Any mathematicians out there?

[wibblywobbly]

Here's a question for you.

If I have a 7.2v 2000mah battery, how long can I have 10 x 5v leds connected to it before it expires? In reality they will not be on constantly, so whatever time you come up with will be extended considerably, I am just curious as to whether it would last a few hours?

The led's would require resistors but that's not a problem, though could I use one resistor and feed all of the led's from it?

Cheers

Rob G

[tankman]

Errrrrr a long time ?. Sorry Rob just couldn't resist that one
Alan

[NigelDerEnglander]

Hi Rob,

If these are similar to the Maplins CJ64U/CJ65V Kingbright LEDs (p763 of Winter 2009/10 catalogue) then as they each require 13mA then ten will take 130mA. So assuming your pack is in fairly good condition you will easily get 10 hours out of it (10 x 130 = 1300mAh) with some left over.

I've never used these regulated LEDs before, but I guess you _might_ be able to use a single resistor for all ten. If so, then it has to drop (7.2-5=) 2.2 volts at 130mA. Which using V=IR means you need a resistor of 2.2/0.130 = 17 Ohms.

The resistor will get a bit hot though and may burn out (W=I^2R) as it will eat up 0.130x0.130x17 watts ~= 300mW and most leaded resistors are usually rated 1/4 watt. For safety parallel up four 68 Ohm 1/4 watt resistors. Just wind their legs together in a 2x2 bundle.

Hope this helps.

Nige

[tankman]

Hi again Rob to make up for the earlier post one of the guy's posted this on the site which may be of some use to you. Sorry to say I can't remember who posted this up as they deserve the credit http://www.theledlight.com/ledcircuits.html" onclick="window.open(this.href);return false; . All I did was to book mark it. Best of luck with whatever you are planning ?.
Alan

[wibblywobbly]

Thanks guys, much appreciated.

Rob G

[madrob]

Heres another that may help
http://led.linear1.org/led.wiz" onclick="window.open(this.href);return false;

[fleshpedler]

No help Rob ... but it begs the question WHY..................
Fleshy.............................

[madrob]

Its supposed to help with wiring diagrams and resistor values, on the warship forum i go on everyone raves about it, i aint got a clue cos i am thick, thought it may help