How important is the forward voltage rating on a crystal radio diode.?

I have been researching how to build a crystal radio, and have built a few from schematics I have found on the web (though none have worked so far). The thing that is confusing for me is the range of forward voltage ratings I have found. Some of the part numbers I have come across such as 1N34A show a forward voltage of 1 volt. While others recommend 500mV and others as low as you can get.

With so many instructions saying different values for best results. Is there a recommended rule of thumb for choosing a diode for a radio detector?

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iceng3 years ago

Trying to visualize the circuit.

Any way the diode has to pass a tiny ( small voltage ) audio signal and a low forward germanium semiconductor P N junction ie Diode helps in that respect.

The next thing is; the diode should not be constructed to carry large currents which would make a capacitive absorption of a weak signal. You want a small signal diode much like the 1N34A.

I read somewhere the 1N34A is a point contact device which you would want..

You say you are having no working results. Perhaps the materials you are using for the back board have a weak resistive leakage upsetting your efforts.

Even building on glass a finger swipe can leave an oily residue to upset a circuit with unwanted electrical pathways.

Keep your parts clean and isolated.

rickharris3 years ago

You need to look for a germanium diode. It looks like this

You can find projects to make foxhole radios that will tell you how to make a contact diode from raw materials.

PS I am an electronics engineer and have never done well with building radios - It's very hard to get right.


Great picture Rick, I was not able to follow it to the source article, can you help ?


atrumblood (author)  rickharris3 years ago

Thanks, I was already aware that I needed a germanium diode. I should have mentioned that in my question.

However I am still curious as to the forward voltage variances I've seen.

atrumblood (author) 3 years ago

Thanks for the replies guys.

I dug through my assortment of diodes and found 3 Germanium Diodes. I am having trouble locating the data sheet on one of them.

Its part number is 0A70.

I Also found two of 0A79

and one 1N87.

Hopefully I will get better results with these.

framistan3 years ago

The 1N34 is a pretty old SIGNAL DIODE. I used to have a pill bottle full of them. You might try calling gateway electronics, a local hobby-electronics supplier here in st Louis, Missouri. their website is gatewayelectronics.com and their phone number is 1-800-669-5810. They might have some in their very large parts counter, and they won't cost much, if they have them.