Transistor Mysteries - Identifying and Replacing

Thread Starter

briesmith

Joined Mar 20, 2021
3
Just retired and I'm trying to get into electronics, particularly repairing broken keyboards and I've come up against a bit of a show stopper. I've done several online courses on transistors but while I understand PNP and NPN pretty well what I don't understand is why, if there are only 2 types, there are so many different units - 1,000s of them apparently - and why it is so hard to find replacements even when the ID numbering is clear.

So my question is, given you have a number (in my case D1028 BK411), why can't I find a replacement and, more generally, why can't I substitute a universal type replacement?

I have the same problem with surface mounted capacitors but I'll save that for another thread. Generally, I am finding it all very frustrating. Electronics is all about precision but when it comes to identifying bits and pieces that seems to go out the window.)
 

DickCappels

Joined Aug 21, 2008
8,169
The reason that there is not a universal replacement is that the some applications need one quality more than others and what makes a transistor good for one kind of use makes it unacceptable for another kind of use. My favorite example (but there are many) is that transistors with high current gain usually have difficulty operating at high frequencies -I'm talking radio frequencies. For some high frequency amplifiers a geometry of the transistor is important but might penalize, for example, high power applications.

You can often swap transistors and get away with it but it helps a lot to know what the transistor is expected to have to do.

There is a MOSFET (yet another kind of transistor) that is names "D1028" but it is an RF part. Do you know anything about the circuit the defective transistor is in and what it does, for example, Martin Eberhard used to love to drive status LEDs on his keyboards with metal can 2N2222 transistors. Driving status LEDs is one application in which a discreet transistor might still be needed in a keyboard.
 

MrChips

Joined Oct 2, 2009
25,046
There are probably more than 1000 different transistors, maybe more like 10,000 or even 100,000, just guessing.
Transistors are used in a wide variety of applications that demand certain specifications:

  1. polarity
  2. material
  3. current, voltage, power, frequency ranges
  4. gain
  5. packaging
  6. manufacturer

The first thing you want to look at is the application.
What is the operating current, voltage, power, frequency? If these are not stringent requirements, look for a general purpose (GP) transistor that fits these requirement. Then look at the packaging required.

9 out of 10 times you can substitute the bad transistor with something else. The key thing here is matching the transistor to the application.
 

Papabravo

Joined Feb 24, 2006
17,615
There are many reasons for the presnet situation. Time is probably the biggest one. Invented in 1947, the first commercial part available in quantity, the CK722, was available a few years after that in 1953. From that point until the demise of the vacuum tube a decade and a half later, transistors were at the forefront of innovation and the transistor data books were thicker than the major metropolian phone directorries of that era. Several companies even specialized in generic replacements at 10 times the price of the original. Gradually over time certain parts were used in ever larger quantities and designers gravitated to those parts due to their lower costs. As the level of integration on chips progressed, designs with individual transistors declined. Today they are essentially niche parts for specialized uses.

You talk about precision, but that is mostly an illusion. This is because the parameters of an individual device are not constant or well controlled. Even parts from the same silicon wafer can exhibit different parameter values. That is why most, and especially the best, designers create designs that do not depend on a device having specific values of their parameters. IMHO, to effectively repair an electronic device you need to understand the circuit well enough that you can pick and obtain suitable replacement parts. Short of that standard you need a purchasing department to track down and obtain exact replacements. Time is also running out on the soldering iron as an effective repair tool.

https://en.wikipedia.org/wiki/CK722
 

dl324

Joined Mar 30, 2015
13,846
I've done several online courses on transistors but while I understand PNP and NPN pretty well what I don't understand is why, if there are only 2 types, there are so many different units - 1,000s of them apparently - and why it is so hard to find replacements even when the ID numbering is clear.
Transistors of a particular polarity vary in important parameters such as collector-emitter breakdown voltage, collector-base breakdown voltage, current gain, current handling capability, maximum power dissipation, frequency of operation, etc.

If you have a schematic or specification for how the device is being used, you can look for a transistor that will satisfy those requirements with some margin for safety. If a transistor needed to handle a current of 1A, that would eliminate many thousands of general purpose transistors. If you required high current gain, that would eliminate a number of power transistors.

You also have to consider pinout. For transistors in a TO-92 package, there are 3 common pinouts (EBC, CBE, and BCE).
 

Thread Starter

briesmith

Joined Mar 20, 2021
3
I just wanted to thank everyone who troubled to reply. I've saved the responses to my OneNote folder and I am sure I will be referring to the links and other content as I go on my way towards being able to actually fix stuff! :)
 

Externet

Joined Nov 29, 2005
1,737
Another detail is your 'D1028' transistor may well be actually a 2SD1028. Welcome to tricks you will hardly learn anywhere and untold abbreviations for saving ink and make life harder.

----> https://www.google.com/search?q=2sd...WYZ80KHRSQDPAQ_AUoA3oECAEQBQ&biw=1362&bih=597

More hidden tricks... 2SA and 2SB are PNP; 2SC and 2SD are NPN.

Other numbers can be week and year of manufacture.

Applications are for power, audio, switching, radiofrequencies, microwaves, digital...

There is european, japanese, US numbering, chinese do anything they want...

Replacement transistors were five "epoxy" in the sixties* ... then ECG... then SK..., later NTE... Now cannot even find their website :(

Borrowed from the web:
Follow these general rules for substitution to stay out of trouble:

Use a device with equal or greater breakdown voltage,
Use a device with equal or greater operating current,
Use a device with equal or greater power dissipation,
Use a device with equal or greater gain bandwidth product,
Use a device with equal or lower switching time,
Use a device with equal or lower trigger current,
Use a device with equal or lower reverse current.


* A couple were NPN, a couple were PNP, a couple RF... EP-1, EP-2, EP-3... looked like :

1638223750023.png

An unobtanium publication that I really, really respected more than others :

----> https://books.google.com/books?id=71sUAQAAMAAJ&dq=editions:LCCNsn80008573

Every manufacturer used to publish fat data books; I had 18 metres of them in shelves.
 
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