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Topic: My "where do babies come from?" of electronics. (Read 2 times) previous topic - next topic

bwoogie

I'm still new to electronics and dont fully understand even the basics. I get what most of your basic components do, resistors, caps, transistors etc. But Voltage and Current kinda throws me for some reason. I get the whole water through a pipe analogy but for some reason it doesn't feel like it's clicked with me yet.

I think my biggest question is where does the current come from? Or how do you find it?
If I have a 9v battery, the battery is supplying the voltage of 9 volts, but what current is that? Is that battery specific? I could use Ohm's law to find the current (I = V/R), if I knew the resistance... But I don't, and dividing by zero, well, that's no good.

My understanding, and back to the analogy of the water pipes, is voltage is the pump and current is the direction and speed of the electricity. Does that mean the more voltage the faster the current (assuming no resistance)?

First off, I'm a little confused which way the current actually flows. Does it go from negative to positive or positive to negative? I've heard it goes from - to + but then why are diodes "backwards" - the current flows the direction of the arrow. Really confusing.... x_X

Anyways, back to voltage and current.

Let's say I plug the voltage and ground rails of a breadboard into an arduino's 5 volt and ground pins. Then I add one LED to the breadboard and power it up. *Let's ignore resistors for right now to try and hopfully keep things simple.
Let's say the LED is this: https://www.sparkfun.com/products/531
3.4V forward drop
Max current 20mA

What do those specs mean? 3.4V forward drop. Does that mean if I apply 5V to the anode it will drop to 1.6V out the cathode? So if that were true then if I put a second LED in series it should be less than half as bright, no?

Max current makes sense... That's the max amount of current you can give it before it fries. But again, I don't know how you figure current.

If I wanted to light 10 LED's how would I figure out what I need?

Also, I've seen people but resistors on the anode side (which makes sense to me, if that's the way current flows) and then I've seen them put them on the cathode side - whats going on here?

Maybe I dont really understand the relationship between voltage, current and resistance.

I'll leave it at this for now. And give you guys a chance to respond if you think you can help me. I know I have a lot (and a little scattered) of questions but I dont really know exactly where to start asking.

MarkT

Well I'll tackle a few points:

Current is deemed to flow from positive to negative through a resistor or other "dissipative load" (meaning
something that converts electricity to another form, be it heat or movement or light).

In reality in metals it is electrons that flow (and they are negatively charged so flow in the opposite direction
from what we call current - a historical accident as "positive" and "negative" charge were defined before
discovery of the electron.

In semiconductors charge carriers can be both electrons and holes, holes are positively charged.

Ohm's law applies to most conductive materials from metals to semiconductors (when just a pure slab of
one kind of semiconductor).  The greater the resistance, less current flows (for a given voltage difference).
The greater the voltage, the more current flows (for a given resistance).

In a battery we have chemical reactions converting chemical energy into electrical energy - in effect a voltage
difference.  The battery has its own "internal" resistance, which limits the maximum current should you short
the outputs of the battery - the more current from the battery the greater the voltage loss across the internal
resistance.
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KirAsh4


Let's say I plug the voltage and ground rails of a breadboard into an arduino's 5 volt and ground pins. Then I add one LED to the breadboard and power it up. *Let's ignore resistors for right now to try and hopfully keep things simple.
Let's say the LED is this: https://www.sparkfun.com/products/531
3.4V forward drop
Max current 20mA

What do those specs mean? 3.4V forward drop. Does that mean if I apply 5V to the anode it will drop to 1.6V out the cathode? So if that were true then if I put a second LED in series it should be less than half as bright, no?

It gets divided equally.  In this case, it's likely they won't light up at all because 3.4V + 3.4V > 5V.  However, you can connect them in parallel and both will be just as bright.


Max current makes sense... That's the max amount of current you can give it before it fries. But again, I don't know how you figure current.

There are some battery manufacturers that disclose the capacity of their batteries, and there are also some conservative numbers that people go by, http://www.techlib.com/reference/batteries.html

However, what you want is the voltage supplied, the voltage drop, and the max current of the LED.  Based on that you calculate the amount of resistance you would need so that you don't blow up the LED.  What the battery can supply in current/hr doesn't matter in this respect.


If I wanted to light 10 LED's how would I figure out what I need?

That would depend on how you plan on having them wired.  In parallel or series?


Also, I've seen people but resistors on the anode side (which makes sense to me, if that's the way current flows) and then I've seen them put them on the cathode side - whats going on here?

Nothing is going on.  The resistor will restrict the amount of current flowing through it, and therefore also through the LED, whether it's placed before or after the LED: the LED is still getting the same (restricted) amount of current flowing through it.  It will make a difference if you have two LEDs and one resistor for both, but you don't need to delve into that right now.

fungus


I get the whole water through a pipe analogy but for some reason it doesn't feel like it's clicked with me yet.

I think my biggest question is where does the current come from? Or how do you find it?


Imagine the pipe is vertical. The taller the pipe, the more pressure it generates to push the water through (get a pipe and try it...!)

Same with voltage. More voltage means more 'pressure' pushing the electrons through, and that means more current.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

majenko

For you rednecks you can also imagine it as driving cattle.

The cowpoke at the back with the bullwhip is the voltage - the harder he whips the faster the cattle run.  But they reach a gully, where the cattle get compressed into a tighter bunch to squeeze through.  This is like resistance, making it harder for the cattle to run.  As the cattle rub up against the walls of the gully they generate heat.  This is power dissipation ;)  The current is the speed at which the cattle run - hence the gully reduces the current as the cattle have to slow down.

If you know how hard the cowpoke is whipping, and how wide the gully is, you can predict how fast the cattle will be able to move.  If you know
how fast your cattle are moving, and you know how wide the gully is, you can tell how hard the cowpoke must be whipping.  Etc.

Ohms law is at the core of most things.  Resistance, current and voltage are all related.  A power source typically has one constant - either voltage or current.  The load on it (resistance) sets what the second parameter on the power supply will be.  If you have a constant voltage supply the resistance will set the current.  If you have a constant current supply the resistance will set the voltage.
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naut


BackWoodsBrewer

Look at some Ohm's Law tutorials on YouTube.  This gentleman has some pretty nice videos explaining things in layman's terms:
http://www.youtube.com/watch?v=Ubj41bNadHE

I keep a printout of different Ohm's Law formulas taped to my workshop wall, I suggest you do the same :D

Henry_Best



Also, I've seen people but resistors on the anode side (which makes sense to me, if that's the way current flows) and then I've seen them put them on the cathode side - whats going on here?


Using the water and pipe analogy, a resistor is equivalent to a constriction in the pipe. It doesn't matter which side of a flow meter the constriction is (before or after), the flow meter will still give the same result (assuming that the pressure doesn't change).

bwoogie

Mmmk. I think I'm starting to get a grasp on it. Basically, you're almost always going to know 2 of the variables to find the 3rd. At least in most "simple arduino projects"
Quote from: KirAsh4


Also, I've seen people but resistors on the anode side (which makes sense to me, if that's the way current flows) and then I've seen them put them on the cathode side - whats going on here?

Nothing is going on.  The resistor will restrict the amount of current flowing through it, and therefore also through the LED, whether it's placed before or after the LED: the LED is still getting the same (restricted) amount of current flowing through it.  It will make a difference if you have two LEDs and one resistor for both, but you don't need to delve into that right now.


Oh. I think that's whats confusing me. I think I have to remember that it's because the circuit has to be complete before anything can happen. So you have to take account for the entire circuit before you can decide on what resistor to use for that one LED. Basically the whole circuit is going to be the same the whole way around right? It doesnt have more current in one spot and less in the other?

Docedison

#9
May 23, 2013, 09:31 am Last Edit: May 23, 2013, 09:43 am by Docedison Reason: 1
No in a series circuit the total current is the voltage divided by the resistance or V/I. The 3 Primary "Laws" or relationships are, R = E/I, I = E/R and E = I * R. There is a similar one for power..
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ryan27968

what i would reccomend you do is go on youtube and search for arduino tutorial. there should be one right near the top by sciguy or something like that. he made a very good arduino tutorial series and in his third episode he explains in detail electrical engineering basics.

fungus


Basically the whole circuit is going to be the same the whole way around right? It doesnt have more current in one spot and less in the other?


Correct.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

majenko


I keep a printout of different Ohm's Law formulas taped to my workshop wall, I suggest you do the same :D

I find this is a very useful one to have up on the wall:
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MisterResistor

The question is a classic! All of the other responders gave you great advice. I'll recommend another. A copy of the 'ARRL Radio Handbook' (any year) has always provided an understandable introduction to basic electronics. Check out Amazon or powells.com and snag a used copy. I've used them for years as a reference.


bwoogie

So I learn best by trial and error. I got out the breadboard and a bunch of LEDs. first I setup up about 12 LEDs and a 1k resistor for each (what I have in mass quantities) and connected them in parallel. gave it some power. they werent as bright as they can be but they lit up. next I tried them in series. I believe I was able to get about 4 to light up. I had one 1k resistor on that circuit. then I decided to live dangerous and dropped LEDs all along the voltage and ground rails. 117  leds give or take. connected a 9v with no resistors and they all lit up nice and bright. I don't know what I learned but I like it.

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