Beginner Questions: AC, Impedence, Transistors, and Salving

[budidharma]

Alright, so lets try and go from easiest questions to most difficult (thought I know their all beginner level).

The first, salving equipment: I found an old PCB yesterday, without hundreds of resistors, capacitors, and ICs. I started removing the resistors and capacitors by applying a soldering iron to one leg, then pulling on that leg with a pair of needlenose pliars till it came out of the solder and hole. Then I did the same to the other side. Is this a good technique? Is there a better way? I assume most of these components will work. I have not been removing the ICs or transistors, though. I assume that they would not be as versitile or easily identifiable as caps and resistors. Am I wrong, should I try to salvage those as well?

Second Question: Say I have a big pile of transistors. I don't know if they are NPN or PNP, and I have no idea how much voltage or current can be applied to them without destroying them. How would I go about testing them, to find whether they are NPN or PNP, and what they are made for?

Third question: I don't really understand Alternating Current. It travels in a sine wave, from 0 voltage to + peak voltage to 0 voltage to - peak voltage and back to 0 voltage. How can it have negative voltage during the second part of that sine wave? Wouldn't this mean that the source voltage has reversed directions and is pushing current the reverse direction through the circuit? The current does have to follow voltage, right? On this, I am very confused. It's covered in TAB electronics Guide in chapter 3, but I still dont' really understand.

I know that changing the amount of current in a wire causes changes in the electromagnetic force around that wire. Does it exert this force on itself? I've seen the term self-impedence mentioned a few times, but not clearly explained. It seems that if the voltage was constantly changing in a condutor with an AC, it would constantly be changing its own impedence as well, what affect does this cause? Can someone explain this to me more clearly?

Third question: Capacitors store voltage. When connected to a DC circuit through a battery and a resistor, a capacitor will continue allowing current to run through the circuit until it has reached its maximum charge and then current will stop flowing. If you then disconnected the capacitor and connected it to a separate circuit using a resistor and an LED, it would power the LED until it's charge had run out. That'd be the basic of a CAP, however, when you apply them to an AC circuit, I don't understand how they work. They are used for filtering, I've read. I don't really understand what this means, or how they work. Can anyone clarify this?

Fourth Question: I can use transistors as switches in a DC circuit. In that sort of environment, I understand how they work. Apply a voltage across the base-emittor and it will allow a larger current to flow across the collector emittor. Again, when applied to an AC circuit, I don't understand.

Alright, all these questions spawn from a diagram I've encountered in the sixth chapter of my TAB Electronics book. It's for an amplification circuit (if you have the book) found on page 163.

Unfortunetly I can't post the schematic (I don't know how I'd get it from my book to this page, I don't have a scanner) and I can't find any similiar ones with google. Like I said it's a simple amplification circuit, you probably all know what those look like.

Thanks!

 

[jrz126]

Re: Beginner Questions: AC, Impedence, Transistors, and Salv

Alright, so lets try and go from easiest questions to most difficult (thought I know their all beginner level).

The first, salving equipment: I found an old PCB yesterday, without hundreds of resistors, capacitors, and ICs. I started removing the resistors and capacitors by applying a soldering iron to one leg, then pulling on that leg with a pair of needlenose pliars till it came out of the solder and hole. Then I did the same to the other side. Is this a good technique? Is there a better way? I assume most of these components will work. I have not been removing the ICs or transistors, though. I assume that they would not be as versitile or easily identifiable as caps and resistors. Am I wrong, should I try to salvage those as well?

I like to use the air compressor to blast the solder out of there, I'll blast a bunch of leads at one time then I'll go back through and use my needle-nose to pull them out. Works pretty good.
For the IC's, It all depends. you can use the part number printed on top to find the datasheet, nearly every IC has one and can be found all over the net.

Second Question: Say I have a big pile of transistors. I don't know if they are NPN or PNP, and I have no idea how much voltage or current can be applied to them without destroying them. How would I go about testing them, to find whether they are NPN or PNP, and what they are made for?

I had a program, cant think of the name right now, I know it was like ntc2000 or NTE2000, but it had a great cross-reference search for transistors (using the part number), I dont know how accurate it was for like the beta value, max voltage and whatnot, but it would atleast tell you if it was pnp or npn.

Third question: I don't really understand Alternating Current. It travels in a sine wave, from 0 voltage to + peak voltage to 0 voltage to - peak voltage and back to 0 voltage. How can it have negative voltage during the second part of that sine wave? Wouldn't this mean that the source voltage has reversed directions and is pushing current the reverse direction through the circuit? The current does have to follow voltage, right? On this, I am very confused. It's covered in TAB electronics Guide in chapter 3, but I still dont' really understand.

It just means that the polarity is switched.

Sure do have a lot of questions...hope you get the answers. hope I helped as well.
[/quote]

 

[checkmate]

Re: Beginner Questions: AC, Impedence, Transistors, and Salv

I'll see what I can help.

Third question: I don't really understand Alternating Current. It travels in a sine wave, from 0 voltage to + peak voltage to 0 voltage to - peak voltage and back to 0 voltage. How can it have negative voltage during the second part of that sine wave? Wouldn't this mean that the source voltage has reversed directions and is pushing current the reverse direction through the circuit? The current does have to follow voltage, right? On this, I am very confused. It's covered in TAB electronics Guide in chapter 3, but I still dont' really understand.

I know that changing the amount of current in a wire causes changes in the electromagnetic force around that wire. Does it exert this force on itself? I've seen the term self-impedence mentioned a few times, but not clearly explained. It seems that if the voltage was constantly changing in a condutor with an AC, it would constantly be changing its own impedence as well, what affect does this cause? Can someone explain this to me more clearly?

In an AC, the voltages reverse periodically. But what current flows is dependent on the load.
Impedance is simply the ratio of the potential difference across the conductor to the current flowing through it. It's a generalization of resistance, except it takes into account capacitance and inductance as well. In linear circuits, this allows us to use equations somewhat similar to Ohm's Law.
Self impedance refers to the impedance of the conductor itself. The word self, is to differentiate it from mutual impedence, which is a conductor's impedence brought about by another conductor.
Impedence is dependent on most occasions to the frequency of the signal flowing through it. "Frequency" is the term used instead of "voltage was constantly changing in a condutor with an AC". You should get used to it. By understanding this behavior, you could design filter circuits.
Lastly, you don't have need to have a changing current to generate EMF. Any moving charges, even a constant current, would generate EMF. That's how a DC relay works. And you could say that it exerts a force on itself, or rather each moving charge exerts a force on all other charges in the vicinity on the microscopic level. But on the macroscopic level, such interactions get clumsy and we assign a value called inductance to encapsulate the net behaviour.

Third question: Capacitors store voltage. When connected to a DC circuit through a battery and a resistor, a capacitor will continue allowing current to run through the circuit until it has reached its maximum charge and then current will stop flowing. If you then disconnected the capacitor and connected it to a separate circuit using a resistor and an LED, it would power the LED until it's charge had run out. That'd be the basic of a CAP, however, when you apply them to an AC circuit, I don't understand how they work. They are used for filtering, I've read. I don't really understand what this means, or how they work. Can anyone clarify this?

Capacitors store charges, not exactly voltages. Imagine a capacitor similar to a bucket. You give it an AC current, ie you pour in water, and scoop out water repeatedly. If you do it slowly enough, the bucket gets full before you switch actions, and no more transfer occurs until the next switch. But if you speed up, you'll notice that the bucket never fills up and there is transfer at all times. That's your filtering.

Fourth Question: I can use transistors as switches in a DC circuit. In that sort of environment, I understand how they work. Apply a voltage across the base-emittor and it will allow a larger current to flow across the collector emittor. Again, when applied to an AC circuit, I don't understand.

I don't quite get what you want for this question. Ideal transistors have no memory effect. Their behaviour at any one time depends on V(BE) at that particular instant. So just apply it in the AC context.

 

[zachtheterrible]

Fourth Question: I can use transistors as switches in a DC circuit. In that sort of environment, I understand how they work. Apply a voltage across the base-emittor and it will allow a larger current to flow across the collector emittor. Again, when applied to an AC circuit, I don't understand.

before you can understand how a transistor operates in an AC circuit, you have to understand AC!!!!!! but now that you understand AC:

a transistor is used to amplify a signal, in analog electronics, usually an AC signal. so basically (very basically), the AC signal comes through the base-emitter junction, which the allows current to flow through the whole transistor, less current when the original AC signal goes low and more as it goes high. so what you end up with is an AC signal out of the transistor that is bigger (amplified) than the origianl AC signal.

this is a very simple explanation. in order to really understand it, you need to see the transistor in an amplifier configuration and have it expained toyou.

 

[sonaiko]

Re: Beginner Questions: AC, Impedence, Transistors, and Salv

Third question: Capacitors store voltage. When connected to a DC circuit through a battery and a resistor, a capacitor will continue allowing current to run through the circuit until it has reached its maximum charge and then current will stop flowing. If you then disconnected the capacitor and connected it to a separate circuit using a resistor and an LED, it would power the LED until it's charge had run out. That'd be the basic of a CAP, however, when you apply them to an AC circuit, I don't understand how they work. They are used for filtering, I've read. I don't really understand what this means, or how they work. Can anyone clarify this?

Thanks!

u wont be able to understand this well until u hear about smthng call Fourier Transform..
Now u r dealing with time.. and voltages and frequencies change due to time change in ur circuit..
later on, u will use the Fourier to deal with frequency.. u send a frequency to a circuit and there will exist a corresponding votlage and current to that frequency..
Filtering means for some frequencies there will be a response. else the respomse is zero.
Capacitors are High pass filters. that means they allow high frequency to pass and have respnose but they block the low frequencies from having a response.

u will under stand this better when u be more advanced in the frequency domain ciruits..

 

[Dngrsone]

Re: Beginner Questions: AC, Impedence, Transistors, and Salv

Alright, so lets try and go from easiest questions to most difficult (thought I know their all beginner level).

The first, salving equipment: I found an old PCB yesterday, without hundreds of resistors, capacitors, and ICs. I started removing the resistors and capacitors by applying a soldering iron to one leg, then pulling on that leg with a pair of needlenose pliars till it came out of the solder and hole. Then I did the same to the other side. Is this a good technique? Is there a better way? I assume most of these components will work. I have not been removing the ICs or transistors, though. I assume that they would not be as versitile or easily identifiable as caps and resistors. Am I wrong, should I try to salvage those as well?

I tried salvaging in my early days, but I ended up not using anything except hardware and connectors... depends on whether you have money to buy components with, I guess. I use a solder extractor or copper braid, though I had a solder iron tip with a huge pit in it that was good for pulling excess solder out

Second Question: Say I have a big pile of transistors. I don't know if they are NPN or PNP, and I have no idea how much voltage or current can be applied to them without destroying them. How would I go about testing them, to find whether they are NPN or PNP, and what they are made for?

Use a cross-reference to get the specs:

http://nte01.nteinc.com/nte/NTExRefSemiProd.nsf/$$Search?OpenForm

Third question: I don't really understand Alternating Current. It travels in a sine wave, from 0 voltage to + peak voltage to 0 voltage to - peak voltage and back to 0 voltage. How can it have negative voltage during the second part of that sine wave? Wouldn't this mean that the source voltage has reversed directions and is pushing current the reverse direction through the circuit? The current does have to follow voltage, right? On this, I am very confused. It's covered in TAB electronics Guide in chapter 3, but I still dont' really understand.

Keep in mind that a wire (or any other electronic component) is already filled with electrons, so an AC signal running through a wire is analogous to shaking a pan filled with water... you get waves, but ultimately no water is added or subtracted. The electrons dash back and forth a short distance.

The negative voltage is relative to an arbitrary measuring point, generally the zero reference is provided by ground. Have I confused you yet?

I know that changing the amount of current in a wire causes changes in the electromagnetic force around that wire. Does it exert this force on itself? I've seen the term self-impedence mentioned a few times, but not clearly explained. It seems that if the voltage was constantly changing in a condutor with an AC, it would constantly be changing its own impedence as well, what affect does this cause? Can someone explain this to me more clearly?

As the current changes in a wire, the electromagnetic field surrounding the wire will change accordingly (I increases, the field expands and vice-versa). Since the wire itself is in the center of the field, any changes in the field would have little or no effect on that wire... assuming a straight wire.

In coils/transformers where the wire is coiled up and the EM field from one area of the wire crosses another area of the wire, the motion of the field across that section of wire will cause a current to flow... in the opposite direction. This effectively reduces the total amount of current running through the coiled wire. This effect of added resistance is called Impedance. Total impedance for a coil is affected by the frequency going through it and the value of the coil itself (in Henry's). Best way to think of it is that coils resist changes in current. Self impendance refers to any device that has a wire that has a significant bend to it (or many bends) where a change in current will cause some impedance. Does this adequately answer your question?

Third question: Capacitors store voltage. When connected to a DC circuit through a battery and a resistor, a capacitor will continue allowing current to run through the circuit until it has reached its maximum charge and then current will stop flowing. If you then disconnected the capacitor and connected it to a separate circuit using a resistor and an LED, it would power the LED until it's charge had run out. That'd be the basic of a CAP, however, when you apply them to an AC circuit, I don't understand how they work. They are used for filtering, I've read. I don't really understand what this means, or how they work. Can anyone clarify this?

This one is more difficult. Capacitors don't allow current to pass through (there may be a little leakage, but for simplicity's sake we'll ignore that), since the wire effectively ends and is capped off with an insulator (the electrolyte). But since voltage (electrical potential) induces current when there is a path and a resistance, the potential difference between the battery and the end of the wire (that side of the capacitor) when the battery is initially applied, a current flows through the resistor until the potential on either side of the resistor is the same (voltage at that side of the cap is the same as the battery on that side).

Here's where it gets hairy-- the potential on one side of the capacitor has an effect on the opposite side. Just the same way static potential can raise your hair, a pile-up of electrons on one side of the insulator will drive away a corresponding amount of electrons on the other side. The capacitor ends up with more electrons stacked up on one side than the other-- your voltage.

Pull the cap out of the circuit and create a new circuit with a resistor and an LED... the potential between one side of the cap and the other want to equalize, so a current flows through the resistance until the potential is gone. Equal amounts of electrons on either side of the cap.

Because of the electrostatic effect that capacitors have, the effect is that an AC signal will pass through, though there is no current flow. Using our pan of water analogy again, the cap would be a membrane sperating two sides of the pan... a splash on one side would cause waves on the other side, though no water passed from one side to the other. This is useful when filtering DC since a cap can pass an AC component from positive to ground while maintaining the voltage level between the wires.

Capacitors and inductors are in many ways polar opposites-- where an inductor opposses a change in current, the capacitor opposes a change in voltage.

Fourth Question: I can use transistors as switches in a DC circuit. In that sort of environment, I understand how they work. Apply a voltage across the base-emittor and it will allow a larger current to flow across the collector emittor. Again, when applied to an AC circuit, I don't understand.

Alright, all these questions spawn from a diagram I've encountered in the sixth chapter of my TAB Electronics book. It's for an amplification circuit (if you have the book) found on page 163.

Unfortunetly I can't post the schematic (I don't know how I'd get it from my book to this page, I don't have a scanner) and I can't find any similiar ones with google. Like I said it's a simple amplification circuit, you probably all know what those look like.

Thanks!

**broken link removed** Something like this?

Ok, the input and output capacitors will allow the AC sine wave to pass through while isolating the circuit from everything else. This is a typical NPN common-emitter inverting amplifier. As the input voltage rises (in the AC component), the amplifier will create an amplified matching downslope (inverting, remember) until the input signal peaks out and runs in the other direction... you get the idea, the size of the output sine wave will be dependant on the amplication factor of the amplifier.

The cap from the Vcc to ground is there to keep the signal from causing a ripple on the DC power lines.

Hope this helps, somewhat.