Author Options:

How many resistors do I need to use for this configuration? Answered

I am soldering 5 standard Radioshack LEDs together to a 9v battery. How many resistors should I use and where? Should I go Battery-resistor-LED-Resistor-LED-Resistor-LED-Resistor-LED-Resistor-LED-Resistor-Battery? Or something else? Thanks. :)



Best Answer 9 years ago

You would need 1 330 ohm resistor, or 1 110 ohm resistor and 1 220 ohm resistor. Solder the LEDs in parallel, ald solder the resistor(s) to the positive, and a switch to the negative and the other LED lead. Hope I helped!

I personally like to use http://ledcalc.com/ to calculate the resistors needed. You need to know the specs on your resistors, it should be on the package. I agree with frollard, each led should have its own resistor.

xoiio's answer is actually a terrible solution - no offense xoiio.

Running multiple leds in parallel with only one resistor in series is a bad plan because each led is essentially a short circuit - and only a SLIGHT variation between manufactured leds will mean one will draw more current than the others - leading to it being brighter - and getting slightly hotter. The hotter it gets, the less it resists, the more power flows, and it quickly becomes a completely short circuit until it burns out.

EVERY parallel circuit needs its own series resistor.

Contrary to XOIIO - do not solder the 110 and 220 in parallel, solder them in series.


depends on the leds

Each led has a voltage drop - the amount of voltage it needs to run itself. For reds its in the 2 volt range, and blues/whites are in the 3.5 volt range. The packaging will say.

You cant hook up more in series (add up the voltages) than the source - so if you have for example, red leds, 5 will be 10 volts (too many) - so you'd have to run 2 on one series string, and 3 on the other. Alternately, use 4 in one, and 1 in the other

use something like http://led.linear1.org/led.wiz to figure out your resistors.
In your case,
input values of 9 volt source, 2 volt drop (for red, adjust as necessary), 5 total leds, and between 10-20 ma per led. (I used 15) Select wiring diagram for a pretty picture.

It offers up 4+1, 3+2, or 2+2+1 or 1+1+1+1+1 arrays - the 3+2 looks most efficient.