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Back when I was younger I got a crystal radio set kit which included a 1N34A Ge diode (as far as I can remember, I don't have it anymore). I wanted to build one again recently and all I had a was a Schottky diode with a Vf of around 0.4V at 1A. I heard no sound from it through a piezo buzzer pressed against my ear and figured it was maybe because of the diode, so I looked around for the datasheet for the 1N34A to read its characteristics. It lists Vf as 1V at 5mA.

It is my understanding that Ge diodes were the choice for crystal sets because their low Vf does not load the tuned circuit. Perhaps I am misunderstanding the datasheet? Thank you.

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Besides that the signal currents are far below 5mA you might want to just analyse a circuit using it – PlasmaHH Mar 21 at 11:23
2  
I'm YANWAS (Yet Another Noob Without A 'Scope!) :) – MichaelK Mar 21 at 13:13
    
It's worth noting that the parameter is listed as maximum voltage at If=5mA. Also worth noting is that Vf is dependent on temperature. – W5VO Mar 21 at 14:43
    
Interesting you mention temperature. I guess it didn't help that I tested this radio outside in -5 degrees Celsius. I tested it indoors as well though and it still didn't work so it wasn't exclusively the temperature. – MichaelK Mar 21 at 15:05
up vote 4 down vote accepted

The answer lies in a combination of things.

  1. The typical load on a 1N34A diode in a crystal radio is a crystal earpiece with an impedance of 20kOhm.
  2. The 1N34A conducts much smaller currents with a lower Vf.

The trick lies in the combination. With a lower If, Vf is also lower. Since a crystal earphone draws very little current, Vf is also lower.

As you can see from this diagram (from here,) enter image description here

The 1N34A will conduct 0.5mA at about 0.3V.
0.3V divided by 20kOhm is only 0,015mA, so that's plenty of currrent to drive a crystal earphone.

Due to the I/V curve of the diode, the forward voltage of the 1N34A in a crystal radio is lower than you would expect from the Vf entry in the datasheet.


You need a low Vf because of the modulation scheme, not to avoid loading the tuning circuit.

You need the crystal earphone to avoid loading the tuning circuit and to keep the Vf low enough for the diode to demodulate the signal.


In reponse to comment: It may well be the diode that is causing problems. The 1N5822 is made for lower frequencies and higher currents than a signal diode. Here's link to a site about crystal radios with some hints about schottky diodes: http://www.techlib.com/electronics/crystal.html

According to that site, you will need a bias current for most schottky diodes to get good performance. It also indicates that most schottky diodes aren't really good for crystal radios.

From my own experience in building crystal radios, you would be well advised to build the first one using exactly the parts recommended and exactly the schematic of someone who has built one successfully. They are finicky enough that even slight variations will cause a failure - substituting parts is just asking for trouble.

Another thing to keep in mind is that they don't have fanstatic range. My biggest problem when building one as a kid was that we lived so far away from any transmitter that you pretty much had to get everything perfect and have a good antenna in order to pickup anything at all. Being a kid, of course, meant that I never got things really good enough.


Please, please, please!
Use a proper crystal radio earphone!
It makes a WORLD of difference in all of your experiments with crystal radios!

A piezo buzzer is meant to BUZZ at a particular frequency. How well it will do anything else is anybody's guess.

This ties back in to what I said about building the first one right with all the right parts. Once you have a working receiver, you can try using the piezo buzzer and see what it does and how it changes things - but that is an experiment for AFTER you have a working receiver.

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Thank you for your answer and the clarification about why I need a low Vf and crystal earpiece. I looked at the datasheet for the Schottky diode I'm using (1N5822) and the Vf vs If curve doesn't even show values of <100mA. But at 100mA it looks like it conducts 0.22V. Would this still be a suitable replacement for a Ge diode? It is completely possible that another part of my circuit is to blame, I just want to rule out the diode. – MichaelK Mar 21 at 13:09
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I think that "a piezo buzzer" today isn't comparable to a crystal earpiece. Try locating an actual crystal earpiece and give it a go. – rdtsc Mar 21 at 13:46
    
Oh, crap. How did I miss that? – JRE Mar 21 at 13:49
    
Isn't a crystal earpiece just a piezo buzzer? If not, I hath been lead astray by this link and videos. Thanks again for your replies. – MichaelK Mar 21 at 15:01
1  
They both use a piece of piezoelectric crystal to work. A buzzer is made to work specifically at a particular frequency. Whether (and how well) it works at other audio frequencies is anybody's guess. It may work well, it may not. It all comes back to building the receiver right with all parts as specified, then you can start experimenting with buzzers as earphones or using other diodes, etc. Get it working right, first, though. – JRE Mar 21 at 15:05

It lists Vf as 1V at 5mA

It's never this black or white: -

enter image description here

All diodes have a VI relationship that is non-linear in their forward conduction area and will never have a boundary where they conduct/non-conduct. Look at the graph above - with 100 uA flowing I bet the forward volt drop is probably 0.1 volts. Look at this one: -

enter image description here

For the 1N34A (green) the volt drop is about 50 mV with 30 uA flowing.

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