Basic Electronics


Step 9: Transistors

Picture of Transistors
A transistor takes in a small electrical current at its base pin and amplifies it such that a much larger current can pass between its collector and emitter pins. The amount of current that passes between these two pins is proportional to the voltage being applied at the base pin.

There are two basic types of transistors, which are NPN and PNP. These transistors have opposite polarity between collector and emitter. For a very comprehensive intro to transistors check out this page.

NPN transistors allow electricity to pass from the collector pin to the emitter pin. They are represented in a schematic with a line for a base, a diagonal line connecting to the base, and a diagonal arrow pointing away from the base.

PNP transistors allow electricity to pass from the emitter pin to the collector pin. They are represented in a schematic with a line for a base, a diagonal line connecting to the base, and a diagonal arrow pointing towards the base.

Transistors have their part number printed on them and you can look up their datasheets online to learn about their pin layouts and their specific properties. Be sure to take note of the transistor's voltage and current rating as well.
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srutkowski2 years ago
To Livntoasty..

You ask about the transistors in your RC car. Think about it this way..

The batteries in your controller are probably very small. Much smaller than the ones in the car. Those batteries are used to produce a very tiny radio signal that is received by the car. That signal would be far too weak to do a job such as steering the car. Some of the transistors in the car are used to amplify that radio signal so it can be used by other circuits to power the steering mechanism.

As for amplification. The radio signal itself doesn't get any bigger or stronger. The transistors are actually allowing the car battery to make a larger 'copy' of the original signal, much like the way a sign-maker would use a pantograph to produce a large sign using a small pattern.
Actually, this makes much better sense. Still a' lil confused but I believe my mind is starting to gear in the right direction Thank you. In fact, thanks to all who helped out.
R.ik2 years ago
I don't really get what's the difference between NPN and PNP... What is the difference, except for the current flowing the opposite direction?
mrmerino R.ik2 years ago
I've been over this many times with different people, but I am mostly sure that the NPN is a "normally off" until current is applied, while a PNP is "normally on" and then when current is applied, it turns off. I think.
randofo (author)  R.ik2 years ago
The PNP is wired with an opposite polarity to the NPN.


NPN --> Collector = power ---> emitter = ground
PNP --> Collector = ground ---> emitter = power
hottroc randofo2 years ago
Yeah but why not just turn one around, why do you need two different parts?
Microbe2 years ago
This instructable, along with the video on transistors has been great.

So I have a scavenged transistor which according to the datasheet is rated at 160V DC off-state voltage. Would it work at 12V? Is the 160V it's maximum rating or the voltage it needs to work at all?

Perhaps an instructable on how to read datasheets would be a good thing for us noobs.
Microbe Microbe2 years ago
Also, the datasheet describes the pins as anode, cathode and gate.

Does that correspond to emitter, collector and base?
livntoasty2 years ago
Lemme see if I understand this some (I'm a wee-bit corn-fused, too)
NPN: collector (input?) ---> emitter (output?)
PNP: emitter (output?) ---> collector (input?)
I see 3 pins on the transistors in your pic..from left to right on NPN with part number facing you, are these collector, emitter and ground? If so, are the pins the same in the PNP transistor? If they aren't in a specific order, then how does one tell which is which? Or, are the connections assumed because they perform specific behaviors? In other words, the transistor "knows" the difference between the small amount of electricity and the large amount of electricity?
When I hear the word amplify, I think of sound. I removed the electrical components from the circuit board (car had no body or light covers) and ended up with quite a few transistors. Does this mean that car was designed to include sound? Or, can transistors be used to amplify the energy in non-audible objects? For example, would I find a transistor in a variable speed rc car with no lights or sound?
Thank you for your instructable.
randofo (author)  livntoasty2 years ago
An amplifier is an electrical device for increasing electrical signals. Transistor amplify an electrical signal. Sound amplifiers are amplifying and electrical signal. They are found in most complex electronic devices (sound or not).

To figure out which pin is which on the transistor, you would need to look up the datasheet for the part number that is written on the transistor. The datasheet will have a picture or diagram to tell you which pin is which.

In terms of NPN versus PNP... the PNP is wired with an opposite polarity to the NPN.


NPN --> Collector = power ---> emitter = ground
PNP --> Collector = ground ---> emitter = power

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