# An Absolute Beginner With A Few Questions

• posted

Hey, everybody!

I figured the best place to get my feet wet was with one of those electronic kits like Radio Shack sells. Bunch of components, instruction manual, compact size, 30 projects to get you rolling.

So I'm working on it and paying attention, and some questions are forming (after just four projects, too. I'm not sure if that's good or bad).

1) In a simple light circuit, using an LED, batteries, and a resistor, does it matter where the resistor is placed? The book cautions that LED's can burn out from too much power. Does the resistor stay before the LED, can it be placed after, or does it matter as long as one immediately follows the other in series?

2) The kit features two..."regular" (I guess they might be called disc) capacitors and two electrolytic capacitors. I noticed the schematics, when they depict the electrolytic ones, include a positive symbol by the flat line instead of the curved. So they can obviously only be connected in the circuit one way, positive current to positive terminal on the capacitor. But what about the smaller disc ones? Does it matter for them?

3) The next project is building a crystal radio, powered by radio waves. It mentions attaching a wire to a ground. Buying a shortwave antenna for simple experiments strikes me as overkill, and the pipes in my place are PVC plastic. Then I remembered my computer. The case that I attach my anti-static wrist strap to is a ground. Can I just attach the ground wire to the case of my computer (with the computer powered off, of course)? Failing that, I was told that the negative terminal of a battery is considered a ground. Can I just wire it to that?

4) What gives something the ability to be a ground, anyway? Just that it can readily absorb an electrical current? It seems to me that, if the computer case idea is workable, then as long as the voltage is small, almost any sizable metal object would make a sufficient ground.

5) Big question: I was taught in science class years ago the electricity actually flows from the negative terminal to the positive. Even though the book says the other way around and schematics for things like diodes imply power goes from positive to negative, when it gets to transistors, it says the electrical path is emitter to collector or emitter to base. Did they just mix the words up, since the schematic shows the flow going the other way, or am I missing something?

Sincerely, Pete Holland Jr.

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• posted

The same current passes through all components in a series loop, regardless of the order.

No. They have ceramic insulation between their plates and it handles voltage in either direction, equally. The insulation between the "plates" of an electrolytic capacitor is formed by a chemical action between the internal liquid and one of the plates (a roll of aluminum foil that has been chemically etched to make it very porous, to have a huge surface area) driven by a DC voltage. I say "plates", because the other one is the liquid.

The case of the computer, and any other thing connected to the ground prong of a 3 prong AC receptacle should connect to ground. Of course, you could also drive a big nail into damp ground and run a wire inside from that.

No. Any point in a circuit may be considered a common node with which to measure other voltages, (and is commonly referred to as circuit ground), but it has nothing in particular to do with Earth ground. Current from an aerial has no incentive to flow in and out of a floating battery terminal.

A planet sized mass will do fine. ;-)

But seriously, you are right. Any metal object that is at least a quarter wavelength long in every direction from the connection point can act as ground for an aerial. But at approximately 1 megahertz (broadcast AM band) that is a pretty large object. At 27 MHz (citizens band) a car body is a fair ground for a quarter wave whip..

That would be electron (or any other current carried by negative charge carriers... Got any muons?)

When Ben Franklin was experimenting with batteries, he assigned the polarities and guessed the current direction. He was right for protons, holes, positrons and positive ions. He had no idea what carries the current. And as long as you use one concept, consistently, the math all works out, though thinking about the current through electron tubes gets pretty silly using positive current. But for semiconductors (diodes and junction transistors, there are both positive and negative charge carriers in play, so either convention is half wrong. Just remember that a positive charge going one way is the same current as a negative charge going the other way.

• posted

On Tue, 13 Dec 2005 19:46:55 -0600 in sci.electronics.basics, "Pete Holland Jr." wrote,

Does not matter. The voltage across the resistor, plus the voltage across the LED, must equal the voltage from the battery.

a + b = b + a

The ceramic disc capacitors are unpolarized and can be connected either way.

Use whatever long piece of wire you have for an antenna. You can re-use it for other things after the experiment.

I would say use the grounding connector (in the USA it's the round "third" prong) of a wall outlet for ground, which is what your computer case is connected to. But maybe somebody has a better answer.

People needed to be able to talk about current flow before anybody figured out whether an electron was charged plus or minus. So, they guessed. They just happened to guess "wrong".

Conventional current is always described as flowing from positive to negative. The actual electrons are going the other way. It rarely matters unless you are using vacuum tubes where the electrons flying through space carry the current, or if you make your own semiconductors.

• posted

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The battery, resistors, LED are in series. The same current goes thru = each so it doesn't matter what sequence they are in

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• posted

OK. yeah.

good. definately. When I started out I had noone to ask.

As long as it's in the path that current takes to go through the LED you're doing it right.

this is true.

It doesn't have to be immediate, but it does need to be in series.

for the disc capacitors it doesn't matter. The manufacturuing process for the disc capacitors prouces symmetrical devices, that can be connected in either orientation.

the computer case is a pretty good earth, but crystals sets can be fussy. A battery's negative is only a nominal ground.

The radio needs a real ground connection, and the computer case (whilst plugged in) is connected to the protective gpound in your house's wiring so it's provides an indirect path to the real ground.

That's a pretty good description for general use. but crystal sets need a conductor that's connected to actual earth

crystal sets work by detecting the current flowing in and out of the end of the antenna wire. for that the need somethinfg that, as you said "can readily absorb an electrical current", for the crystal set this could be another antenna pointing in the opposite direction, or the ground

any that's connected to the ground,

some examples, the hot water cylinder, the refrigerator, the stove, a train track

yes electrons carry a charge that we call "negative" and move from the flow from the negative terminal of the source to the positive.

power is not electricity. power is a rate of work (eg horse-power) the unit for measuring power is the watt (or horse-power, BTU/hr etc)

anyway the book implies that _electricity_ (or to be more precise electric current) flows from the positive terminal to the negative terminal.

yes it does, this mythical current that flows in the opposite direction to the actual electron flow is called "conventional current" and for the most part it is fine to treat it asif it is the real thing.

there are two major clases of (bipolar) transistor PNP and NPN I'm going to assume your kit has an NPN transistor because for some reason NPN transistors are significantly more common than PNP transistors. if you use a monospaced font like courier this diagram will look somewhat like an NPN transistor

/ / | / C |/ B | ------------| | |\\ | \\| E ~\\ \\

here as you say the conventional current flow is into the base and out the emitter or in the collector and out the emitter the electron flow is ofcourser the opposite

with the PNP transistor it's the opposite

/ / | / C |/ B | ------------| | |\\___ ||\\ | \\ E \\

electrons flow in at the collector or base and out at the emitter conventional current flows the opposite direction

When discussing the details of how transistors function often electron curent is considered instead of conventional current. fortunately it's not essential to know how transistors function to use them one only needs to know how they behave. much like to drive a car one needs only to know how to drive, not how the engine and transmision work. it is acceptable to treat transistors like a plack box that responds in a predictable way to a known stiimulus.

With the crystal set a good earth connection is important if an earthed appliance like your computer's case is less than ideal because the path from it to the earth peg is not direct best results are had with a wire directly to an earth peg thas gose deep into the dirt.

try the compuuter case first, if it's not giving satisfaccction you may need a better earth connection.

Bye. Jasen

• posted

It's good. It shows you're thinking, and have a desire to understand. That's very cool to us old-time geeks and tinkerers. :-)

Since they're in series, what will the current be through each? It doesn't matter where in the loop they are, unless there has to be some sort of reference that constrains the design. (like, you need the LED to have a "ground" reference, for some reason. (more on ground soon :-) )

No. Any cap that it makes a difference will be clearly marked. Oh, and just FYI, what gets connected to the positive terminal is the positive _potential_ WRT the negative terminal. But current can only flow for a little while, which is where T = RC comes in. The non-polar ones can "pass" an AC current, even though nothing goes through them, except maybe charge - the electrons pile up on one side, then the other, so in effect AC is going "through" it. This is where you get "reactance", but that's another lesson. ;-)

You probably can - when I was a kid we had hot water heat so we'd just clip it to the radiator. ;-)

But before you get too far, I want to clarify "ground". There are at least three, count'em, three common usages of the term, and that's only in electronics! Anyway, there's the "safety ground", which is the third, green wire in outlet boxes and the third prong on the plug. This has as low a resistance as possible connection to the actual Earth. What that does is, if there's an internal short in the equipment, it will blow the fuse rather than electrocute, because the case will be (or is supposed to) at zero potential relative to the wet basement floor. :-)

Then there's the kind of Earth ground that's a counterpoise for an antenna. When the radio wave induces a current on the antenna, at the end where the receiver is, the other input terminal is ground, because for any current to flow, you have to complete a circuit. In this case, the circuit is completed by the capacitance between the antenna and ground; or maybe just the EM field - I drank too much when I got to that class. ;-)

And then there's "circuit ground", which is just a common reference point for the power supplies and signal return paths, and doesn't have to be grounded, they just use the term by convention.

Well, for RF, any old metal object can be the "ground" which means a counterpoise for the antenna, one notable example would be the car.

But if you want lightning protection, you need a "good" earth ground, which means a low resistance connection to the Earth itself.

College students are taught "conventional flow", because everyone assumed that current flowed from positive to negative. When some smart-ass discovered electrons, they just left it that way, and say "electron flow" simply flows the opposite direction to "conventional flow". I think it was the techies that used the "electron tubes" that threw the wrench in the works. But they're exactly the same for any practical purpose - all you have to do in any mathematical formula to use one or the other is reverse all of the signs. :-)

Welcome to the Asylum! Rich

• posted

On Tue, 13 Dec 2005 19:46:55 -0600, via , "Pete Holland Jr." spake thusly:

Hey!

1, 2) Doesn't matter.

Oh boy! I built one when I was about five, my older brother and I. I didn't do any more serious experiments with electricity until I was about 12 and plugged an 8ohm speaker into a wall outlet. Tesla would have been proud.

Oh boy again! Grounding is fun.

Yes.

No. No terminal of a battery can be considered a ground until a ground is attached to the terminal. For instance, communication gear is typically attached to 48VDC batteries, with the POSITIVE terminal grounded.

A sizeable metal object that carries electrical current is best described as a chassis, as in the chassis of a car or motorcycle, which is commonly used as a RETURN PATH for electrical current. GROUND is the term that is in common use, because Mother Earth is extensively used as a return path for electrical currents.

Specifically in the case of your computer (double entendre?), the chassis makes a ground because the chassis is connected to a prong on the input power cable, which allows current to find real earth ground through the house wiring.

Now, ground is a lot more complicated than most people in the various electronics fields realize. One of the most useful and conversely most incorrect concepts is that a common point on a schematic is "the same point electrically." For theory that works great. In reality, well, the reality is much different. It's like ignoring the effect of wind and air resistance when doing falling body calculations. It makes the math easier and teaches the concept, but actually has precious little resemblance to the real world.

The topics of electron flow and energy flow are a big-assed can of worms. Instructors blame Ben Franklin for misunderstanding the direction of electron flow, but he actually had a good grasp of how electrical energy flowed. (His "kite and key" experiments were intended to figure out a way to protect dwellings from burning down from lightning strike, and were a real success.)

Thus we have energy flow from positive to negative, whereas actual electron flow is from negative to positive. In applied electronics, energy flow is more important than electron flow. Here's a heads-up for you - when you get to transistor theory and design, they'll throw "hole flow" at ya. Yikes!

Good luck.

• posted

A crystal radio is a passive receiver that needs power from the transmitter and as such it needs two "wires" to the transmitter. These wires are the radio waves (signal) and the earth (return). You should be able to use your computer as ground if your computer is connected to the ground of your electrical installation.

BTW, I think that your anti-static wrist strap is probably overkill as well.

--DF

• posted

On Tue, 13 Dec 2005 19:46:55 -0600, via , "Pete Holland Jr." spake thusly:

Missed this bit the first time. It's wrong if stated as you say. Here's kinda how it works: A potential is developed between collector and emitter, but there is no current because of a phenomenon called the "depletion region," which is a transistor's way of resisting current. The emitter flow (either "electron" or "hole" flow) wants to move to the collector, but is impeded by the depletion region.

A potential is applied at the base in order to influence the depletion region to shrink, until finally it is small enough to allow energy to pass from emitter to collector. However, the energy injected at the base also flows to the collector. Again, based on type, NPN or PNP, the transistor changes the type of flow from "electron" to "hole."

Well, you asked for it, so there it was! Check this:

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On Wed, 14 Dec 2005 10:52:45 -0000, via , Jasen Betts spake thusly:

Not even nominal. No electrical cell terminal is a ground, unless and until it is connected to ground via a conductor.

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Hey, Deefoo!

Believe me, I suspect that myself, but I always wimp out and go with the strap anyway, just in case and all that jazz.

Dobre utka, Pete Holland Jr.

• posted

Hey, Rich Grise!

A question related to your response, and then a general question on the subject of transistors.

I built one circuit. It was to show capacitors at work, storing and releasing electicity. I decided to test if the battery could be a regular ground by attaching the positive terminal on the charged capacitor to the LED and the negative to the negative terminal on the battery. As I'm sure won't surprise you, the LED didn't light up. I'm figuring both poles of the power source have to be in the circuit, you can't divide the polarity between two different sources. So I was still learning the limits when all the responses hit.

But that brings me a question that has puzzled me since I was a kid trying to understand the concept of RF ground--as a kid, I had a little transistor radio (this was before IC's and stuff, around the early 1980's). Ground was supposed to be important, but where was it on the transistor radio? It was completely self contained, and encased in plastic, too. I took one apart, and couldn't figure it out. How do they do it?

The other question concerns what makes transistors necessary. One of the projects connects the two batteries, LED, and the collector and emitter of one transistor (the symbol shows the arrow on the emittor line pointing away from the base, not towards) in series in one part of the circuit. In the other, it is one of the batteries, the key (cheap but effective momentary switch), and the base of transistor in series. The LED doesn't light up unless the key is pressed. Thanks to Big Mouth Billy Bass, I completely understand why the circuit works the way it does, the question I have is why is a transistor necessary? Take a powered radio, for example. Wouldn't it be more efficient to simply attach the battery in series with the radio signal, boosting it? Or does the transistor act more as a safety barrier (receives the signals and passes them to the next part of the circuit without the two parts overloading or interfering with each other)?

"Welcome to our humble madhouse. I trust you'll find yourselves at home."

--The Marqui de Sade in the movie "Quills". Seems an appropriate sentiment. ;-)

Dobre utka, Pete Holland Jr.

• posted

Just for the sake of pedantry, "nominal" only means that it's called that; it's derived from the Latin "nomen", which means "name", i.e., "nominal" pretty much translates to "in name only".

And of course, the appropriate rejoinder here is that the "ground" that the battery's terminal is connected to is also "only" a "nominal" ground. :-)

Cheers! Rich

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Hey! :-)

Well, the operative word here would be "circuit". There has to be a path for the electrons/charge to go all of the way around, back to the source. The reason "ground" works in this case is that that "ground" provides a "return path".

In the water hose model, it looks like just cutting the hose lets all of the water flow out, but the part of that analogy that's missing is that all of the water flows out of the ground, down to the water table, and is sucked up again by your well pump.

For electrons, you have to provide a path for them, because when you cut a wire, it's more like pinching off a hose.

Open the transistor radio again, and notice the "loopstick antenna". It's a ferrite slug, either a cylinder about 3/8" (1 cm) diameter x 5" (12 cm) long, or a rectangular solid, about 2.5" (6 cm) long, about 1/2-3/4" (12

- 20 mm) wide, and about 3/16" (~4mm) thick. Whichever of those it is, you'll see that it's wound with hundreds, maybe thousands of turns of wire. In this case, the "return" is simply the other end of that coil: it's actually, conceptually, acting like the secondary of a very large, nebulous transformer. (the ferrite slug picks up the H field - you'd have to study some antenna physics, or maybe just a textbook: my explanation falls way short.)

It's because it can amplify. If all you want to do is light a bulb, then, yes, just flip the switch. But to use a radio signal, the signal needs to be amplified, because by the time it reaches your antenna, (with those

50KW distributed throughout millions of cubic miles of space), you're picking up a signal that's in the microvolts range.

Here's a practical experiment: Connect a speaker to a lamp. Yell at the speaker. See if it lights the lamp. Now, get a power supply, and put the

• terminal to one lead of the lamp, put the other lead of the lamp to the collector of your NPN transistor, and connect the emitter of the NPN to the - lead of the supply. Connect the speaker directly from the emitter to the base of the transistor. Now shout at the speaker. Please report your results. :-)

Have Fun! Rich

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I've even seen designs with two tuned ciruits - one tuned into the nearby high-power station, that just gets rectified, and becomes a "free" power supply for the other section, which can be a much more sensitive radio, because with a power supply, you can have gain. ;-)

Cheers! Rich

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At least I used my 2000 ohm headphones and Mom's sewing machine cord. ;-) (I had learned many years earlier, the thumb-between-the-prongs trick. ;-P )

No Tesla effects, but the whole family did rush to investigate the impromptu Klaxon alarm. ;-P

Cheers! Rich

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This corroborates that the charge injected into the base flows to the emitter. The emitter current is the sum of base and collector currents, coming out instead of going in (or the opposite). In the NPN example, the conventional currents are flowing into the collector, with a little control current flowing into the base, and they're both coming out the emitter. (well, that's "conventional current": in a PNP, the flow is the opposite direction, and with electron flow, it's all reversed. That's not confusing at all, is it? ;-) )

There are those who proclaim that a transistor is "voltage controlled", but to model it as a voltage controlled device, you have to do some nasty exponential arithmetic. I (and probably lots of other techs) prefer to think of it as, "once the base-emitter junction is forward biased enough to actually conduct, the collector current is equal to beta times the base current (with a few correction factors that we don't need to worry about if we're not designing TVs or cell phones from scratch, but only flashing an LED.)".

Cheers! Rich

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On Thu, 15 Dec 2005 21:45:42 GMT, via , Rich Grise spake thusly:

Bah. That bit's always confused me, and I *thought* I'd looked it up and verified it the other way round. "Collector" just seems the right term for collecting all the current. Oh well. Stupid transistors.

Meanwhile, for the sleep impaired, I found this tidbit:

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• posted

electrical energy doesn't always flow from positive to negative, else how would AC appliances operate?

It is traditional to use a positive supply and negative ground in many designs (especially digital) but as you noted "communications" equipment (telephone?) uses positive earth.

Bye. Jasen

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