Resistor getting hot

[stephenstern]

Hi all,

Hopefully someone can help me out here.
Ok, so I have:

9 x 1w LED, its 12v, 300ma, 3,4v.
9 x 3w 33R resistor.
Power supply 12v, 200ma (testing with this one but i intend to use larger ma when all 9 leds are in use).

I believed this resistor to be good enough (its a fair size) to cope, but it seems to get very hot..

Using this guide..;

 

[pc1966]

Resistors get hot - it is their nature!

Typically a 3W cermaic resistor is reated to something like a 200C or more surface temperature. If you need something cool running use a much larger resistor, or a metal cased on bolted to something that acts like a heatsink.

 

[stephenstern]

Resistors get hot - it is their nature!

Typically a 3W cermaic resistor is reated to something like a 200C or more surface temperature. If you need something cool running use a much larger resistor, or a metal cased on bolted to something that acts like a heatsink.

Thanks, Yes, i guess i cant tell if hot is too hot, but if the math is correct, and its not going to burnout/catch on fire then im happy ?!

 

[pc1966]

Resistor will be fine (check any data sheet for air flow though). But you need to keep it a safe distance from anything that is heat-sensitive. You get ceramic tube spacers, etc, to raise them off PCBs for that sort of reason.

 

[davesparks]

You would do far better to use an LED driver circuit for this rather than resistors.
A constant current driver as the power supply could power all of the LEDs in series.

Or else small constant current driver circuits for each LED, I think you can get these as small PCB modules for not much more than a higher powered resistor.

 

[ipf]

Resistors get hot - it is their nature!

Always wondered why my 3 bar electric fire used to get so hot....

 

[stephenstern]

So the real issue here is 12v for 9 x 1w leds.
I have found a 9v power supply. 2.1a

Can i use this, with the resistor i already have? (it does power it as i tried but if they will just burn out that wont help!)

 

[pc1966]

With a 9V PSU and 2 LEDs in series you need to drop the 9 - 2*3.4 = 2.2V for your 200mA so you need 11 ohm, and could do so with 3*33R in parallel if not buying new resistors.

Repeat 4 times and have one as originally planned for 9 LEDs with less total power wasted in the resistors.

But it is still not as efficient as a dedicated LED driver!

 

[Wilko]

Hi - you can have fun experimenting, but if you haven’t got test equipment (a multimeter say) then it’s going to be hit and miss. Take your 12V supply, many of these actually output 14V (ish) on no load, reducing to 12V (ish) at rated load. So if your led drop is 3.4V (it may not be) then you might get 10V across the resistor with a 300mA load, which is 3W in the resistor. As PC1966 has said, this will make it hot, possibly too hot for the other things around it. But it’s always fun letting the smoke out of components when you’re experimenting .

 

[stephenstern]

Thanks all
I cant put the leds in series as each of the 9 are on 5m wire back to a box (which just joins power to the runs) - think decking lighting. so i can only power 1 led in a circuit.

if i use the <10v supply i have found, the math says a 22ohm resistor. but will 33 be safe - the less volts should help with the heat?

 

[telectrix]

less volts can make the LEDs dimmer, or even not lighting, if it's too much less.

 

[stephenstern]

Thanks, the leds was 3.4 so surely im not going to have an issue with that.
i can see with the 10v psu, its a touch dimmer with my 33r resistor, but then i assume that resistor is now too big for the job,, but its still way bright so happy to keep that if its safe!

 

[stephenstern]

Rethinking this. I originally used the psu for the original leds what was in place. Which was 12v. But I don't really get why it was ever made with 12v..

Anyway. I now have a 5v 2a supply. This means I can buy a much smaller resistor and displace little. Meaning no heat.
The 33ohm one I have now limits current to 50~60ma. Which weirdly enough is still super bight on the led (rated for 300ma).. That I can't work out? If anything I'd like to restrict more current and dim the led further

 

[pc1966]

Rethinking this. I originally used the psu for the original leds what was in place. Which was 12v. But I don't really get why it was ever made with 12v..

Anyway. I now have a 5v 2a supply. This means I can buy a much smaller resistor and displace little. Meaning no heat.

Meaning less heat.

The 33ohm one I have now limits current to 50~60ma. Which weirdly enough is still super bight on the led (rated for 300ma).. That I can't work out? If anything I'd like to restrict more current and dim the led further

The human eye is not a linear response to light intensity, it is more of a logarithmic curve (as for hearing) so a x2 change in light intensity is not perceived as twice as bright/dim, etc.