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How good is the 1N4004 diode as a varactor

 

What is your experience with using the 1N4004 diode as a varactor in tuning circuits?? How does it compare to the MV209? In the same circuit, what is the comparison between the 2 diodes in tuning range?
 

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Not that particular part but useful info here. Plus a circuit to take some data.?
 

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Hi Doug,

One normally doesn't run a varactor down to 0 volts (as Han's chart showed), but at minimum 1 volts. I tested two brands of rectifier--1N4003 and 1N4007.? The 4003 at 1V was 10.6 pF and at 12V was 5.2 pF. The 4007 at 1V was 7.8 pF and at 12V was 3.8 pF, so quite a difference, relatively speaking. I don't have a MV209 to test but the specs say it's a nominal 29 pF varactor at 3 volts; the data shows? a graph with about 44 pF extrapolated at 1V. So comparing the 1Nxxx to MV209 show quite different total capacitances which of course would also give a smaller tuning range for the 1Nxxx vs. for the MV209.

73,

Steve AA7U


On 4/30/2025 4:10 PM, Doug Hendricks via groups.io wrote:

What is your experience with using the 1N4004 diode as a varactor in tuning circuits?? How does it compare to the MV209? In the same circuit, what is the comparison between the 2 diodes in tuning range?
 

Thanks for the info.? Very interesting that a 5mm red led works so well.? I'm going to do some testing.? ?Again, I appreciate the help.? Doug
 

What circuit did you use to test?
 

I have no experience with the 1N4004 as a varactor, but I did use a 1N4001 as one in a simple NE602-LM386 DC receiver on 40M. The inductor was 3.1?H (27 turns on a T50-7). With a voltage span of 3.2V-7.9V on the 1N4001, I was getting a tuning range of~7000-7054KHz.? That would equate to a (rather small) capacitance variation of ~164-167pF.
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If you're interested, I can open it up, fiddle with a few things, and provide the frequency swing for voltages on the 1N4001 below 3.2V. The write-up on the receiver is here -?
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Dave
AA7EE
 

Please ignore my last post. I wish these things could be edited after posting, but it doesn't work that way here! I was forgetting the influence of the 270pF cap connected to pin 6 of the 602. My brain is somewhat fried this evening. I need to learn not to post quite so quickly.
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Dave
AA7EE
 

Hans provides a circuit to measure its varying capacitance. Of course you can also install candidate diodes into your vfo that may be easier.?
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I realize in the old days folk sought a huge range. But note the venerable nc20 used a variable cap in parallel to expand the range. Or switching in a fixed cap. I am confident Doug has seen all these approaches.?
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My all time favorite may be the vcxo in the 38S.?
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Curt
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I'm assuming you've seen this Doug, in the old QRP-Tech group?
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Dave
AA7EE
 

If anyone has a project that needs an MV209 (40pF at 1V to 10pF at 9V) or an MV2101 (10pF at 1V to 5pF at 9V), I have some extras. But to be clear I'm not moving them from my spare stock to your spare stock. I will let you have 1 at a time for a specific project or repair.
You can contact me at thomas.b.seeger(at)gmail.com
73 Tom
 

Like Dave mentioned, there is info in the old qrp group from the 25th anniversery NC-40 build...
Here is what I wrote (message #14308):
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"I am running a 1n4007 in mine, works great.
I am using Chuck's crystals so my VFO is at 2.9mHz but I get 60KHz range out of it.
Don't think it is as stable as Chuck's but I tested from cold room start and for longer
time... ?about 100Hz.
Didn't buy the specified varactor but tried LEDs, 1N4001, 1n9148, etc. ?best was the
1N4007."
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73,
Gary
WB6OGD
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