FIND YOUR PROJECT. This schematic index page
allows you to find project schematics. Just use the buttons in the right
sidebar. On this page, I also include basic information about
transistors for your projects.
WHAT TRANSISTORS TO USE. If you aren't picky about the transistor
device number, you can often save money. That's where the table
below comes in handy. It will help you to achieve the same performance
with economical parts. How do you pick an equivalent device? Start with
circuit requirements: Most of my schematics call for general-
purpose, small-signal transistors. These devices must develop a current
gain of about 200. Plastic T0-92 or similar package metal can transistors
will function just fine.
SILICON. I've designed the circuits for silicon transistors.
Germanium, gallium arsenide, and other semiconductor materials require
different bias resistor values. I'll leave the circuit changes up to you.
(Maybe a gallium arsenide FET will turn up in some future project. I
haven't found a need for one yet.)
I prefer through-hole devices to surface mount, but the two
types are equivalent. If you have very small, nimble fingers, you can
experiment with surface mount.
THE TABLE BELOW provides several equivalent device types. You can purchase
such devices at hamfests or from first-line or surplus vendors. Some "house"
type transistors will also work. Just be sure that you're using silicon
devices with a gain (beta or hfe) of 200. You can find out the beta by testing
the device. Testing is superior to a spec sheet lookup. The spec sheet usually
states a range. But you want the beta for the device in your hand. Time to
Equivalent Transistor Table
Note: Check pinout of replacement part.
You can buy several grades of parts. I've already
mentioned a few. Below, I describe the common grades in detail...
END-RUN PARTS. Sometimes we call these "tape and reel" parts.
Vendors often sell these devices on strips of cardboard. Tape holds the
transistors to the cardboard. If you're wealthy, you can buy a "reel" with
the cardboard strip wrapped around it. A reel might contain thousands of
transistors. You'll be set for life! The reels are what remains of a factory
production run. In the factory, one or more reels feed pick-and-place
machines that stuff circuit boards.
End-run parts may contain a
brand name and searchable part number. Or you might receive a strip of
house parts. Either way, the parts cost more than off-spec parts. As you'd
expect, within a part number, the quality tends to be more uniform. Watch
the price, though. Large catalog houses sell new parts for close
to the surplus price.
HOUSE-NUMBER PARTS are a bit more expensive than off-spec parts. But
within a type number, house number parts tend to operate uniformly. You
won't be able to check a datasheet, because house parts have no accessible
reference sheet. By testing a few samples, you might discover that your
house part works just like something you have a data sheet for. If you got
a good price, that's great.
PULLOUT PARTS are used. Usually the term "pullout" applies to
tubes, relays, modules, meters or power capacitors. Most catalog
houses and Web vendors sell new semiconductors. Pullout transistors turn
up at hamfests. Pullouts might be a valid way to acquire exotic devices.
For example, an old circuit project might call for a low-gain device.
Buying a pullout transistor might be the only way to make the circuit work.
Otherwise, you don't need to buy a used part. New semiconductors are common.
On the other hand, the used price is right. Besides, pullouts can be more
reliable and uniform than off-spec parts. Before you build with used parts, be
sure to test them.
OFF-SPEC PARTS(usually with no
package ID) sell at "bargain bag" prices. These parts are dropouts from a
factory production run. Along with the bargain bag comes a
serious cost: You'll have to sort your treasures, because device
performance varies wildly.
Some transistors may have the leads wired backwards. Many in your group
won't work at all. You might find other devices with amazingly high
gains. Not bad for a one-transistor project that you never need to
duplicate. But what if you need to service your project? If the
transistor blows, you can't match it with any other device. Because
of the non-uniformity, I usually avoid off-spec parts.
RETAIL PARTS. You can go to a storefront and buy new or
surplus parts. The walkup convenience factor costs you, though. Instead
of paying a nickel to 20 cents per device, you might pay a buck. If you're
buying "bargain bag," off-spec devices, the deal is even worse. For the
same price, a major Web vendor will sell you new, brand-name devices.
Retail's strongest advantage is convenience. If you need something now,
then nobody beats retail. Retail also makes possible easy merchandise
returns or exchanges. Otherwise, the Web and the delivery service are your
friends. Don't forget to factor delivery charges into your cost.
WEB AND CATALOG PARTS. Web vendors are giving walkup retail a
run for the money. That's only fair. The markup for some retail parts might
be 100 to 1 or more. Also, retail has had the market to itself for a long
time. It could compete, but the creativity isn't there anymore. Plainly,
somebody's asleep at the switch.
On the Web, nobody cares if you're a mechanical TV geek. Buy what you want,
and pay the down-to-earth price. Tired of back-ordering from the walkup store? Web stores
stock tens of thousands of items. The selection is fabulous, and you can choose
whether to buy new or surplus. You never have to drive downtown, and comparison
shopping couldn't be easier. Don't buy anything heavy though, or the
shipping tab wrecks all your fun!
EBAY ELECTRONICS COMPONENTS are part of the Web flea market. But
eBay is in a class by itself. Chinese vendors compete with locals for best
quality, delivery speed, and lowest shipping price. New-old stock and obsolete
parts are available again, often at bargain prices. Even Walmart and Sears sell
electronic components, such as miniature speakers and semiconductors! Overall,
the prices are typically below wholesale. The only way to beat these prices
would be to buy in lots. Click: