Electronics theory question about RESISTANCE

[Rodgre]

Even though I learned this stuff 20 years ago in school, I can't seem to wrap my head around the concept of RESISTANCE and it's relationship to VOLTAGE and CURRENT.

In the simple equation of trying to figure out what size resistor to put in series with an LED to get it down to a safe voltage. Of course this is the kind of rabbit-hole thinking that will drive a person crazy, but this resistor is technically a "current limiter" in this circuit, is it not? How does that relate to the fact that I have a 9vdc source and an LED that only wants to see about 3vdc?

Am I wrong in thinking that it's not as simple as "this resistor will convert that voltage to this voltage"? 

Am I wrong in thinking that the resistance is limiting the current that the LED is able to draw? I thought that circuits draw what they need, and no more. It's never a problem to have too much current AVAILABLE to a circuit, whereas too little will be unstable.

Looking at the role of a volume pot in a guitar, isn't this variable resistor in fact reducing VOLTAGE, and not current? That is to say, the pickup is putting out such-and-such a voltage, and the volume pot makes this smaller (lower volume=lower amplitude=less voltage swing).

I realize that this is a Basic Electronics 101 question, but somehow I've been able to go through life without fully understanding this concept. Silly me.

Roger

[Roobin]

In the simple equation of trying to figure out what size resistor to put in series with an LED to get it down to a safe voltage. Of course this is the kind of rabbit-hole thinking that will drive a person crazy, but this resistor is technically a "current limiter" in this circuit, is it not? How does that relate to the fact that I have a 9vdc source and an LED that only wants to see about 3vdc?

Am I wrong in thinking that it's not as simple as "this resistor will convert that voltage to this voltage"? 

Am I wrong in thinking that the resistance is limiting the current that the LED is able to draw? I thought that circuits draw what they need, and no more. It's never a problem to have too much current AVAILABLE to a circuit, whereas too little will be unstable.

You are correct about the LED example. In such a circuit, assuming the cathode of the LED is tied to ground, ie 0v, then there are several constants - ie things which do not change. 1) the battery voltage 2) the voltage across the LED 3) the resistance of the resistor.

The voltage of the battery is, say, 9v. The LED forward voltage is, say, 2 v. The resistance is, say, R ohms. If the LED and resistor are in series, then there is a voltage across the resistor, since there is a current flowing with a resistance. The resistor converts current to voltage. (If the resistance or current were 0, then there would be no voltage across the resistor.)  The resistor determines the current flow in the circuit (the current is the same at all places, Kirchoff's Law). This current is given by (9-2)/R. The circuit has drawn what it needs - in fact this is the maximum current that can 'flow' through such a circuit. The only way more current can flow is by reducing the resistance of the resistor. If the resistance were 0, then the current flowing is far higher, thus the LED blows.

In this instance, one could think of the current draw as pulling a piece of string along the ground - the string will only go as fast as you pull it. Sure, the tail could go faster, but it'll just bump into the front bit and have to slow down. So overall the string moves at the same pace. How do we increase this pace (current)? Use a smoother surface (lower resistance) or by pulling harder (greater voltage).

About your second question, Mr R G Keen does a far better job explaining voltage dividers with respect to current/voltage than I ever could: http://geofex.com/circuits/Biasnet.htm

[OnLyTNT]

All electronic components have a DC resistance. So, if we get a led which operates at 3 V and need 25 mA current, that means this led has 3/0.025 = 120 Ohm resistance.

If we run such a circuit that contains  a resistance and a led in series with 9 V supply, we have to limit overall current to 25 mA to protect the led. Also, in serial circuits overall current is fixed for all components.  In this case we need a proper resistance value which allows 25 mA to go through it self. Another important point in serial circuits is that overall voltage is divided over the circuit components.

We need to know resistance voltage. We calculate that by subtracting led voltage from supply voltage. 9-3 = 6 v

Now we have resistance voltage and current . We can calculate resistance value: 6/0.025 = 240 Ohm.

But probably you wont be able to find such resistor in the market. That' s because resistor values are fixed to mostly used ones. Best option to get right resistor for your circuit is to get bigger but nearest value like 270 Ohm.

Potentiometers behave like two resistors connected in series. That means they operate like voltage dividers. You get what you need by adjusting resistor values so you get a dividing ratio. And the overall current on two resistors never changes.

I hope you will get what you need in my reply and sorry for crappy English.

[ayayay!]

Oh are you kidding?  It's fun to remember this stuff and see if you (meaning I) can still explain it without sounding like a moron!

My approach has always been to segment out each little tiny circuit, then look for a) how much voltage is coming in b) how much potential/current (I like to think of it as "damage") it will provide and c) if the resistance in the next component/combination will keep it from going "ka-BOOM."   I always have to write it out on paper and do the long math, but Ohm's law is my advocate! 

It may oversimplify things, but it sure has kept me from destroying things so far! 

A guy like you, Roger, who has a studio is obviously thinking beyond just pedals because of things like impedence and mismatching in cabs, mic cables, inputs, etc. Right?      So I think it's a good topic. 

[R.G.]

This thread really started my day right.   
One of the things I get back out of participating on line is seeing people grow up (in the EE sense) and understand the electronics we play with. Getting three out of three coherent replies to Rodgre's question is a real kick! In fact, having Rodgre actually thinking about the nature of resistance itself is a kick.

He's in good company. Back when people were first experimenting with the lightning-magic, it was very difficult not only to figure it out from nothing, but to make good measurements. It's not widely appreciated, but humans knew more about calculus than we did electricity in Georg Ohm's day. There was actually a debate going among the inventor/researchers/discoverers of scientifica in Ohm's day about resistance. It was known that resistance related voltage and current, but the nature of the relationship was the matter of some debate. Most of the electronics intellegentsia (remember, these were guys who figured out electricity and radio waves without meters or oscilloscopes!) thought that the true relationship for a resistor was something like
V = R* I ^x +k
where x is the power current was to be raised to and k was a constant of some kind. They believed that X was something like 1.2  and had various values of k. Ohm's position was that x=1, and he was roundly criticized for thinking it was so simple. I for one am incredibly grateful that he was right!

Rodgre, a resistor does take just whatever current it needs from a voltage. It's just that we know exactly how much that is: it's always I = V/R. What confused you was that, as roobin and OnlyTNT pointed out, using ayayay!'s method of subsetting down to a tiny particle of a circuit, the LED demands a constant(ish) voltage. The LED is not a resistor (although, as OnlyTNT notes, we can compute an "equivalent resistance" at these conditions) so it demands all the current in the world to keep its voltage constant. This subtracts the LED voltage from the available battery voltage, and the resistor does what resistors always do, I=V/R, and that's all it needs, and all it lets flow.

A resistor is simple minded. Mother Nature has told it to do V=I*R no matter what. If you force a current through it, it demands that  you supply a voltage sufficient to cause that current or it will not play. If you apply a voltage across it, then the current through it is fixed by its nature.

Good one guys!!

[Rodgre]

You guys are better than my 4 years of electronics shop in high school!

To think they were trying to teach us how to fix TVs. Who fixes TVs nowadays? Puh-leeeze.

Thanks all!

Roger

[drewl]

You guys are better than my 4 years of electronics shop in high school!

To think they were trying to teach us how to fix TVs. Who fixes TVs nowadays? Puh-leeeze.

Thanks all!

Roger

I do.
My 4 yr old likes to bang on them and I don't feel like shelling out several hundered dollars every year or so.

They usually just need the pc boards reflowed (like amps and pedals) mainly due to heat failing solder joints.
See the resistance increases with a bad connection..........