DIODE LED as DIODE Varicap Varactor

[rigmo]

DIODE LED as DIODE Varicap Varactor Measuring - FM VARICAP DIODE - VHF UHF Varicap Varactor DIODE most common use
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https://www.on1bes.be/varicaps_meten.html
DIODES AS VARICAPS
Varicap diodes in the field Varicap diodes are very useful components in HF electronics and prove their usefulness especially in radio receivers and transmitter circuits. However, they are not always easy to find in the trade and it is often a matter of searching for the appropriate type that has the correct values ​​of capacitance to use in a VFO, oscillator circuit of a transmitter or receiver. It will be unknown to many that in principle every DIODE is also a varicap. This knowledge therefore arouses the curiosity to test for ourselves which types of diodes we can also use as varicaps.
Roughly speaking, we can make a list of varicaps that are needed in eg receiver frequencies:
AM - 15 to 500 pF
VHF - 8 to 30 pF
UHF - 2 to 18 pF
Testing varicaps
There are different methods for testing varicaps, but usually there is a frequency meter to measure the effect of a varicap in an oscillator circuit. The capacitance of a varicap cannot be measured directly. If one includes a varicap in an oscillator with a crystal of eg 10MHz, the effect of the varicap on the known frequency (10MHz) can be measured with the frequency meter, and the value of the varicap can be calculated approximately.
However, if you have a C-meter (or combination L/C meter), then with a small addition each diode as a varicap is easy to measure!
Here a small auxiliary circuit to measure the diode "test" with a C-meter :
The problem is that a diode or varicap needs a voltage to perceive the capacitance effect.
This voltage must be "reverse" on the diode - so in reverse direction or in "reverse voltage".
At 0 volts, a varicap gives the most capacity, at 9 to 12 volts the least capacity.
Results
The measured diodes from the junk box can be seen in the picture at the top of this page.
What is very striking in the measurements of the different diodes is that with LEDs, for example, the values ​​are very different
and are very useful for replacement varicap types.
This is probably due to the large surface area of ​​the junction junction in the diode.
If we compare physically with a real capacitance, the condition applies: the larger the surface of
a capacitance, the greater the capacitance value at equal distance from the plates.
It is also noticeable that, for example, with a 4148 diode, no appreciable capacitance is measured; this diode is
made more like a switching diode, and has a small junction junction.

FM VARICAP DIODE - VHF UHF Varicap Varactor DIODE most common use
1T397 2.60-51.0pF M-235 sony
MV209-MMBV109-MBV109 (BB103-BB109) 5-32pF
S167D = BB106 4-20pF
BB107 15-300pF
BB109G 4.3-32pF SOD23
BB132 2.3-75pF SOD323 pcselectronics
BB133 2.2-46pF SOD323
BB148 2.8-33.5pF DO-34
BB204B 14-42pF SOT-54
BB209 2.6-31pF SOD23
BB409 4.5-32pF DO-35
BB439 4.3-32pF SOD323
BB502 4-32pF 10B2
BB504 4-32pF SOT23
BB609A-B 2.8-33.5pF DO-35
BB610 3.35-69pF SOD123
BB619A-B 2.5-39.5pF SOD123
BB620 3.35-69pF SOD123
BB640 2.9-72pF red S SOD-323 pcselectronics
BB709B 2.7-41pF SOD23
BB809 4-46pF DO-34
BB909B 2.8-33.5pF DO-34
BB910 2.6-40 pF sod323 - axial TO92S-2 https://www.ebay.com/itm/222547098230 The winner
BBY40 6-32pF SOT23 100pcs 8e
Jaeger electronic catalog 1999
MV2105 6-30pF SOT23 TO92
KV1471E 6-40pF 1-4.5 V
MA2S304 6-29.8pF SS-Mini Type Package panasonic
MA2S357 2.5-34.3pF
MA2S374 2.7-49.5pF
MA322 4.5-34pF SOD23
MA325-MA326 2.6-32.5pF
MA337 2.6-42.6pF marking code: 6F SOD123
MA338 2.6-32pF 6H
MA353 2.6-38.7pF 6S
MA357 2.5-34.3pF 7K
MA357J 2.5-34.3pF SOD323
MA361 2.6-32.6pF 6B
...https://www.web-bcs.com/diode/dv/ma/MA2S304.php

[rigmo]

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[Albert H]

The supposition in the article is mostly correct. The capacitance effect is due to the thickness of the "depletion layer" which forms at the boundary between the N-type and the P-type silicon (or germanium or a couple of other technologies). The larger the surface area of the depletion layer, the bigger the capacitance.

I've used all sorts of diodes as varicaps over the years.