Electronic – Inserting Capacitor into CIrcuit causes LED to fade – Why

batteriescapacitorledseries

My components are

  • 2x 1.5 V batteries
  • 25V 2200uF capacitor
  • 3v Led
  • copper cables

My circuit looks like the following.

schematic

simulate this circuit – Schematic created using CircuitLab

My understanding here is that as the two batteries are in linked in series, we now have 3v for our circuit.

When completing the circuit the LED starts bright then slowly fades and fades until there is no light at all.
Also interestingly if I dissconnect the batteries, wait a minute, and reconnect them completing the circuit, the LED stays off – It does not start bright and fade again.

What is actually happening here? Is the capacitor consuming all the power?
If I left this circuit complete for a while, would the capacitor gradually consume all the power until it reaches its 25V and then suddenly release 25V instantly? (blowing the LED of course)

I actually ran this test because I wanted to try to fill the capacitor up, and then dissconect the batteries hopeing to see the LED stay alight for a while running from the capacitors consumed power.(if this is even possible, perhaps my understanding is not correct)

Best Answer

As cowbydan stated, placing the capacitor in series to the LED will block DC currents. To achieve the desired effect of a glowing LED after removing the batteries you need a circuit similar to this :

schematic

simulate this circuit – Schematic created using CircuitLab

The capacitor needs to be in parallel to the LED and the polarity of the capacitor needs to match the polarity of the connected battery. The resistor R1 is not mandatory in your particular application but it reduces the current which the LED consumes. A larger resistor value will increase the time the LED glows after you remove the battery but will also decrease its brightness. Just test it with some different values. Choosing a LED with a lower voltage rating would require a resistor in series to it to reduce the voltage.