how hot are resistors suppose to get?

[Chris M.]

Power (watts) = Voltage (volts) x Current (amps).

Power dissipated in your resistors = Voltage dropped accross them (ie, input voltage - voltage accross the LS) x current.

So if your supply is 4.5 volts, the power dissipated in the resistors is (4.5- 2.95) x 0.324 = 0.5 watts (to 2 decimal places).

Higher power resistors won`t necessarily be cooler, they will just be better able to withstand the heat generated. Larger physical sized ones will be cooler to the touch cos there is more of a mass to heat up- but still, they`ll be just as warm overall- as there`s the same amount of power to dissipate in them. It`s just spread out over a larger area.

Any help?

 

[Silviron]

Hey Bart-

Are these the amber LSs that you are running on your car?

And are you running BOTH LSs through the same resistors, or one set each?

I'll assume for now this is about the car mounted ones....

They really shouldn't be getting that hot- Maybe if you are using one set of resistors for two LSs, but not for one---

The only things I can think of is that either that you have a weak short to ground, or that your battery / alternator is just pumping out too many amps for the resistors to handle, even though the LS is only drawing a reasonable amount of current. My reasoning here could be pretty flakey, but that is the only thing I can think of offhand...

If this IS in your car and you are using the resistors to drop 14 - 12 V to ~ 3V, why don't get a couple of those little regulators that you can plug into your cigarette lighter and run a radio, CD etc off of, rip em open and wire them into your LEDs, and you will have somewhat better / safer voltage dropping... I think you can find those at Hosfeld or All electronics for about $3.95 each, or Walmart for about $5.95. Whatever the reason for your hot resistors, I would think this would eliminate it.

 

[Bart]

Chris... thanx for your help. it works out to 1.9 watts. I'm sure that sounds right.

Silviron, I don't have them in my car yet. It's in the shop getting tranny fixed

but I should get it back this week. Right now I'm just testing it with a regulated 9v power supply. I'm going to take it apart and use the regulator in my car. I use regulators for all my LED assemblies because of the fluxuations. This is just one LS that I'm using right now... when I hook it up, I am going to use seperate circuits for each LS because I want each LS to operate isolated from the other so I can use them with the turn signals as well. (on all the time but turns off when turn signal comes on). The voltage regulator can put out up to 800mA so the LS's can't be overdriven by much... and the resistors as well. All in all, I think it will be OK because I ran it for 45min and the resistors didn't quit or burn. Who knows, I may even mount a heatsink on the resistors.

 

[Bart]

I'm sure someone else has asked this before... but to make sure I don't burn out my resistors while running the LS at 330mA, I am using 5 1/2 watt resistors in parallel but they are too hot to leave my finger on it for more than a split second. I can smell them slightly, but not that burning resistor smell. I can't remember the formula for acquiring the number of watts... is it V/I=watts? if that is so, then I'm using 8.93 watts... but that would only come out to 4.5 lumens/watt. That doesn't make sense. Anyways, the resistors should be able to handle 2.5 watts total. Basically I'm just wondering how hot these resistors usually get. BTW, LS is at roughly 2.95v @ 324mA.