If you don't have a clue, try to get the exact replacement part. To properly answer this question involves understanding the circuit design. The obvious parameters to satisfy are breakdown or max collector voltage, next is beta (hfe), finally for more specialized applications noise, or gain bandwidth. Just like you can replace a bolt with a stronger bolt, your can replace a transistor with a stronger (better) part 99% of the time. A low voltage low current GP device, and higher voltage GP parts, and very low noise parts for preamp front ends. I still have hundreds of the 3 legged buggers sitting in my back lab. In circuit design there are (were?) many transistor applications where a design engineer used the GP NPN or PNP device that he already has in stock. The rules for substituting is pretty much the same as the rules for initially selecting transistors, while it's a little like replacing nuts and bolts in a car, you want the replacement to be strong enough and fit in the hole. Is there a general rule for value deviance (I'm thinking hFE and power dissipation: if higher no problem?) or does it exclusively depend on the circuit the transistor is used in? However, even if I had the schematics, I still wouldn't really know how to find close enough substitutes. I haven't been able to source a free copy of the schematics yet, so not sure exactly what the transistors do, but I thought I might give it a try anyway. Most of the transistors I'm looking for are designated as "general purpose", used in the (motor) control section of an old CD player (i.e., 2SA933 = 2SA733, 2SA970, BC556 - so probably all fine).īut there are a few I can't find equivalent matches for that look close enough. A quick search on the net gave me these pages. I also bought the Tower's FET selector as well The internet is great, but I still like to browse through specifications in a book now and then.I need to replace a few obsolete transistors. For GBP12, it's a bargain so I have placed an order. I just took a look on Amazon.the Tower's International Transistor Selector is still available The latest revision seems to be December 2008 and it includes SMT transistors. Some circuits just will not behave without a very close equivalent part. This is crucial for good equivalent selection. One thing I did learn all those years ago, was to read the schematic and decide how critical the components specifications were to correct operation. SMD stuff is too modern for it though so I then have to fall back to internet searches. The internet has changed searching for equivalents but I often find the Towers book faster as I grew up with it and know many favourite equivalent transistors. ![]() It lists the transistors specifications and any equivalent (but not all) You can flick through various ranges of transistors and select a suitably specified part. I still have my copy from the 1980's and it still serves me well. In the UK when I worked as a 'Saturday lad' in an electronics shop, I used the 'Towers Transistor selector' book. Substitution of RF power devices needs to be approached with a great deal of caution,& preferably with a Spectrum Analyser on hand. Modern devices as a rule,are better spec'd than older ones,so a common modern type may replace an old special type. In more complex circuits,have a look at what devices other manufacturers use in similar circuits-there may be a common device which will do the job. If it is something simple,like a lamp,LED,or relay driver,you can probably replace it with a more common type without any drama. If you can't find a listed equivalent,one good philosophy is to look at the circuit the device is used in. Not only did they often disagree,but by the time you had tracked from one book to another,the specs on the final one you turned up were many times quite different to the original. ![]() At one of my earlier jobs,we had semiconductor equivalents book from several sources. Good luck finding anything that will do that!(though it must be out there somewhere) Many times the equivalents shown in the various books are harder to find than the original component.
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