Capacitors not behaving as expected

Thread Starter

NewDCguy

Joined Feb 7, 2021
6
Hey everyone, I have a very basic DC circuit set up on a breadboard running 1 LED with a push button switch (+220 ohm resistor to keep LKED from blowing). When I add (1) 100 microfarad capacitor in parallel, I would expect the light fade off rather than turn off when the switch is released, but had no effect.

So I added another 100 mF in parallel - no effect

So I added a third, and when I do the breadboard power supply board no longer provides power (the green light won't go on when switched on). Power comes back on again when I remove the 3rd cap.

Hoping one of you smart, wonderful people can explain why I'm not getting the 'fade' effect adding caps in parallel as expected, and why power no longer turns on with 3 in place.

Note: AC/DC converter output is 5v/10A
caps are 100mF, 50v rated
LED is just a basic

Thanks!
 

Tonyr1084

Joined Sep 24, 2015
5,969
Welcome to AAC.

Can you provide a schematic?

5V ÷ 22Ω = 23mA Is the cap parallel to the LED and the Resistor? Or just parallel to the LED? If it's just parallel to the LED then it will only charge to the forward voltage (Vf) of the LED. When power is cut - the let will blink faster than you can see. The cap should be parallel to the series resistor and LED.
 

ericgibbs

Joined Jan 29, 2010
12,902
hi New,
Welcome to AAC,
Adding a cap in parallel with the LED will not work as you expect.
What colour are the LED's?
E
 

Audioguru again

Joined Oct 21, 2019
3,177
The capacitor discharges some of its voltage into the LED when the power supply is disconnected. But an LED has a narrow voltage range to work. A modern bright green LED is bright at 3.4V, is dim at 3.3V and is off at 3.0V. It takes a very short amount of time for the capacitor to discharge only 0.4V.

Your 3rd capacitor is shorted or is connected backwards.
 

Marcus2012

Joined Feb 22, 2015
403
The switch is downstream from the capacitors so they won't discharge through the LED on turn off anyway. Place the switch upsteam of the capacitors.
 

Thread Starter

NewDCguy

Joined Feb 7, 2021
6
After moving the button I got it to fade - but needed 5 100mF caps to do it =)
Attaching pic for everyone to reference laugh at.

I'm (obviously) new to this, goal is to build a talking/flashing toy for the kids, but trying to learn a bit in the process. Thanks for your help, I'm sure I'll be visiting again as things progress.
 

Attachments

ericgibbs

Joined Jan 29, 2010
12,902
hi New,
You are using mF as the Cap notation it should be uF for microFarad, a milliFarad is 1000 times bigger.
E

BTW: If the Vinput to that 5V regulator is say 9V or so, you could use that 9v to charge the Caps and drive the LED's using a higher value resistor
 

Tonyr1084

Joined Sep 24, 2015
5,969
How long a duration do you want the LED to take to fade out?

Instead of using five 100µF caps you can use a single 470µF cap more closely rated for 5 volts. A 10 volt cap will give you plenty of protection while keeping the cap size smaller. Two of them will give you even longer a fade time. A 100µF cap at 10V is a lot smaller than one at 50V.
 
Last edited:

djsfantasi

Joined Apr 11, 2010
7,689
μ
After moving the button I got it to fade - but needed 5 100mF caps to do it =)
Attaching pic for everyone to reference laugh at.

I'm (obviously) new to this, goal is to build a talking/flashing toy for the kids, but trying to learn a bit in the process. Thanks for your help, I'm sure I'll be visiting again as things progress.

Five 100μF caps is about right. I built a control panel with a 470μF cap across the 5V supply and when I cut power, the indicator LEDs faded out.
 

Thread Starter

NewDCguy

Joined Feb 7, 2021
6
hi New,
You are using mF as the Cap notation it should be uF for microFarad, a milliFarad is 1000 times bigger.
E

BTW: If the Vinput to that 5V regulator is say 9V or so, you could use that 9v to charge the Caps and drive the LED's using a higher value resistor
Ah yeah, sorry - couldn't find the special "u" symbol so incorrectly used an 'm'
 

Thread Starter

NewDCguy

Joined Feb 7, 2021
6
How long a duration do you want the LED to take to fade out?

Instead of using five 100µF caps you can use a single 470µF cap more closely rated for 5 volts. A 10 volt cap will give you plenty of protection while keeping the cap size smaller. Two of them will give you even longer a fade time. A 100µF cap at 10V is a lot smaller than one at 50V.
I'm just testing out the principles I learned on capacitors, though I have read that you want to put a 1000uF before an LED strip to smooth out V drops which I will want to do at some point.

I wound up adding 7 total 100uF caps to the circuit, and though I do get a small (quick) fade, it's nowhere near the fade I saw in the video I watched:


I may have to check my resistor values, maybe it's lower than I thought.
 

Tonyr1084

Joined Sep 24, 2015
5,969
Well, with 5V and a 220Ω resistor your running 23mA (0.023 amps). You could use a 470Ω resistor. That would give you a little under 11mA, which, depending on the type of LED you're using should still be bright enough. The lower the current the longer the caps will push the LED.

[edit] In the video he changes from a 220Ω resistor to one of 1,000Ω (1KΩ). At 1KΩ he's running 5mA through the LED. As you can see it's still bright enough to be easily seen. Probably not so in full direct sunlight, but plenty bright.
 
Last edited:
Top