MEASURING THE VARIABLE CAPACITANCE OF LEDS

The circuit used to measure LEDs' capacitance included a 100nF capacitor in series with the LED and its current limiting resistor.

The value of the series capacitor needs to be taken into account when assessing the capacitance of the LED. That said, it is likely that any tuned circuit using the varicap LED will need that capacitor for DC isolation.

With the circuit built on a piece of matrix board, a green minature LED was soldered onto the test point terminals.

My trusty old Fluke DVM was used to measure the voltage from the ponteial divider to the LED.

I used an Atlas LCR40 from Peak Electronics (http://www.peakelec.co.uk/) to measure the capacitance.

Since there were 21 of these green LEDs in the box, the measurement was performed over 21 iterations.
 

The results, taking simple arithmetic averages for voltage and total capacitance, were recorded as follows. Since the actual value of the series capacitor was measured at 995pF, the capacitance of each LED was calculated, per voltage measurement, using

1/C_LED = (1/C_Total) - (1/995)

all values in pF.

With a total capacitance range from 50pF to 130pF, this circuit provides a great basis for varicap tuned circuits.

The rate of change of capacitance with voltage is quite linear, as the graph below shows.

Click on the graph to see a larger image.