Hey electronics guys!

[saint]

If I am going to wire up 4 LEDs to a 12V power source (or 9 or 6 for that matter), is there a difference in how long the batteries will last if the LEDs are in serial vs. parallel?

[HaRuMaN]

If I am going to wire up 4 LEDs to a 12V power source (or 9 or 6 for that matter), is there a difference in how long the batteries will last if the LEDs are in serial vs. parallel?

Are they 12V LED's?  Then you would need to wire in parallel.  If you did series, each LED would only get 3 volts, vs. 12 volts in parallel.

[saint]

They are 3.2V LEDs that don't have resistors. I'm putting them in the circuit. I will be using 4 (possibly 6) LEDs rated at 35 mA, 3.2V. I will either be using a 9 or 12 v power supply (basically stacking 3-volt button cell batteries). I'm using the LED calculator at http://ledcalc.com/#calc to determine the appropriate resistors. Seems like I have 3 real options - series, parallel, or a mix.

Assuming 4 LEDs: The mix uses 62 mA. The parallel uses 129 mA. The series can't be done with 12V, I have to bump up another battery to 15V, which then uses 32.4 mA. Does that all seem right, and if so do you have a recommendation? My goal is maximum battery life. I'm assuming with the resistors used as shown, that all 3 circuits will achieve equivalent brightness?

Thanks!

Mix


Parallel


Series

[MonMotha]

The answer to your question is yes.  Since each "string" (resistor plus however many LEDs you put on that branch in series) needs the same amount of current, and the input voltage is fixed, each string draws the same amount of power (since P = VI).  This means you want to minimize the number of "strings" by putting as many in series as you can, then putting the "strings" in parallel.

For 4 LEDs at 3.2V each, go with 2 parallel strings of 2 in series.  If you need 6, go with 2 strings of 3.  Note that you'll need a different resistor for those two configurations.  Battery life will be essentially the same.  In the latter scenario, the resistor burns less power.  In the former, the resistor takes up the power that would be taken by the 3rd LED.

As you've noted, you can't quite put all 4 in series.  It'll work, but they'll be fairly dim.  I also advise against connecting LEDs up with no resistor at all, as any fault will pretty much instantly destroy them all.  The resistor, even if it's small, can provide a surprising amount of buffer if something goes wrong.