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Topic: Transistor Specs (Read 609 times) previous topic - next topic

jarrod0987

When substituting Transistors, could someone give me an idea which specs usually need to be the same and which ones can be the same or better then the original etc. Obviously need to check the pin arrangement as well.

Thank You

Grumpy_Mike

Look at the maximum collector current and look at the gain ( Hfe ) those are the main ones.
If it is for a high frequency application look at the bandwidth and base capacitance.

aarg

If you have no knowledge of the circuit, a replacement transistor should meet or exceed all the specifications of the transistor it is replacing. If you feel confident that you understand the circuit, the replacement only has to meet or exceed the requirements of the application. This is because seldom is a transistor a "perfect fit" for an application, instead, it will meet the minimum requirements of every spec, and exceed it in others. Thus the second option allows for much greater flexibility in choosing and sourcing a replacement.

There is no absolutely reliable rule about which ones are important, because that depends on the application.
  ... with a transistor and a large sum of money to spend ...
Please don't PM me with technical questions. Post them in the forum.

Grumpy_Mike

Quote
a replacement transistor should meet or exceed all the specifications of the transistor it is replacing.
True but only if you understand what exceeding a specification means. It does not necessarily mean having a higher number against a specification. For example when looking at the collector / emitter saturation voltage a lower number here is better. 

aarg

#4
Mar 19, 2015, 10:52 am Last Edit: Mar 19, 2015, 11:01 am by aarg
True but only if you understand what exceeding a specification means. It does not necessarily mean having a higher number against a specification. For example when looking at the collector / emitter saturation voltage a lower number here is better. 
Yes. Also it bears mention, that some circuits are designed in such a way that exceeding the specifications in what would ordinarily be a good way, will lead to failure. Magic numbers and sneaky tricks, if you like. It's rare, but it happens. So you might see, for example, a circuit that has a flaky design, but parts were swapped in and out until it worked. I have seen cases where we had to use such and such a part only from a certain manufacturer, because of this. Generally, it is because the circuit lacks compensatory measures like feedback. An example would be a common emitter amplifier stage that is DC biased by feeding current to the base with only one resistor (and emitter grounded). The DC bias then depends on gain and other factors like temperature, etc. Obviously, you cannot replace it with a transistor with a "better" gain, because it will upset the bias. You need one with the same gain.

Thus, you should know some electronics before replacing a transistor. But there's a lot of wishful thinking going around these days, it seems. Also a lot of stuff on the market with absolutely pathetic documentation and/or sketchy specs.
  ... with a transistor and a large sum of money to spend ...
Please don't PM me with technical questions. Post them in the forum.

jarrod0987

These are just for Audio applications so RF is not necessary. So HFE and max Collector Current are the ticket? I'm having a hard time finding some of the 180v and 200v TO-92's from older amps :(

Wawa

Most part suppliers have some sort of search engine.
High voltage NPN
Leo..

TomGeorge

Hi,

Vceo is another parameter to check.
It is the Voltage from Collector to Emitter with open Base.
This should be as high or higher than what you are replacing.
If you don't know it, then get a Vceo of at least 50% higher than the supply to the transistor.

Tom.... :)

Everything runs on smoke, let the smoke out, it stops running....

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