30.1 ohm resistor gets hot w/2 Rcr123a, Why?

[lukevsdarth]

I was wondering while doing a modification to my Ultrafire WF-502B with a drop in Cree bulb 3.6-9V, I tried putting in a 30.1 ohm resistor. This resistor got hot real quick. I bought several from two different buyers. The forward voltage peaked at 8.36 from 2 Rcr123A (1000ma). I wanted at least a good 60ma for low setting. I had some other resistors 300 ohms ended up with about 14ma real low like powersaver low. Well two settings is what i wanted from this light. Used the LED PRO 2.12 I get 156 hours on low 2.7 on high.

Thanks for any input, FRED:candle:

 

[elgarak]

What's the wattage rating of the resistor? Ignoring internal resistance of the batteries, you're running ~270 mA through the circuit, and burn 2.3 W on the resistor. So if you have a low wattage rated resistor, say, less than 1W, it will get hot and die pretty rapidly. For long-term running, you'll need a 2W, better 3W rated resistor, but I guess people with more practical experience than me might have better input.

If you want 60 mA, something around 100 Ohms, 1W should work (depends on the actual forward voltage of the LED).

 

[milkyspit]

What's the wattage rating of the resistor? Ignoring internal resistance of the batteries, you're running ~270 mA through the circuit, and burn 2.3 W on the resistor. So if you have a low wattage rated resistor, say, less than 1W, it will get hot and die pretty rapidly. For long-term running, you'll need a 2W, better 3W rated resistor, but I guess people with more practical experience than me might have better input.

If you want 60 mA, something around 100 Ohms, 1W should work (depends on the actual forward voltage of the LED).

Everything he said!

 

[greg_in_canada]

Also, I think resistor power ratings are based on being in open air (mounted on a printed circuit board). So in an enclosed space they will get hotter than designed for, unless they are in contact with the flashlight body.

Greg

 

[lukevsdarth]

Thank you for your input in this matter. I bought Dale Earnharts 30.1ohm 1% resistors. The way I was looking at it and tried them out to is by using also a 17670 3.7v batt too in this application it worked fine. I had a few other resistors left over from a 12volt hobby I had they worked great real low mode but nothing from the 3.7v only 8.19v. I figured since 8.19 was closer to the 12 volts the 300 ohms resistors worked but real low. Im pretty happy with the mod maybe I will fish for a slightly better resistor from other computer parts until i find a brighter solution.
3.6vf , 2X 4.18v batt, 1000ma, 14ma to P4 Cree, 30.1 (1%) or 300% (5% i think)
I do greatly respect your intelligent input from everyone.
Thanks, Fred:twothumbs