the touch, and 1 or 2 of the lets were flickering just a bit.

i am using a 12 volt power from an older server power supply.

should i try running the bars with a smaller power supply, or should i change out the resistors to 1/2 watt ?

thanks

 

[ericgibbs]

I am making a few led strips, they contain 12 leds, in sets of 2, for every 2 leds there is 1 resistors, i needed it to run on 12volts, i did the math and came up with 330ohm 1/4 watt resistors, i got everything soldered together, and it lights up just fine, but last night i noticed after 2 or 3 minutes that the resistors were getting really hot to the touch, and 1 or 2 of the lets were flickering just a bit.

i am using a 12 volt power from an older server power supply.

should i try running the bars with a smaller power supply, or should i change out the resistors to 1/2 watt ?

thanks

hi,
I assume that they are BLUE leds, say about 3Vfwd nominal at 20ma.

So that is 6v to drop at .02A, that gives 0.12W. A 0.125W will run hot.
Use either a 0.5W or 1W rated resistor

 

[MikeMl]

Why nor recalculate the resistor for groups of 3LEDs? Then it would be dropping only 3V @ 20mA, which means the dissipation would drop to 60mW, so a 1/4W resistor would be fine.

 

[joshua17ss2]

im reusing some PCBs from old exit lights they are grouped in sets of 2, so i dont really have much of a choice on that.
i can pick up some 1/2 watt resistor and solder them on and see how it does

 

[MikeMl]

If you are driving lots of them, why not either get an 8 to 9V power supply, or modify the existing supply to put out 8V?

 

[Hero999]

Why not connect three LEDs in series?

The forward voltage is probably about 3.5V so that'll only giver you 1.5V headroom but it'll be more efficient and run cooler.

 

[ericgibbs]

Why not connect three LEDs in series?

The forward voltage is probably about 3.5V so that'll only giver you 1.5V headroom but it'll be more efficient and run cooler.

im reusing some PCBs from old exit lights they are grouped in sets of 2, so i dont really have much of a choice on that.
i can pick up some 1/2 watt resistor and solder them on and see how it does

OT:
How did that LTS 555 project go.?

 

[joshua17ss2]

What is the actually current that a board like this would draw, i have 12 leds, in sets of 2, at 20ma, for each led, does that mean 40ma for each set, and 240ma for the whole board, or is it calculated differently ?

 

[ericgibbs]

What is the actually current that a board like this would draw, i have 12 leds, in sets of 2, at 20ma, for each led, does that mean 40ma for each set, and 240ma for the whole board, or is it calculated differently ?

hi,
If the LED's are set for 20mA current, then for LED's in series its 20mA for that series chain.

So its 20mA * number of chains, in your case 20mA * 6 =120mA