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# What resistor should I use? 9 volt battery to a 3.6 volt @ 30 mA LED Answered

What resistor should I use? I have a 9 volt battery that is connected to 30 LEDs in parallel and i need a resistor. the LEDs are blue and are at 3.6 volts with 30mA. Each LED is in parallel I want to put one resistor on the positive side of the battery before all the LEDs.What size resistor do you think i need? The two online calculator say i should use a 220 ohm at 1/2 watt  but another one says I should us a 220 ohm at 1/4 watt resistor. WHat do you think I should use?

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## Comments

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Each of the 30 leds would need it's own 180 ohm series resistor. There is no other way to power 30 leds from a 9 volt DC source. And yes a small 9 volt battery won't have the current capacity to power 30 such leds at one time.

If you're using 30 leds, don't use a single 9v battery or you'll get VERY abysmal runtime. If you ABSOLUTELY need 9 volts, use 6xAA batteries in a holder.
220 ohms for a single led on 9 volts is correct.
3.6 volts for the LED means 5.4 volts for the resistor in series (current is the same through both series devices)...that's a BIG waste, because for every watt of led you get 1.5 watts of resistor.

A better design would be to put two leds in series with each resistor, so the LED voltage is 7.2v and the resistor voltage is 1.8. 1.8 volts @ 30mA is a much more gentle V = IR, R = V/I
1.8/0.03 = 60 ohms.

9v alkaline battery = 550mAh.

Single leds in parallel = 30x 30mA = 900mA = 37 minutes
Double leds in parallel = 15x 30mA = 450mA = 74 minutes

Alternately, a 220 ohm with single leds on 3AA batteries would have a similar effect on efficiency.

As for what wattage, single leds = 30mA @ 5.4 volts = 0.16 watts. A quarter watt resistor would do, the half watt suggestion was for 'just in case it gets too hot'.

But if I do two LEDs in series then a resistor wouldn't one LED be not as bright as the other? I can't have one LED brighter than the other

In case there was any confusion with how I phrased it, look at mpilch's pictogram. The resistors ensure each parallel strand gets the same(ish) current, and the leds in series themselves will ensure they get the correct voltage.

NO ! They BOTH carry the same current, they will both be the same brightness, within the limits of the LED quality you bought. If you buy them from Chinese supplier, all bets are off, but quality LEDs from people like Nichia, Toshiba, Cree are "binned" into different brightness and colour grades so you can be sure they are the same brightness at the same forward current

One calc is being more conservative then the other. The higher the wattage rating on the resistor the more power it is able to safely dissipate. So you can't go wrong with the higher wattage rating

But you shouldn't have them in parallel only. They need to be in a series parallel array like the picture i've attached. Like frollard said a series parallel array 60 ohm to 68 ohm resistors is what you should go with.

You can't do it like that. You could put two LEDs in series, and a resistor on each pair, but you will destroy your LEDs with a single resistor, also, a standard 9V battery has a life measured in a few minutes taking 900mA out of it.