capacitors - Page 1
Forum section: electronics / projects

author: grifftech
author_rank: Frequent Contributor
timestamp: August 19, 2015, 12:30:40 am
content: What is a good brand/size of capacitor for quick charging and slow draining through a blue LED?

author: c4757p
author_rank: Super Contributor
timestamp: August 19, 2015, 12:36:01 am
content: Define 'slow'.

author: grifftech
author_rank: Frequent Contributor
timestamp: August 19, 2015, 04:51:34 am
content: 5 minutes

author: mariush
author_rank: Super Contributor
timestamp: August 19, 2015, 05:56:06 am
content: I think you basically want to fill a capacitor with energy and then discharge it through a LED, basically you want to make a flashlight.However, I think you have some misunderstandings about how capacitors and leds work so you should first understand this:1. A capacitor doesn't work like a battery. A battery charges slowly and then the voltage of the battery remains within a certain range (1.2 - 1.35v for a rechargeable AA battery) as the battery is discharged. This doesn't happen with capacitors. For example, if you connect a 3300uF 16v rated capacitor to a 9v battery, it will charge super fast and when you check the voltage with a multimeter, you will read around 9v. But, with every drop of energy you take out of the capacitor, the voltage will decrease until it's all the way down to 0v.Because isn't relatively fixed like with regular batteries, there has to be an electronic circuit to always give the LED the voltage it needs and restrict the amount of energy going through the LED.2. Compared to batteries, the amount of energy in a regular capacitor is very small. For example, capacitors like the ones used in computer power supplies can typically only power a LED for a few seconds. Supercapacitors however can hold enough energy to keep leds happy for hours.1. A LED doesn't behave like a regular flashlight bulb. If you connect a regular 3v flashlight bulb to a couple of AA batteries (connected in series), the bulb will be happy and work until the batteries discharge, because the wire inside the bulb has its own resistance so it only lets a particular amount of energy go through it.LEDs however don't have any wires, there's nothing to restrict the energy flowing through the LED so if you connect the LED to a power source that can provide a lot of energy (like a couple of AA batteries or a capacitor), the LED can be damaged by all that energy flowing through it, it can "burn out".So there needs to be some resistance to limit the energy going through it. The most basic resistance would be a simple resistor in series with the LED which basically would guarantee at any point only so much energy can go through the LED. This is fine when you have a power source with stable voltage like a couple of batteries connected in series, but it won't work well with a capacitor: as the voltage decreases, so will the brightness of the LED.This is a cheap simple but not very efficient method, a lot of energy would be wasted inside that resistor which restricts the energy to the LED. To get the most out, you'd need a led driver circuit.For 5 minutes, regular capacitors would be too large. You'd need a super capacitor or a couple of supercapacitors connected in series, to store enough energy to last for 5 minutes.Get back to us if you need more details or clarifications, me or someone else will try to explain. Try to use more than 2 words though.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: August 19, 2015, 06:13:24 am
content: At the risk of actually answering the OP's question....10 Farad, 2.7 Volt supercapacitor, plus a "joule thief" circuit like this:

author: hayatepilot
author_rank: Frequent Contributor
timestamp: August 19, 2015, 06:59:11 am
content: Yes, alsetalokin4017 is right.A LED (20mA) for 5 minutes needs about 18 Joules.This energy equals about a 5F 2.7V capacitor.If you discharge the cap to 1V you have used ~86% of the energy in the cap. And assuming that the LED driver is about 70% efficient means that you need a supercapacitor with at least 8.2Farad.With a little bit of safety margin gives you 10F.Greetings

author: rickselectricalprojects
author_rank: Regular Contributor
timestamp: August 20, 2015, 12:05:16 am
content: look at super caps on eBay. the minimum you would want would be about 10fhope this helps