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Varicap diodes (varactors) PDF Print E-mail
Written by Hans Summers   
Tuesday, 01 September 2009 04:18

Varicap (a.k.a. varactor) diodes exhibit a variable capacitance when they are reverse biased with a variable voltage. Unfortunately specific varicap diode part numbers are often very hard to obtain and seem to go obsolete much more quickly than other types of semiconductor. Hence if you are attempting to build a project more than a few years old you might find it hard to get the specified varicap. However, in fact all diodes have the variable capacitance in reverse bias. On this page are the results from various experiments with ordinary diodes used as varicaps.

My early experiments in 1994.

Way back in 1994 (actually I think it might have been 1993) when I'd recently built a simple 8-digit frequency counter, and had been reading about people using 1N4001 rectifier diodes and such like as varicaps, I decided to put the frequency counter to good(?) use and make some measurements of my own. I had the idea of using 5mm red LED's as varicaps, they tested well, and I happily used them thereafter in all my projects, never bothering with "real" varicap diodes ever again. Read More...
LED's as varicaps.

Here we are in 2007 and the distinguished David VK6DI, who knows that I am fortunate to own an AADE LC Meter, requested some further measurements on LED's. Read More...
Rectifier diodes as varicaps.

No sooner than I had published the 2007 LED testing results here on my web site, than the (also distinguished) Arv K7HKL pointed out that for completeness it would be interesting to compare those LED results with rectifier diodes such as the 1N4007. I measured all those I found in my currently somewhat depleted junk box cabinet drawer, alongside a couple of small signals and, GASP, a "real" varicap diode! Read More...
Forward biased diodes as varicaps.

A few months ago (late 2006) I accidentally discovered that LED's exhibit a variable capacitance even when forward biased. Since I have by now already measured so many MANY diodes, I thought why not add a few more results to the mix. Here are results for some forward biased LED's and 1N4007. More questions than answers. Read More...
Photovoltaic(?) effects with LED's.

Roger K7RXV (of course, also distinguished) wasted very little time in warning me that the capacitance of a diode is sensitive to the amount of light falling upon it. I was already aware of this possible problem with using LED's which naturally are designed to expose the junction so that the emitted light can be seen, though I have never encountered any problems with my own projects. Thanks to Roger's email, I decided to test this once and for all. Read More...
Further LED results from John VK6JY.

My good friend John VK6JY in Albany, WA made some further LED measurements in both forward and reverse modes, using his AADE LC-meter kit. Read More...
Dave G7UVW measures less usual LED's.

Dave sent in some detailed measurements investigating the variable capacitance of three slightly less usual LED's which haven't been tested before (that I know of): White, Infra-red (IR) and Ultra-violet (UV). Read More...
ZL2PD Varicaps

Andrew ZL2PD has an interesting page on Varicaps:


These excel spreadsheets contain all the data and charts used in these pages.

CLICK HERE for original 1994 measurements

CLICK HERE LED and rectifier Diode measurements from 2007

CLICK HERE for Fwd Bias measurements

CLICK HERE for Dave G7UVW measurements

Last Updated on Wednesday, 04 January 2012 13:42
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