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Transistor usage question

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Types of transistors(basics)


most common transistors are BJT or MOSFET(FETs)

we will be just going into BJTs, FETs should be discussed another time :)

Bipolar junction transistors(BJT) are used to amplify current, but a side effect is they can be used as a switch.

If you would like to know exactly how they work you may want to consult Wikipedia or your local libray or google.


*transistor = BJT transistor for this document/post*

transistors come in 2 common polarities, NPN and PNP, they both function on the same property but are biased differently

NPN - voltage/current flows from the emitter to the collector and is controlled by the base current/voltage

when the base current is applied to the base, more current flows from E to C

PNP - current/voltage flows from the collector to the emitter and is also controlled by the base current/voltage

when the base current is 0v current flows from C to E, but when voltage is applied, the current will be limited

*the arrow on the symbol is the way the current flows


Pins on the Transistors, some power transistors have 2 pins and one connected to the housing/case

collector - think of it as a label, but refers to how the transistor works

base - your control current/voltage

emitter - think of it as a label, but refers to how the transistor works


some sample circuits to get you started

Basic use

NPN switch




an LDR or photo-resister or CDS cell can be substituted for photo-transistors

sample 1 - when dark, more current flows through

sample 2 - when light, more current will flow through


R2 = is your gain, you can substitute a Potentiometer for the resister to adjust the gain



Darlington pair + Szikai pair

darlington is more useful then the szikai.

I like to use darlingtons for touch sensors or anything else that has a low voltage, that needs to be read. also you can buy darlington arrays, in a DIP package, single TO92 packages or build your own.

personally i have never used the szikia in a project. Its just a reference, usually used in audio amps.

Ignore my BCE labels on the darlington i had mess it up........the E and the C are just reversed, sorry.



values to consider

IC max. - max collector current

VCE max. maximum voltage collector to emitter

hFE min. minimum gain



When using s switched load like a Coil, solenoid, motor, relay

- add a diode across(parallel to) the load to prevent voltage spikes when the load is switched off

(IE the diode should be opposite of current flow\)









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Nice post! One question though...


I tried out the NPN switch circuit with the LED, using a 2N3904 (from GeekDoc :D) and found that it didn't work with the LED on the emitter side, but rather only on the collector side. When the LED was on the emitter side, actually, the transistor heated up ridiculously fast while there was current/voltage applied to the base. I'm not sure why, any ideas?

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you may need a resistor on the collector side when the led is connected to the emitter, to limit the current going through.

the other thing is are do you have a load on the base current when activating it? I usually have to put a resistor on the base to prevent it from going straight to ground, and burning your transistor


diagram comming in a few mins

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