Will my Resistors catch fire? (luxeons + car) *NEW

Ok, I got a RBLuxIII I'll be running at 1amp, and a White 1W I'll be running at 700ma in my dome light of my car. The biggest power rated resistors I could find at RatShack was 33ohm 1/2watt... grabbed a pack of 5. A few quick calculations in my head came up with this; Using 15v as reference to my car voltage (just in case it goes that high, which is possible, and at normal loads it will be less than the max current I have specified for the luxeons):

2 x 33 ohms in parallel = 16.5 ohms for the 700ma White. Good!
3 x 33 ohms in parallel = 11.0 ohms for the 1 amp RBIII. Good!

That gives me ~ 1watt resistor rating for the white, and 1.5watt rating for the RBIII. Of course I'll be pulling ~2.5watts for the White, and ~4watts for the RBIII.

Should I be concerned about this? I don't think I can find the right values in larger resitors or even larger resistors period. The 5w ceramic resistors are quite huge and won't fit with all my stuff I'll be putting in my dome light (3way toggle switch, & dimmer)....

Ideas? BTW I'm looking to use only resistors for this project, no buck circuitry.

Re: Will my Resistors catch fire? (luxeons + car)

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Re: Will my Resistors catch fire? (luxeons + car)

catch fire? probably not. It will most likely melt away including its surroundings though. You will need higher wattage rating for the resistor.

Re: Will my Resistors catch fire? (luxeons + car)

Catch on fire? Probably not, but they might smell funny and make the inside of your car smell funny too. /ubbthreads/images/graemlins/jpshakehead.gif
And you might see the magic smoke getting let out of them. /ubbthreads/images/graemlins/icon15.gif

Re: Will my Resistors catch fire? (luxeons + car)

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Re: Will my Resistors catch fire? (luxeons + car)

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Re: Will my Resistors catch fire? (luxeons + car)

DrJ, I said I'd like to stick with resistors. I have no money for such niceties as those boards..

Looks Like I'm going to have to go back to the drawing board for this one.. /ubbthreads/images/graemlins/frown.gif

I hate having everything but one important part holding you back...

Re: Will my Resistors catch fire? (luxeons + car)

Hmm anyone have any 10 or 5ohm 10w power resistors? on hand?

Re: Will my Resistors catch fire? (luxeons + car)

Issac,

Are you going to be running both of these in the same fixture off the same switch?

If so, just run them in series at 700mA. You won't see any real difference between 700mA and 1A on the royal blue.

Running them in series will make things simpler. You could then use an 8-ohm resistor, and it would only be dissipating under 4 watts

Or better yet, since voltages will fluctuate so much, run the two in series with an LM317 set up as a current regulator. It's only 2 parts (regulator and 1 resistor), and you would use about 1.7 ohms of resistance, which would dissipate less than 1W (get a 1 or 2W resistor for that). The LM317 will dissipate the rest, and will keep the current steady regardless of voltage fluctuation.

Just a couple more options there.

Re: Will my Resistors catch fire? (luxeons + car)

yes..evan has a good idea..and cheap

Re: Will my Resistors catch fire? (luxeons + car)

With the LM317, you need to heatsink it well.
The same power will be dissipated by the LM317 (and its feedback R) instead of the power resistor. However, you can affix it to the metal in the car and use it as a heatsink. A heatsunk 317 can dissipate 20 watts max, so 10 shouldn't be a problem.

Re: Will my Resistors catch fire? (luxeons + car)

Not in Series. Either White will be on, or Blue will be on, or neither. I have a center off DPDT switch for it to select what color the dome light will be, or off.

Could I get the LM317 at RatShack? How would I set it up for parallel then?

I have a large aluminum plate I will be using to mount everything (luxeons included) on. So heatsinking is good to go. Then 2 wires clip onto the terminals where the bulb normaly goes and the cover is placed over the whole thing.
Car is a Beretta.

Re: Will my Resistors catch fire? (luxeons + car)

[ QUOTE ]
IsaacHayes said:
Not in Series. Either White will be on, or Blue will be on, or neither. I have a center off DPDT switch for it to select what color the dome light will be, or off.

Could I get the LM317 at RatShack? How would I set it up for parallel then?

I have a large aluminum plate I will be using to mount everything (luxeons included) on. So heatsinking is good to go. Then 2 wires clip onto the terminals where the bulb normaly goes and the cover is placed over the whole thing.
Car is a Beretta.

[/ QUOTE ]

The LM317T is available at RShack (276-1778) for $1.99. You would just have to place the DPDT switch between the output of the LM317T and the Luxeons. I hope this helps.

Dan

Re: Will my Resistors catch fire? (luxeons + car)

Thanks for the help guys! :thumbs up:

Could you perhaps tell me how I would set it up to give each luxeon ~700ma? They won't be in series. Only one on at a time... Like what ohm resistors, and what power ratings on them? Or is it easy to understand instructions on the package?

Still what power rating resistor for the "sense" resistors would be called for?

Re: Will my Resistors catch fire? (luxeons + car)

The LM317 is probably the best way to go since it will regulate the voltage. But if you are in real need of 10W resistors, www.altronics.com.au has some. They do delivery to overseas countries via Australia Post airmail.

Or you can PM me with what you want and I can pick them up (the shop is 5min from home) and send them in a padded envelope (may be cheaper that way). (No charge, "Pay it forward" principle.)

Re: Will my Resistors catch fire? (luxeons + car)

Here's the circuit diagram to follow:



The switch changes which luxeon is lit up.
If you bolt the 317 to the plate, make sure you electrically insulate it from the plate - as the metal tab of the 317 is also serves as a connection to its output.

Re: Will my Resistors catch fire? (luxeons + car)

You can use the other section of your DPDT switch to change the current from 700 mA to 1 Amp (or whatever you decide is right) at the same time. You can use the contacts to select one of two resistors, short out a small one in series with a larger on (to raise the current) or put a larger value resistor in parallel with a fixed smaller one (adding more current again).

3 different ways to wire it to get the same result.

Doug Owen

Re: Will my Resistors catch fire? (luxeons + car)

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Re: Will my Resistors catch fire? (luxeons + car)

Yep. There's no return path to ground in constant current mode with no load connected, so there's exactly 0 current anywhere in the regulator...so nothings happening with the regulator (it's as if the regulator isn't even connected to a voltage source).

Re: Will my Resistors catch fire? (luxeons + car)

Thanks guys, looks like this is the way to go. Got allmost all my bases covered.

Doug, I was wondering what I could do with the other side of the DPDT switch. But I think I'll just leave it the same for both Luxeons now. 700ma should be plenty. But you gave me the idea of using the LM317T and a potientometer for a dimmer. I may want the RB to be on while driving, but it may be too bright at times..

Evan, thats great thanks. /ubbthreads/images/graemlins/smile.gif What will the output be using 1.5ohms? Is it 700ma?

Any ideas though on how to electrically isolate it but yet still have it mounted on the alum plate? I have arctic alumina and normal epoxy, but I don't know if I trust that to isolate it alone...