# What resistor for uneven amount of LEDs in series, in parallel

I have an IR LED array which is powered by 12VDC. There are 10 chains wired in parallel. Each chain has 9 IR LEDs wired in series with a 10ohm 1/4 watt resistor. I also have 2 more chains wired in parallel, however these only have 6 LEDs.

simulate this circuit – Schematic created using CircuitLab

I've used this tool to try and figure out an answer, but want to be sure I'm using the correct values: http://led.linear1.org/led.wiz It does suggest using a 1 Ohm 1/4 watt resistor for the chains of 9, however I only have 10 Ohm as my minimum.

The LEDs have a forward voltage of 1.5V and a current draw of 20mA (to the best of my knowledge).

I want to calculate the correct resistance value to use for these shorter chains.

• Please draw a schematic. – winny Aug 18 '18 at 22:09
• Nine of 1.5V LEDs need 13.5 V input nominal. – Ale..chenski Aug 18 '18 at 22:31
• That’s not a schematic, but rather some variation of a fritzling connection table. There is a built in schematic editor here on EE.SE. Press edit on your question and press the schematic symbol and it will launch the editor. – winny Aug 18 '18 at 22:41
• That's a far better tool, thanks for that. I've again updated the question. – Alex Holsgrove Aug 18 '18 at 22:53
• @TonyEErocketscientist - so if I use the 100 Ohm resistors for the strips of 9, I don't know what you're saying to use for the strips of 6? Where do you get 40 volts from please? – Alex Holsgrove Aug 19 '18 at 0:23