HowTo-Color (8) / Hacking Electronics / Simon Monk / 236-3 / Chapter 3
CHAPTER 3: Basic Hacks
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transistor. If for some reason (and this does happen) we wanted to switch the positive side, then we would need to use a PNP equivalent of the NPN 2N3904, such as the 2N3906. NPN stands for Negative-Positive-Negative, and yes, you can guess what PNP stands for. That is because transistors are kind of semiconductor sandwiches, with material of either N or P type as the bread. If the bread is N type (the most common), then the base voltage needs to be higher than the emitter voltage (by about 0.5V) before the transistor starts to turn on. On the other hand, a PNP transistor turns on when the base voltage is more than 0.5V lower than the emitter voltage. If we wanted to switch the positive side, we could use a PNP transistor (as shown in the PNP alternative to Figure 3-21) displayed in Figure 3-24. MOSFETs also have their own equivalent of PNP transistors called P-channel, their version of the more common NPN being called N-channel.
Figure 3-24 Using a PNP bipolar transistor
Common Transistors The transistors in Table 3-3 will cover a wide range of transistor applications. There are thousands and thousands of other transistors, but in this book we only really use them for switching, so these will cover most “bases”!
Name
Appendix Code
Type
Max Switching Current Notes
Low/medium-current switching 2N3904
S1
NPN bipolar
200mA
Current gain about 100
2N3906
S4
PNP bipolar
200mA
Current gain about 100
2N7000
S3
N-channel MOSFET
200mA
2.1V gate-source threshold voltage; turns on when gate is 2.1V higher than source
N-channel MOSFET
30A
2.0V gate-source threshold voltage; turns on when gate is 2.0V higher than source
High-current switching FQP30N06
TABLE 3-3
03-ch03.indd 47
S6
Really Useful Transistors
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