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I am in a little trouble and seeking some help. I got a bunch of mixed up diodes from an old collection. I know there are few diodes which are Germanium Diodes. But they look so similar to 1N4148 and similar transparent case diodes. The problem is, the diodes are old (but working) and it's very difficult to read the numbers printed on them. How can I identify and distinguish Germanium diodes? Can I measure something with a multimeter, or create a simple circuit to identify the Germanium diodes. I am looking for identifying diodes like 1N60 and 1N34A. I would highly appreciate your help!

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    \$\begingroup\$ First google on germanium diode characteristics gives me: "For silicon diodes, the built-in potential is approximately 0.7 V (0.3 V for germanium and 0.2 V for Schottky)." (Although I don't like the term built-in potential. I prefer forward voltage drop.) \$\endgroup\$ – Oldfart Feb 4 '18 at 16:23
  • \$\begingroup\$ Forward voltage will be lower on germanium diodes IRC. \$\endgroup\$ – Wesley Lee Feb 4 '18 at 16:23
  • \$\begingroup\$ What do you need these for? Xtal Radio? \$\endgroup\$ – Sunnyskyguy EE75 Feb 4 '18 at 16:42
  • \$\begingroup\$ Typical small signal Germanium diodes have a needle contact and look like this: da.wikipedia.org/wiki/Fil:Germanium_Diode_OA85.JPG \$\endgroup\$ – Janka Feb 4 '18 at 16:42
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    \$\begingroup\$ Funny way to test if you have any germanium diodes is to make an old AM radio circuit. If it is germanium the circuit would work. 🔌 \$\endgroup\$ – Bix Feb 5 '18 at 2:25
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Use this schematic to test the diodes. You can easily distinguish Silicon and Germanium Diodes. Silicon diodes should read approx 0.7V and Germanium diodes should read 0.3V. A little difficult to distinguish Schottky diodes though. They should show approx 0.2V which is close to 0.3V. If you have a very stable power supply and a good meter you can distinguish this as well!

Good Luck!

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Thanks for the schematic! It worked. I am able to distinguish the diodes easily! \$\endgroup\$ – Amanda Miller Feb 5 '18 at 19:22
  • \$\begingroup\$ You can also likely tell the difference between Ge and Schottky diodes by reverse-biasing it and looking at the leakage current. With a Schottky diode, you will see a current on the order of microamps, and the 0 if it is Ge. \$\endgroup\$ – Caleb Reister Feb 14 '18 at 18:19
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Germanium diodes have a lower forward voltage drop than silicon diodes. Rig up something that puts a little current thru them, and measure the voltage.

For example, a 5 kΩ resistor in series with a 5 V supply should do quite well. The current will is limited to 1 mA, and the reverse voltage to 5 V. Neither should hurt any of the diodes you have.

Silicon diodes will have around 650 mV forward drop. Germanium will have about half that.

Note that silicon Schottky diodes have about the same voltage drop as germanium diodes. If you think there might be some Schottky diodes in the mix, then it gets more complicated.

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    \$\begingroup\$ For Schottky reverse current is useful to. There are cheap devices on eBay called "transistor tester", based on some electronics forum design. They measure that and capacitance too, and despite being not very accurate I find that little thing a very valuable tool in parts identification \$\endgroup\$ – PlasmaHH Feb 4 '18 at 16:35
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    \$\begingroup\$ @Plasm: Yes, you'd probably need to use reverse current to distinguish between germanium and silicon Schottky diodes, especially at elevated temperatures. I wasn't going to get into that unless the OP came back and said that there might be Schottky diodes in the mix. \$\endgroup\$ – Olin Lathrop Feb 4 '18 at 16:39
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    \$\begingroup\$ Aren't germanium diodes rather leaky as well? \$\endgroup\$ – ThreePhaseEel Feb 5 '18 at 4:37
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    \$\begingroup\$ last time I did the sums, 5v/1k was 5mA, not 1mA \$\endgroup\$ – Neil_UK Feb 5 '18 at 9:47
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    \$\begingroup\$ @Neil_UK: you mean last time you did the quotients ;-) \$\endgroup\$ – Curd Feb 5 '18 at 11:16
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The continuity test function on many multimeters has a "diode" setting that will tell you what the forward voltage is, from which you can infer the type of diode.

http://en-us.fluke.com/training/training-library/test-tools/digital-multimeters/how-to-test-diodes-using-a-digital-multimeter.html

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  • \$\begingroup\$ Thanks for this, I was wondering what the diode symbol on my MM was for. \$\endgroup\$ – Russell Borogove Feb 5 '18 at 3:16
  • \$\begingroup\$ Thanks for responding. Yes now I am effectively using the Diode setting on my DMM. Thanks for the clue! :) \$\endgroup\$ – Amanda Miller Feb 5 '18 at 19:23
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Using diode test mode on a DMM is the best way. It will use some standard fixed current like 1mA to measure voltage up to maybe 3V. This is also useful for comparing LEDs. If you dont have a DMM, get a good one.

Opinion

There is no real need for old Ge , as Schottky performs better and diode capacitance * forward Rs= (ESR) resistance , which is relatively constant is better on Schottky and ESR=k/Pd for power rating Pd.

In fact some manufacturers are making the 1N60 with Schottky Silicon instead of the original Germanium.

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    \$\begingroup\$ If you want to produce a noise signal, some low-voltage circuits specify Germanium diodes because their breakdown voltage is very low. But today one could use a blue LED for the same purpose. Or use a voltage doubler somewhere. \$\endgroup\$ – Janka Feb 4 '18 at 16:45
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Real world germanium diodes (even recent production) almost always come in a larger glass body (diameter about equal to or even thicker than a 1N4007, slightly longer. Not unlike a small reed switch), either left clear or painted black. With clear case examples, the insides will appear mostly see-through hollow (instead of mostly filled with red/orange copper tampers, like you will see on a 1N4148 or similar), sometimes with a visible hair thin wire going towards the actual semiconductor element.

This old case style has been used for silicon parts too, but is VERY uncommon for them.

Germanium semiconductors in molded plastic cases are an ABSOLUTE exception (only one I am aware of is the AF279 HF transistor), since most germanium parts were made in processes that require the part to be kept in a clean, hermetically sealed case (which plastic molding does not reliably provide). So, anything plastic molded will be silicon.

For power diodes, the same style of metal cases have been used for both Si and Ge devices.

If the labelling is partially readable: european parts whose designation starts with "A" are always germanium, "O" is so old that is is LIKELY germanium, "B" is silicon.

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