View Full Version : Resistor Question

Brian Anderson
02-23-2011, 08:43 PM
Hello All,
Thank you all for your helpful replies. I messed around with a set of Green LED's and found out my math was wrong. I posted "Green 1.22k ohm going to use 1.5k ohm 2w" well that actually took my resistor value to 3k. The lights did light up but they were very dim. What I should have done was to put 611 ohm resistor on each side to equal the 1.22k ohm I needed. So I think I am almost done and now can fix the entire 60 broken strings. Once I'm done I'll try to put all the information I'm gathered into a how-to post so those with bad LEDs can fix their own. Thanks everyone again for you post, solutions, and suggestions!

I do have a few more questions. If I need two different resistors to make up lets just say 1.22k ohm can I use one 1k ohm one one side and 220 ohm on the other? See the attached link for the schematic to get a better idea or does each side need to be balanced?


I guess I'm asking does the value of the resistor have to be the same on each side?
what type of a resistor should i use like Carbon Film, Metal Film or something else?

How does the wattage add up? If I use two different 1 watt resistors the value will stay at one watt, but if I use a 1 watt resistor and a 1/2 watt resistor then the vale would be the lower 1/2 watt?

Thanks again!

02-23-2011, 09:06 PM
If you have two resistors and a bunch of LEDs in series, it doesn't matter where they are in the string. The same current will flow through all the components. They also don't need to be half the value; resistors in series add their values. You do have to be careful about the power dissipation.

Power dissipation in a resistor is I*I*R (where I is the current in amps, and R the resistance on ohms). So if the resistors are the same value but one is 1W and the other is 1/2W, your dissipation is limited by the 1/2W one. If they are different, than the dissipation will be proportional to the resistance.

02-23-2011, 09:09 PM
If I can I would use the same value resistors for two reasons:
1. the power consuption if both is the same
2. I don't have to mess with two different values.

Carbon Film of Metal film, all these are good. Stick away from the old one from surplus stores as that may not take solder too well.

Power = Volts x Current OR Power = Current x Current X Resistance From this we deduce that for a same current, the power dissipated in the resistor is proportional the resistor value - a smaller resistance will dissipate a small power. That is another good reason for keeping the resistance values similar, if not the same.

02-23-2011, 09:16 PM
Wattage is current times resistance. Use the wattage that keeps it below 80% on the resistor. The resistors don't have to be the same wattage.

You can also use two resistors twice the value in parallel. The wattage is half of calculated.

Edit: I must type slow or something.;)

Brian Anderson
02-24-2011, 02:38 AM
Thanks guys for all the info. I am going to stick with 1watt for the Blue and Greens and 2w for the Red and Orange. I think this will work out great. I'm glad I didn't give up on this project. I hope someone will benefit from all my trial and error. I got a assortment of 1w resistors to play with and when I figure it all out I will order my parts from mouser. If anyone see's anything I've missed please let me know.
Thank you all again!

02-24-2011, 03:46 PM
it might have been said, just remember resisters in series are additive if in parallel they are not....its better to just use the right one first not two unless cost and power demands are better in using 2.

Brian Anderson
02-25-2011, 12:27 PM
Hey Griffixdc thanks for the comment! Every bit helps, but I'm not sure you read the entire post or looked at the schematic. It calls for 2 resistors so I need to use two. I am just trying to get as close to the calculated value as possible I know I might have to fiddle around a bit with them though. I am aware of the parallel issue but these are in series ( I hope that's the right word.) Thanks again for looking and anything else you have to offer is really appreciated.

02-25-2011, 02:35 PM
well i see your link but was enable to go to it....there are "...."s inside of the link and when i click on it they show up so i get a 404 error.