capacitors

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MX6boii

Copper - Posts: 74

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Joined: October 22, 2002

Location: Pennsylvania, United States

Posted: March 03, 2005 at 5:25 PM / IP Logged

I've been trying for the last 2 weeks to design a circuit that will charge a capacitor up while lighting a LED and then keep the LED lite up for 5sec after the power is cut.

The problems I've noticed is that when I cut the power there is no current to drive the stored power from the cap to light up the LED.

Does anyone have any idea what I'm doing wrong or maybe I'm using the wrong components for this type of application. I thought this would be a simple circuit with components I could get from school.

Thanks for your help.

Francious70

Silver - Posts: 629

Joined: July 26, 2004

Location: United States

Posted: March 04, 2005 at 9:19 AM / IP Logged

You may have the led backwards, even the have a polarity. Reverse the leads and see if that helps.

Paul

MX6boii

Copper - Posts: 74

Joined: October 22, 2002

Location: Pennsylvania, United States

Posted: March 04, 2005 at 9:51 AM / IP Logged

I'm sure I have the LED in correctly because when it didn't work I bypassed the cap and wired it straight to the light and it lite up.

My circuit is wired like this:

12VDC---Resistor----(+)cap(-)---(+)LED(-)Ground

I have the resistor as 1K and the cap is .0068F. With that combination it should take the cap 6.8 sec to fully charge. Once I turn the 12V supply off after it's charged the LED doesn't stay on. Well it's not even on in the first place. So I'm confused. I pretty sure I'm negating a very important electrical concept leading to this circuit not working.

That's for your input though.

Francious70

Silver - Posts: 629

Joined: July 26, 2004

Location: United States

Posted: March 04, 2005 at 11:18 AM / IP Logged

You may have the cap backwards then. You setup should work, unless the cap dosen't provide enough wattage for long enough to light the LED.

Paul

daniel2002p

Silver - Posts: 524

Joined: October 10, 2003

Location: United States

Posted: March 05, 2005 at 5:55 AM / IP Logged

can you draw out the circuit and post it here so that we can see what is going on?

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MX6boii

Copper - Posts: 74

Joined: October 22, 2002

Location: Pennsylvania, United States

Posted: March 09, 2005 at 9:08 AM / IP Logged

Here it is.

Thanks for the help.

Mad Scientists

Silver - Posts: 380

Joined: February 07, 2004

Location: United States

Posted: March 18, 2005 at 8:49 PM / IP Logged

capacitors don't pass DC.. get the cap out of series with the LED and install it in parallel.. like you would for a big cap in the trunk with the amp.

The 1k ohm resistor is limiting overall circuit current to about 6 mA.. lose the 1k ohm resistor too.

Start with just the 12v supply, the 470 ohm resistor and the LED. Verify everything works and the LED lights up. Then add a capacitor into the circuit, in paralled with the LED/resistor, and see if the LED stays lit after power is removed. If not, start going with larger capacitors.

consider talking to someone at school about using a transistor and an RC circuit on the base.

Jim

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MX6boii

Copper - Posts: 74

Joined: October 22, 2002

Location: Pennsylvania, United States

Posted: March 19, 2005 at 1:57 AM / IP Logged

Jim,

Thanks for taking the time to troubleshoot my circuit. I did what you described on Thursday, so everything's working. It's just horrible how I forget this stuff after a year of learning it. I guess it's cause I'm not putting it to use.

Thanks again.

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capacitors

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