Capacitive coupled LED

[umery2k75]

Hello everyone,
I wanted to experiment if I could drive an LED 3mm RED 1.8V forward bias voltage through a capacitor, so I connect function generator on HighZ on 5Vpp 300KHz connect it with different capacitors non-polar and polar and in series with resistor and a LED. All small non-polar capacitors like 0.1uf or 0.01uF shifts down the AC waveform just enough so the LED doesn't turns on, later I experiment with polar capacitor 100uF 25V, I was able to see the LED turning bright but eventually the AC wave shift downs and light intensity lowers down, I increase Vpp slowly then I can see bright light again then light intensity lower down. When I turned my Vpp to my original position 4.5Vpp coming from 10Vpp. I can see LED turns off completely and my majority AC waveforms has shifted down in negative and then I have to remove the capacitor - then short it and put it back again, then it starts to work again. Can anybody shed some light.


Experiment video

 

[danadakk]

Your LED is a diode in function, so when you start with a discharged cap, it slowly
charges up and acquires a V. A V no longer available to turn on LED. Note I simed
this with a regular diodes as I do not have a LED in simulator. 2 diodes to sim a red
LED turned on V.

Green is diode current.


Regards, Dana.

 

[KlausST]

Hi,

A LED works with DC current.
But a (series connected) capacitor blocks DC current.

Thus you can only see the LED light when the capacitor gets charged. As soon as it is charged high enough the DC current stops.

Try to use connect a anti parallel diode to the LED. Thus AC current can flow. In one direction through the LED, in the other direction through the diode.

Klaus

 

[FvM]

Did you review a LED datasheet? It has relative low reverse breakdown voltage. Without antiparallel diode, the LED can be easily destroyed in your experiment.

 

[danadakk]

Did you review a LED datasheet? It has relative low reverse breakdown voltage. Without antiparallel diode, the LED can be easily destroyed in your experiment.

Looked at several datasheets, Blue, Red seem to have 5V as max reverse voltage. So no
margin in design. That spec was, in some datasheets, at room temp. Even less margin in
actual use......

Using a reverse diode to protect LED, can function generator handle driving a forward
biased diode directly ? Consult the datasheet for the f() generator to make sure it
can manage, that is operative word of the day.


Regards, Dana.