Source: http://mrarchive.mrr.trains.com/?iid=94728&startpage=page0000062
Timestamp: 2019-04-20 23:07:15+00:00

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+ ; Input voltage 1 ' C PNP transistor 1. t } About V, Volt I wasted here i • ,·Remaining voltage R ' reaches here Size of resistor determines :he base current 3. Base conducting forward current. -1 R ', 40 '..* 'b"ent •,4 + ---- 1 4. Base blocking reverse current. ligible effect on this base current the amount it allows is much more than the amount of current will depend on the base current, typically 25 times more. available voltage and the size of resistor R. It follows Ohm's law, I=E/R. All the transistor does about this base current is to measure it like an ammeter. Then it used here only because it is typical. Now, controls the other path according to how for example, suppose you draw a current strong the base current is. The base and emitter actually form the transistor to furnish 1 a. via the cola rectifier and you would find this out if you connected the voltage forward and then backward as in Figs. 3 and 4. of .04 a. from the base. This commands lector. When a transistor is regulating in this manner, any small change in base cur- In Fig. 4 the source polarity has been rent, for instance when you turn the reversed and now, except for possible knob just a little, will be magnified as a leakage, no current flows. The full volt- great changein the collector current, age is found across the transistor and no typically 25 times as much change. This voltage across the load. Sometimes this rectifying feature is made use of in a transistor circuit, but more often it is not. To summarize, except for rectification COLLECTOR: Matters are different at the collector, for the transistor has a lot between these limits. You can accomplish this control by adjusting the value of the base current. Transistor action In Fig. 5 you'll find the setup for a current off, on, or regulate it somewhere is how a weak signal can run a sound effects loud speaker. ON: If you turn the pot control far enough, the transistor will be willing to furnish more current than the load will and the 36 v. or so loss, the transistor even take. Then full voltage (almost*) does not affect the flow of current passing through its base connection. of what the load will draw. reaches the load and the load determines its own current just as when a switch is ttuurrned on. This "on" condition is called "saturation" and occurs whenever the to say about the current that flows here. base current is typically more than VEs If you wish, the transistor can turn the PNP versus NPN transistors Most power transistors and many others are listed as PNP types. A few transistors are called NPN types. In practice you can use either in the same circuits, other things being equal, but revealing experiment which demonstrates when you use the NPN type all the electhe three states of transistor control: off, on, and regulating. The resistor R is a Fig. 2 I have done this by keeping plus volume contro1 potentiometer such as tric polarities should be reversed. In at the top but inverting the wiring dia- radio men call a "pot." It can be ad- gram. In practice a simpler way is to justed to draw various amounts of cur_ merely change all pluses to minus and rent from the base and the results will be vice versa (polarized capacitors inone of the three conditions as shown in cluded). Figs. 6,7, and 8. (Resistor Ro will be explained later in the material on leakage.) OFF: If you turn the pot control so no base current is drawn at all, Fig. 6, then the transistor, seeing no base cur- rent, will turn off the collector path too. Only a very small leakage current will flow through the load and this is often unimportant. The load is turned off as added for this purpose. * 1 Power source • *--... E:EE3·taarnal•fs,53 *A,",ilR. -15(&' radio pot up just a little to draw a very small amount of current from the base of the transistor, as in Fig. 7. Then the REGULATING: Suppose you turn that transistor is commanded to let some current also flow from the collector :•nd the \\ L- //2.C 11 \1 /*' /7 6&k&f& 5. Lamp dimmer circuit. R . lc•97:61 11/ * Typically about 1 v. may be lost due to resistance in the collector path in tile transistor when "on." This voltage varies with wits and conditions. + ...1 .. E C 1 Nov 1 drop oltage here •",I•* Vo:kio•tage 8 '- here toward negative. Also note that the arrowhead in the NPN is reversed. The arrowhead for transistors and diode rectifiers alike points in the direction of normal current flow, toward minus. There is a wider variety of PNP type transistors available at prices suitable for model railroad use but occasionally the NPN polarity may offer some advantage, especially in circuits taking advantage of both types working together. Selecting a transistor This 1 to 25 ratio is called Beta or listed as HFE in data tables. It varies with transistor types. So the value 25 will be AVAILABLE TYPES. Many types of transistors are made, but almost all work on the very same principle and almost all could be used in model railroad circuits if it were not for their cost or, in some cases, their low current or voltage rating. The types usually within our price range are power transistors, general purpose types, audio or entertainment types. A particular design of transistor is made in large batches and then the transistors are tested. All those that meet military and o th e r special requirements are given certain type numbers and sold at fairly high prices. Otherwise identical transistors, but which show lower voltage test ratings or other limitations, are given different type numbers and sold at more popular prices. These are the ones we usually use, and an assortment of them is shown in our list. Published tables of transistor data usu- ally use the letter P for power in watts or milliwatts, V for volts, I for current in amperes or more often milliamperes (1000 milliamperes = 1 ampere, etc.). To these letters, small letters are often added to show which connections are being rated. A small letter "o" means the condition when a transistor is not conducting. Thus Vceo max means the maximum voltage allowable between emitter and collector whennot conducting. Sometimes this is written BVceo meaning "Breakdown voltage, collector to emitter, when turned off." A small letter "s" in place of the small "o" means that the base is short circuited to the emitter. This is nearly turned off. In general "Ratings" are limits beyond Note that the P of PNP indicates that the emitter connects to positive while the N of NPN shows this emitter connects which damage or destruction will occur on the weakest of a given batch of transistors. "Characteristics" are given for the purpose of designing circuits with practical current and voltage values. The data in our tables are mostly ratings. VOLTAGE RATING: Some manufac- turers give nonninal voltage ranges for the type of circuits transistors may be used in, such as 6 v. and 12 v. uses. It is more accurate to show the peak or maximum safe voltage for a transistor, and this is most often the rating given. This maximum voltage rating should be chosen somewhat higher than the voltage you expect to control with your transistor circuit. For instance, suppose you want to turn off the full voltage from a power pack. A "12 v." power pack actually has a voltage of 16 v. to 18 v. when not loaded. Further, the half waves of power reach a peak 40 per cent higher than the nominal voltage or total about 24 v. For safety, the lowest maximum voltage rating you should then select for your transistor should be still higher.

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