This article is part of Page 66 of our Basic Electronics Course. The full Basic Electronics Course is available via subscription.
Here are the most common pinouts for transistors:
There is no simple answer to finding a replacement for a transistor.
There are thousands of types, shapes, sizes and specifications.
But if you take things slowly and methodically, you can generally find a replacement quite easily.
There are two reasons why a transistor needs to be substituted.
A. You may be building or designing a project and not have the exact specified type, or
B. You may be repairing a damaged project, and the original type is not available.
Don't worry too much about getting the exact equivalent. Most circuits will operate with almost any substitute. I know this is a bold statement but it's the only way to approach the situation.
There are many transistor-types and numbers on the market and they came about due to "batching" - not because of a radical improvement .
Batching is the process of grading a transistor due to a particular characteristic, such as maximum collector-emitter voltage before "zenering" occurs, - another term for "breakdown" or failure. Originally it was difficult to produce transistors with a high operating voltage and every transistor had to be tested. The good ones were sold with a "commonly-recognized" part-number and the others had a different part-number.
Also, the gain of the transistor was determined, so a higher price could be obtained, and the transistor was given a pre-fix "-A," "-B," "-C," etc.
Then JAPAN came into the picture.
They perfected transistor manufacturing.
They increased the operating voltage enormously, improved its reliability and totally revolutionized the electronics market.
That's why you have to be very careful when replacing a Japanese transistor.
Some Japanese circuits operate on a high voltage and only a Japanese transistor can be used.
But apart from that, our simple approach can be used.
Firstly you need to work out if the transistor has been damaged by overheating or from a voltage spike.
Thirdly, you need to determine if the transistor is a low power
medium power or high power. This can be easily done by
viewing the size of the