| Author | Message | ||
| Gary Stadler (Boogiethecat)
Rating: N/A |
Since this keeps coming up and it also is apparently difficult for some to grokk, here's a simple way to figure out what resistor to hook in series with your 12 volt LEDs to run them on 24. Kind of a "seat of the pants" method that'll work fine... You'll need a voltmeter. Go down to radio shack or equivalent, and purchase these resistors (or as near as you can get) all 5 watt: 100 ohms (two pieces) 240 ohms (1 piece) 470 ohms (1 piece) Start by hooking all four resistors in series (in a string). Resistance adds when you hook resistors in series, so you'll have an overall resistance of 910 ohms to start with. Hook the mess in series with your LED, and hook it all up to 24 volts. Now take your voltmeter and measure the voltage across the LED lamp. It's unlikely that with 910 ohms the voltage will be higher than 12 volts, but if it is you need more resistors. If it's lower than 12 volts which it probably will be, try shorting out one of the 100 ohm resistors and measure again. If it's still too low short out the other 100 ohm resistor and measure again. If it's still too low, unshort the 100's and short out the 240 ohm. Still too low? Short the 100's out again one by one. Still too low? Unshort the 100's and the 240's, short out the 470, and start at the beginning again with the 100's. The idea is to slowly decrease the resistance by shorting and unshorting various combinations of these resistors until you finally get 12 volts or so across your LEDs. Then what resistance is left in the circuit is what you want to use permanently. Once you've figured out the resistance by totalling the values of the ones you have in the circuit (the ones that are not shorted out) you can go back to the store and get a single resistor close to that value and you're done. As far as the wattage of the resistor, you can calculate it using ohms law, or you can simply run the thing for a while and if the resistor doesn't burn your finger, it's ok. My guess is that a 5 watt resistor will probably take care of most any LED fixture although single-LED marker lights will probably only require 1/2 watt resistors. Just as a reminder, if you purchase a 5 watt 12 volt zener diode as Richard so brilliantly suggested in another thread, that's all you'll need (per LED lamp) *instead of* resistors, and it'll pretty much take care of itself and your LED without all the fiddling. Hope that works out easier than using ohms law and current meters for all you non-electronic types..  | ||
| Greg Peterson Rating: N/A |
Be careful of the power dissipation if you use the 5W 12v Zener diode. The Zener dissipates most of its power out the leads so it is very important as to what the leads are attached to. On a printed circuit board with small traces the 5 W zener will not even make 2 W of dissipation. If the leads are connected to heavy copper foil or wires then you can go higher in power dissipation. It is not like a resistor that can get very hot and take it. The zener failure mode is usually a short circuit that would then put the 24v across your lights. Personally I prefer to use resistors when dissipating above 1w of power. If you use film type power resistors they will fail open and the only harm is no lights. I fixed one engineering design screw up by raising the 5W zener off the printed circuit board and adding copper tubing over the leads in order to increase the surface area and allow the zener to dissipate the heat. This was not the cleanest fix but got us into production with out delaying a 20 million dollar program. | ||
| Gary Stadler (Boogiethecat)
Rating: N/A |
Greg is 100% right on the money with every comment. Thanks for the additional insight Greg!!! |