Transistor Substitution

Tony Duell ard at
Tue Jun 9 12:53:39 CDT 2009

> Okay: I admit it, I am sometimes pig-ignorant about basic hardware 
> questions.  :/
> I have a couple of DEC machines which I need to replace a few components 
> on, and also stock up spares of others. With the transistors and diodes, 
> however, I often can't find a direct replacement - and don't know how to 
> figure out what a modern substitute is.
> For a 2N3009, for example, I can find basic information and a datasheet 
> online easily enough - but as for choosing a functional, available 
> substitute for it, I'm honestly not even sure where to begin.
> Is there a basic resource for determining modern equivalents for older 
> transistors and diodes? Can someone helpfully provide me with a clue 
> here?  :)

It depends on what the transistor is being used for.. In general 
transsitors used in the logic (particulalry low-speed ones, for things 
like lamp drivers) and transsitors used in linear PSUs are fairly easy to 
substitute. Transsitors used for high energy pulse work (SMPSU choppers, 
horizontal output transsitors in monitors) are a lot more critical, and 
the data sheet may bot help. 

Basically, you need to get the following right :

Tpye/polarity (NPN or PNP etc)

VOltage ratings high enough to stand voltages in the circuit (if the 
ratings of the substitute exceed those of the original, you should be OK)

Current ratings (particularly Ic, collector current). Again, if your 
substitute is better than the original it should be OK

Gain (hfe and all its varients) is not too critical in most classic 
computer applications, particularly for switich transiustors which are 
driven hard into saturation.  But try not to use a transistor of too low 
a gain.

Max frequency (Ft, etc) is something you should look at in logic 
transistors, particularly in higher speed circuits, clock oscillators, etc.

But my experience is if you pick something of the same polarity and 
similar characteristics, it'll work in most classic computer circuits 
(except for SMPSUs and horizontal output stages). Probaly 99% of 
small-signal transistors can be replaced y 2N3904 (NPN) and 2N3906 (PNP) :-)

Diodes are evem more generic. Most switching diodes can be replaced with
1N4148 :-). For PSU rectiifers in linear PSUs (again, SMPSUs are another
story), choose something with sufficient votlage rating (PIV -- peak
inverse votlage) (2*sqrt(2)*output voltage should be safe, say 3 times the
output voltage or more) and sufficient current rating (If, forward current). 
When there are similar diodes of different PIV ratings, I normally buy 
the highest. It's not much more expensive, and I'm on the side of safety.


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