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# For electronic n00bs Answered

This topic is made to help all who don't know a lot (or even anything) about electronics. Here people can ask simple questons without opening a new topic.

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## 14 Replies

BC-45 (author)2011-02-14

OK you guys i need help with AND , OR , AN and Not gates what are there functions and how do they work in electronics need help on that

Chicken2209 (author)2008-11-02

can someone explain something to me I don't exactly see the resistor's purpose as a nonmoving switch point me out where I go wrong here Lets say you have a basic circuit with a lightbulb as a load On that circuit you have a transistor, hooked up to another circuit with a switch so when its pressed, the circuit works. What's the point of that transistor, why not place the switch on the main circuit

westfw (author)2008-11-07

Presumably you meant "transistor." The point is that the load (light bulb) current can be controlled by a MUCH smaller signal than the load requires, AND it's electronic. So instead of physically making two pieces of metal touch to turn on your desk lamp, a microcontroller can do it by outputing a small signal (0.1W or less; 5V@20mA) on one of it's pins, even if the load is a 20W bulb (12V @ 2A) (or more.) (And it's fast too. Switch that 20W bulb on and off 100 times a second and you can control the brightness by varying the ration of ON time to OFF time.)

11010010110 (author)2008-11-08

as wave maker transistor can be connected to other components that return 'echoes' of current to it. the transistor amplifies the echoes and sends them to the circuit. then gets new ones . . . in this way transistor can be used to make a constant stream of pulses or wave this is used in many circuits. for example flash circuit in a camera. the transistor converts DC (constant current) to AC (wave). then the AC can be amplified with a transformer to the high voltage required to fire the flash. there are some 'ibles here on how to use the flash circuit for various stuff another option is to set up 2 transistors together so they switch on and off each other in a rush. the effect is the same - stream of pulses that can be used for various stuff. such circuit is called multivibrator as part of digital devices transistors can be connected together to make more complex stuff in the circuit you have connect a second transistor in parallel with the 1st one. connect C and E of both transistors together. connect B (the base - the control current) to some other switch now if ANY of the switches is close the led is on now connect the new transistor in series - disconnect E of the 1st transistor from - and connect the new one in between now if any of the switches is open the led is off now leave only the 1st transistor and disconect its base (B) from the switch. instead - connect it to the + thru a resistor. the led should be on all the time now. take the second transistor. connect C to the B of the 1st one. E to -. B to the switch. now if the switch is close the led is off (you kinda inverted the way the switch works) the circuits you made are called AND OR NOT gates and are the 3 basic blocks of which any digital circuit (like programmable controller and computer) is built.

gmoon (author)2008-11-02

A transistor or other silicon switch is used when the source of the switching is a logic (or analog) signal. I.E., when it's being switched by an IC, transistor, or other device, there isn't a mechanical switch being used at all.

Practically, transistor switches are way faster than any mechanical switch or relay, anyhoo.

Chicken2209 (author)2008-11-02

yeah, but to make the transistor act as a switch, it itself needs a switch to turn it on and of

gmoon (author)2008-11-02

Nope, bipolar transistors are essentially current-amplifiers.

In WestFW's diagram, the amount of current flowing through the load resistance (r1) is Ic, and is proportional to the base current (Ib). If there's no current applied to the base, then there's no Ic current, either....

So another device, say a microcontroller or even a 555 chip, will supply the logic signal to the base. No mechanical switch is needed.

Chicken2209 (author)2008-11-02

can you dumb it down a bit?

gmoon (author)2008-11-02

A regular mechanical switch actually physically disconnects the two sides of the circuit (it makes or breaks the connection.)

But a transistor acts more like a valve. It blocks the current (rather than disconnecting it) when there's no current applied to the base (the "input" of a transistor.) It's easy for some other electronic device to supply (or deny) that base current.

Dantex (author)2008-11-07

here's bunch of questions 1. witch is the main difference i work of NPN and PNP transistors 2. can you recommend me some all purpose diodes "for simple projects, no more then 9 v) 3. witch is the difference between linear and logarithmicall potentiometers

Dantex (author)2008-11-02

Which is the difference between two pnp (or between 2 npn) transistors? for example : 2N2222 2N2369 2N2484?

westfw (author)2008-11-02
First, a basic review of what a transistor does. A small current through the "base/emitter" junction (Ib in the picture) results in a larger current flowing through the collector/emitter path (Ic). All other things allowing, the formula is approximately Ic = Hfe * Ib (Hfe is also called the "gain" of the transistor.) So...

• The first thing different between different transistors is the gain Hfe. You can have high-gain transistors or low gain transistors (gain tends to be traded off agains other parameters.)
• Next are some practical considerations. Ic has a maximum related to (approximately) the area of the current-carrying region of the transistor. Try to put to much current through it, and the magic smoke will escape.
• There is a maximum voltage that the transistor can withstand across the collector/emitter leads in the "off" state: Vce-max Apply too much voltage and the transistor will start conducting regardless of base current (this isn't necessarily fatal, but it tends to shortly lead to very high values of Ic, which IS fatal.)
• There is a minimum value of Vce called Vce-sat; this will limit the gain for high values of Ib
• There is a maximum power dissipation for the transistor, related to how well it can get rid of heat generated within the junction. Exceed the maximum power dissipation and the transistor heats up until the magic smoke escapes, even if the other parameters like Ic are within the limits. The power dissipated is approximately Ic * Vce, so for some (switching) applications the power dissipation is going to be related to Vce-sat, and for other (amplifier) applications it is going to be related to the actual values of Vce in the circuit.
• Of course there is the physical package and pinout. If you're working with an existing PCB, you really do want a transistor that fits in the spot provided, and has the holes in the right spots for the right leads to the right holes without having to go through contortions. (It doesn't help that Europe, Asia, and the US all seem to have settled on DIFFERENT "standard" pinouts for some common package types. Sigh.)

Those are the basics. There are more subtle characteristics, like just how close to linear that current equation is - a transistors gain tends to vary with Ic, so circuits that want very accurate amplification have to restrict the range of Ic, or pick transistors that have been specifically designed for wide ranges of Ic. And then there's SPEED, which is limited by a bunch of parameters that are "not shown" in the simple diagram here.

That said, there are WAY more transistors commonly used in hobbyist projects than there ought to be. Everybody seems to have their favorite "NPN General Purpose Switching Transistor"; in most circuits any of them will work equally well. (2n4401, 2n3904, 2n2222, pn2222, MPSA06, BC846, etc, etc...) I've created a "generic NPN" transistor that I use in schematics instead...

Sandisk1duo (author)2008-11-02
ahh, the "magic smoke"

love that stuff

guyfrom7up (author)2008-11-02

Things to add/ make simpler:

I usually use transistors as switches, so look at this page for hfe:
http://www.rason.org/Projects/transwit/transwit.htm
it also tells you where to hook up the load

watts: how many amps times how many volts going through it must be lower than watts disapated

volts: don't exceed it, lol

current: don't exceed it, lol