audio amplifier transistor?

I pulled what looked like a voltage regulator from a PSU, i guess as it turns out its not?
apparently its a audio aplifier transistor, but what would that be doing in a PSU?
if its any use, it was near a fan connector on a small sub-board...

markings are:
UTC
2SD880
(2 small, undistinguishable letters)RKOB (or 0B)
i believe this is the datasheet, correct me if im wrong.
http://search.alkon.net/cgi-bin/pdf.pl?pdfname=utc/2SD880_TO220.pdf

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orksecurity6 years ago
As the spec sheet says, it's designed for audio _frequency_ applications, not necessarily audio applications... and you can use it outside that frequency range, though other transistors would probably be a better choice if that's what you need.

There is no such thing as an "audio amplifier transistor". There are transistors which have response characteristics and power handling capabilities which make them suitable for particular applications. Audio is just one application.

(In any case, part of the art of hardware hacking is learning to look past what something is called and see what it actually is and does.)
zack247 (author)  orksecurity6 years ago
so is this pretty much just a general transistor?
would it work as a replacement to a tip31?
The only answer possible is "maybe". How dependent is the circuit on the characteristics of the tip31?

This is much like asking "can I replace a 10Kohm resistor with an 20Kohm resistor". Depending on what purpose the resistor serves in that particular circuit, it might still work but behave somewhat differently, or it might melt half its components, or you might never notice the difference, or it might make a difference but not under the conditions you intend to use that circuit in.

Generally, if the transistor you want to replace and the one you want to replace it with match pretty closely on the parameters Andy mentions, the substitution is possible but may require tweaking other things. Or, as I say, may not.

Active components are more complicated than passive ones. Which is part why more complicated ICs are a good thing; they don't just pack more function into a small area, they give you a higher-level (abstracted) view of that function which can be described more simply and which is more amenable to substitution with equivalents. This is one reason I suggest that folks who want to learn analog circuit design start with op-amps; the basic concepts are simple, and while there are certainly lots of details you can get into, for many simple applications all op-amps really do behave pretty much equivalently.
AndyGadget6 years ago

 
There are many thousands of different bipolar transistor part numbers, but they can all, by and large, be classified by a few common parameters, roughly :-
NPN or PNP,  power handling capability (collector current ), gain range (HFE), breakdown voltage and frequency response, and also the case style (which determines it's thermal characteristics).
Although the 2SD880 is referred to as an audio amp transistor, it's characteristics make it suitable for its role in the PSU as well.  It has adequate gain and power handling and is a fairly low-frequency device (i.e. cheaper).


I should add that some are better at switching from fully on to fully off (saturation mode) whilst others are better working with smaller voltage swings in the middle of their range (linear mode).