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 Post subject: Diode Delusion
PostPosted: Sep Sat 18, 2021 9:10 pm 
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Hoping that someone can help identify the following diodes, and firmly hoping that they can be used for a probe fabrication for signal generator and tracer? Schottky or germanium? Picked them up at a ham fest. Could barely read the tiny markings, but wrote them down here.

** There are two separate bundles here marked differently. They appear to be glass.
Each bundle has their corresponding numbers written adjacent to them.


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Last edited by Codepug on Sep Sun 19, 2021 1:49 pm, edited 1 time in total.
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 Post subject: Re: Diode Delusion
PostPosted: Sep Sat 18, 2021 9:17 pm 
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Those look to be 1N4148 signal switching diodes. Data sheet available on the web.


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 Post subject: Re: Diode Delusion
PostPosted: Sep Sat 18, 2021 9:22 pm 
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Agree with madlabs. And they would work for your probe purposes.

Here’s a data sheet:

https://pdf1.alldatasheet.com/datasheet-pdf/view/15021/PHILIPS/1N4148.html

Have fun!

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 Post subject: Re: Diode Delusion
PostPosted: Sep Sat 18, 2021 11:09 pm 
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Use the diode function on a DMM. Germanium will show the lowest reading, schottky in the middle, silicon the highest. They won't all show the same reading, but there's a distinct difference between all three.

I went through a bunch of diodes taken off boards some years back, and was surprised to find more germanium than I'd expect. But, once I'd sorted them, I think all the germanium were somewhat chubbier than a samole 1N4148. So that's something to watch, though it needsconfirmation by DMM.


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 Post subject: Re: Diode Delusion
PostPosted: Sep Sun 19, 2021 1:54 am 
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Thank you for the response.
I failed to make clear that the picture denotes two separate diodes.

The dmm diode readings for both diodes shown give the same result of .590
I did dmm test a known germanium 1N34a that i have, and got .263, and as mblack indicated would give low reading.

The one that is clearly marked V4148 and I presume it to be a switching diode, which I think is not a schottky diode? Is it silicon even though it looks like glass?

The other diode is marked (double tilde, 26,6-,-) as drawn in the picture. Is this also a 1N4148?


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 Post subject: Re: Diode Delusion
PostPosted: Sep Sun 19, 2021 4:24 am 
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The 1N4148 is a silicon diode. It is not a Schottky. I'm not sure from the markings what the other diode is. As noted, testing would give you some idea of its characteristics.


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 Post subject: Re: Diode Delusion
PostPosted: Sep Sun 19, 2021 1:45 pm 
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Ok. Thank you…


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 Post subject: Re: Diode Delusion
PostPosted: Sep Sun 19, 2021 2:51 pm 
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Codepug wrote:



The one that is clearly marked V4148 and I presume it to be a switching diode, which I think is not a schottky diode? Is it silicon even though it looks like glass?

The other diode is marked (double tilde, 26,6-,-) as drawn in the picture. Is this also a 1N4148?

A germanium or regular diode is formed from two types of semiconductor material, one called P and the other called N. The diode action is actually due to the PN junction. A schottky diode is formed from a junction of a metal and a semiconductor (like silicon.).

The other group of those diodes may not be 1N4148s but they probably are near enough to be assumed to be.
The actual size of active material in those diode is less than the size of a grain of table salt. The glass case is there to protect the active material and support the force from the leads.

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 Post subject: Re: Diode Delusion
PostPosted: Sep Sun 19, 2021 6:39 pm 
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From your original question it would seem you're not sure of the role of the diode in a signal tracer probe. (There are no diodes in signal generator probes). The diode serves as a detector, same as the detector in an AM radio, to rectify an RF or IF signal that has been modulated with audio. The recovered audio is then amplified and heard on the speaker.

Thing is, there are basically two ways a diode can be used in a detector circuit. One of these is linear or envelope detection where the diode is operated as a rectifier. The incoming signal is rectified to DC, the instantaneous voltage of which follows the audio modulation. This method has been used in most radios made since the mid 1930s. It gives low distortion but it needs a fair amount of input signal to work. The other method is called square law detection, which uses the small portion of the diode characteristic curve near the bottom where it is logarithmic. This gives an output proportional to the square of the input, so it responds to the power in the signal rather than its voltage. This gives higher sensitivity, but it is limited to low level signals if distortion is to be avoided.

Your 1N4148 diodes were intended primarily for switching applications and for rectifying small currents. As such they would work fine as envelope detectors. But the square law regions of their response curves are very small so they would not make ideal square law detectors. Practically speaking this means a signal tracer using them might not "hear" low level signals that might be detected with a more suitable diode like a 1N914 or a 1N34A. On the other hand, sensitive square law diodes are very delicate and easily damaged by overvoltages while switching diodes are much more robust. So your 1N4148s may hold up a lot better over the long run.

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 Post subject: Re: Diode Delusion
PostPosted: Sep Sun 19, 2021 8:36 pm 
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The doubled wavy lines in the top batch looks to be a National Semiconductor logo.

EDIT: Was going to guess 1N266 but that's a germanium diode.


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 Post subject: Re: Diode Delusion
PostPosted: Sep Mon 20, 2021 2:41 am 
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Location: Lexington, KY USA
The specs for a 1N914 and a 1N4148 are very similar; the exact same part can meet both. So don't expect much difference between parts with these numbers.

A silicon diode like the 1N4148 may work in your application. Try one and see. As suggested, germanium is the traditional sort of diode to use, and will probably work better on small signals.

It is possible to provide a bit of DC bias for the diode, to move the curve over a little, and get more output on small RF signals.

As to the original question, sometimes you never find out just what type number hamfest finds really are. Many sorts of diodes were and are made using this package. Could be a house-number or a special part. A zener? a diac? Often you can test it enough to see if it will work in your application, in order to make use of it. Probably not something you would build into a Mars lander or medical device.

Ted


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 Post subject: Re: Diode Delusion
PostPosted: Sep Mon 20, 2021 5:42 am 
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Location: Montreal, Quebec
For a lot of hobby use, part numbers become descriptive. "1N34" means you need a small signal germanium diode. It's "chosen" because it was available, so it kept being listed. People were not evaluating every solid state device, they used what was sold to hobbyists, used what other projects used.

"1N914" was specified as a small signal silicon diode, until a switch to "1N4148". Same with transistors, endless device numbers, a relatively small number used most of the time.

You can look at a schematic and get a general feel by the semiconductors used.

So unless these diodes were something exotic, varactors, zeners, diacs, whatever, and a seller knowing that woukd mark them as such, one generic silicon diode is as good as another for most hoby use.


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 Post subject: Re: Diode Delusion
PostPosted: Sep Mon 20, 2021 4:49 pm 
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Joined: May Tue 30, 2006 4:46 pm
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Location: Santa Rosa, CA
Quote:
The specs for a 1N914 and a 1N4148 are very similar; the exact same part can meet both. So don't expect much difference between parts with these numbers.


Years ago, I visited the Telefunken Semiconductor plant in Vöcklabruck, Austria. TFK had an amazing automated assembly system for small signal diodes. Chips, lead wires, glass tubes, etc. were all on the input side. At the other end of this 30 ft. machine, diodes came out, taped and reeled and marked. The same parts were used for 1N4148 and 1N914, just a different marking code.

At that time, TFK were selling 1N914 for about $6.80 per thousand... that's 0.68 US cents each... including the taping and reeling!

Rich


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