# capsitor to make a blink led

Discussion in 'General Electronics Chat' started by chassler53852, Apr 16, 2013.

1. ### chassler53852 Thread Starter New Member

Jan 22, 2013
10
0
ok so I have a 6volt battery from those 1.5 watch cells put together
I have a 4.7uf 35v capsitor when I put the 6 volt to the capasitor
then put the capasitor to the 3v led light nothering happens. what
I cant figure out is that voltage test on capasitor only shows 2 volts
when I put the multimeter to it. Help

2. ### MrChips Moderator

Oct 2, 2009
18,881
6,046
The amount of charge stored in a 4.7μF capacitor is not enough to keep the LED lit for any length of time. If you look carefully at the LED you will see a very short flash the very first time you connect the charged capacitor to the LED. The voltage on the capacitor rapidly falls below the turn on voltage of the LED.

3. ### BillB3857 AAC Fanatic!

Feb 28, 2009
2,490
385
First, you need to put a resistor in series with your charged cap and the LED. Otherwise, the life expectancy of the LED will be short. The 2 volts you are reading, I assume are after you have connected the LED. Right? If so, you are seeing what is left over after the cap has discharged through the LED and the LED has stopped conducting at the 2 volt level. Others may have a better explanation.

4. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
198
The current limiting resistor is calculated like this:

First, guessing on the current, use 10ma.

Then:
Resistor = (battery voltage - LED voltage)/current
=(6-3)/.01= 300 ohms.
A 220-470 ohm would be fine.

5. ### chassler53852 Thread Starter New Member

Jan 22, 2013
10
0
the 2 volts is what I get connecting the mutimeter directly to the capasitor before its connected to the led. do I need a differnent capasitor?

6. ### chassler53852 Thread Starter New Member

Jan 22, 2013
10
0
ok it did blick for 1/2 second. so can somehow i hook up my 10k resistors to it to make it blick on and off. how do I hook it up?

Feb 28, 2009
2,490
385