# How does one use more than one device requiring the same input voltage on one/same power supply?

Lets say I want to power a high output LED powered by the appropriate/included driver as well as a heatsink fan; they both expect a 12V input voltage, but the LED driver expects 900mA meanwhile the fan expects 90mA. The desired power supply would be AA batteries.

What would I need to add to my circuit to work under the same power source, how would I wire it, would I still power it off of 12 volts (therefore 8xAA batteries)?

• put them in parallel? I don't really understand what the question here is.. – PlasmaHH Oct 13 '15 at 13:36
• you don't need anything else, just wire to join them in parallel. The 8xAA batteries will supply as much current as each load needs to reach the battery voltage. you can think of them as different resistors, and ohms law applies V=IR – Icy Oct 13 '15 at 13:44
• This is an extremely recurring misconception - voltage sources don't push current, the load takes it. – Dzarda Oct 13 '15 at 14:23
• @PlasmaHH Think that was the question. Assuming you didn't know about parallel vs series circuits, your question might look like the above. – Matt Harrison Oct 13 '15 at 14:25
• 990 mA is a VERY heavy load for AA cells. You would would get much better battery life, and a more stable voltage, if you used C or D cells. – Peter Bennett Oct 13 '15 at 15:58

If you wire them in series as you suggest, your voltage will vary a lot during discharge.

If you look at the following graph you'll notice that during ~most~ of the useful life of the batteries with a 990mA load their voltage will be closer to 1V. This means that to have a meaningful battery life you would need at least, say, 11~12 batteries in series. That said, this graph also indicates that their avg charge will get you from 50min to 80min life with avg AA batteries with your load.

Also, getting back to the fact that you need more than 8 batteries in series to extend the useful life of the device a bit, you have to find out if it can tolerate (11~12)*(1.5~1.6)V in their input. Most LED drivers probably will but I'm not sure about the fan.

IF they do tolerate, then you can do as others have suggested and add this battery set in parallel to extend battery life. This leads to the next point which is, to get a couple of hours of light you would need more than 20 AA batteries. Have you ever considered using RC LiPo batteries or SLA batteries?

• Where does he suggest wiring them in series? and where has anyone suggested wiring batteries in parallel? I do agree AA batteries are under-rated for a 990mA load, but that's not in the question. – Icy Oct 13 '15 at 14:23
• "power it off of 12 volts (therefore 8xAA batteries)?" to get 12V out ot 8AA batteries you wire them in series, its implicit.. first comment: "put them in parallel?" – Wesley Lee Oct 13 '15 at 15:33
• commenting to clarify that I misunderstood the first comment, he meant putting the loads in parallel.. – Wesley Lee Oct 13 '15 at 15:39
• Alright, as per this comment, I've changed my mind about the 8xAA batteries and plan on doing 8xC instead. Judging on the alkaline mAh ratings, would this mean I'd get ~5.5 hours of battery life as opposed to ~2 hours? – user3125996 Oct 13 '15 at 19:00
• By this datasheet, looking at 400mA consumption and multiplying it by 2.5*, it says that your batteries will be at 9.6V in about 1.5hour. BUT 400mA to 1A cant be linearly scaled I think, so probably a bit worse. data.energizer.com/PDFs/e93.pdf – Wesley Lee Oct 13 '15 at 19:13

Just put them in parallel as shown

simulate this circuit – Schematic created using CircuitLab

• The schematic you gave confused me a little, are you implying I should simply wire my two electronics in series, or should I have more electronics in my chain such as resistors of some sort? – user3125996 Oct 13 '15 at 15:18
• The two 'devices' are the two dashed blocks. I have just shown how the internals of the devices might be modelled. R1 and R2 are the effective internal resistances of the devices. The two devices are in parallel, across the battery. – Icy Oct 13 '15 at 15:25