First post - Capacitor question:

[sydcomebak]

I have 4 LEDs from Radio Shack that I want to keep on for about 5 hrs:

Forward Supply Voltage: 2.1 Typ / 3.0 Max
Forward Supply Current: 20mA Typ / 30mA Max

I want to install a capacitor that will power these lights.

Am I going the wrong route? Should I be looking into a Lion or NiMH battery? I'm going to try to find a female MicroUSB to charge the power source either way.

What size / type of power source should I look into? Anyone have a source / pinout for female microUSB components?

Thanks!

 

[MikeMl]

Hi Syd.

A capacitor will not work. You need a device that maintains nearly a constant voltage as it gives up its stored energy, ie a rechargable battery. A capacitor's voltage decreases more or less linearly with time as it is depleted.

I'd suggest a three-cell NiMh battery. That can be charged from 5V (USB) with a suitable charging circuit, and run four parallel LEDs (with a resistor in series with each led).

 

[sydcomebak]

Mike,

Thanks for the prompt reply!

Space is an issue. I'd really like to find a cheap & rechargeable CR 2032 battery. Also, I've never built a charging circuit. I have Googled them, but I just got more confused.

 

[crutschow]

The four LEDs running at 20mA for 5 hours will require a battery capacity of 20ma x 5 x 4 = 400mA-Hrs. An AAA NiMH will provide that but not a CR 2032, which has perhaps a tenth of that capacity.

 

[MikeMl]

By definition, Rat Shack Leds will not be voltage matched, so you need a separate current-limiting resistor in series with each Led.

As to recharging, read the wiki. How long are you willing to wait for a recharge?