This is my first DIY wiring project, I'm taking an RGB LED and using four potentiometers to control (1) overall brightness, and (3) each color level. Wanted to make it simple.

For my purposes, all it seems I just need to use them all as variable resistors. But there are a couple thing's I don't get which I'm sure are obvious to most, but so incredibly basic it's basically assumed I know. Like it would be insulting for an article on resistors to say what I'm looking for. So these questions may seem dumb and I apologize for that.

  1. If I have a 10K resistor and a hot wire (construction lingo) with less than some amount called f(10K) (that can be found using a function I don't know yet), nothing will pass through right? Or does a relatively small amount pass through, but never zero?

  2. I can't figure out what the difference between a voltage divider and variable resistor is.. kind of. Mostly what I don't get is whether or not the resistance between far left and right is ever less than full (in this case 10K). And if the resistance is always the full 10K, then it seems like the outer one would never change (with knob turn), while the center one would. Which is just the exact same thing as a variable resistor, but with a wire coming out the other side that has 10K resistance on it. Or maybe the divider configuration changes both, but then it would be like two Variable Resistors that just share one knob. Or maybe they could be used like an amplifier, but no.. the knob does the variable resistance. Point being, I don't get what a divider does, or how, or what for.

  • \$\begingroup\$ welcome to electronic SE. It is ok, we all start from some where. RGB LED essentially is 3 LED's. To get you start, do you have a basic LED. It will be easy that way. Also what type of variable resistor do have. Tell your part list so we can draw a circuit for you. \$\endgroup\$ – Mahendra Gunawardena Sep 26 '15 at 18:49
  • \$\begingroup\$ Just to hopefully eliminate some confusion. A potentiometer can be used as a variable resistor, the extra leg lets you use it as a variable voltage divider as well \$\endgroup\$ – crasic Sep 26 '15 at 18:57
  • \$\begingroup\$ A potentiometer can be viewed as two variable resistors connected in series, and arranged so that the resistance of one resistor decreases and the other increases as the shaft turns, with the total resistance remaining constant. \$\endgroup\$ – Peter Bennett Sep 26 '15 at 19:00
  • \$\begingroup\$ So what's the difference between a variable resistor and a voltage divider? They keep seeming more and more like the same thing. \$\endgroup\$ – Seph Reed Sep 27 '15 at 6:23

1) The current through the resistor will never go to zero as long as there's a voltage across it, that current being defined by Ohm's law as:

$$ I =\frac{E}{R}\text { ,}$$

where I is the current in amperes, E is the voltage in volts, and R is the resistance in ohms.

2) A voltage divider is, in its simplest form, two resistors in series, as shown below, where the voltage at their junction is described by:

$$E2 = \frac{(E1-E3)\times R2}{R1 + R2} +E3 \text { volts} $$

enter image description here

There are, basically, two kinds of variable resistors, rheostats and potentiometers, the difference between them being that a rheostat is used as a two-terminal device and a potentiometer is used as a three terminal device.

Variable resistors are made so that their resistive elements can be traversed, from one end to the other, by a sliding contact, and the arrowhead on the slider (or "common" terminal) means that the slider can contact the resistive element at arbitrary locations along its resistive track.

In the following example, a comparison is made between a fixed resistor an equal valued rheostat.

Notice that the fixed resistor has a resistance of 10k ohms between its terminals, and that no provision has been made to change that resistance.

A rheostat with the same total resistance, however, has a conductive slider which contacts the resistive element and traverses its length, and that causes the resistance between the rheostat's terminal 1 and terminal 2 to vary from close to zero ohms to close to the element's maximum resistance. In the drawing, with the slider completely off of the top end of the element it'll be connected directly to terminal 1, with the result that the resistance from terminal 1 to terminal 2 will be very low.

Then, with the slider moved to the midpoint of the element, the resistance between terminals 1 and 2 will be half the total resistance of the 10 000 ohm element, 5000 ohms.

Finally, with one end of the element connected to pin 1 and the other end touched by the slider, the resistance between terminal 1 and terminal 2 will be 10 000 ohms.

enter image description here

Rheostats are two-terminal devices, but they're usually equipped with three terminals in order to allow their resistance to increase/decrease when the shaft is rotated in a clockwise or counter-clockwise direction.

enter image description here

For example, if terminals 1 and 2 of the rheostat shown above are used as the connections to external circuitry, the resistance between them will decrease as the shaft is rotated clockwise when viewed from the knob end of the shaft.

If terminals 2 and 3 are used, the resistance will increase as the shaft is rotated clockwise.

Schematically, the symbology looks like this:

enter image description here

A potentiometer can also be used as a rheostat, but when it's used as a potentiometer a voltage is impressed across terminals 1 and 3, and a voltage is taken from terminal 2 vhich will vary depending on the wiper's position.

Shown below is a potentiometer with 10 volts on terminal 1, ground (0 volts) on terminal 3, and the voltage developed at the wiper on terminal 2, referenced to ground.

The leftmost instance shows the wiper 100% clockwise, the center instance with the wiper at the element's midpoint, and the rightmost instance with the wiper 100% counter-clockwise.

Notice that on the leftmost and rightmost instances the slider is essentially connected first to the 10 volt supply and then to ground, forcing terminal 2 to 10 volts and zero volts, respectively.

enter image description here

With the wiper sitting at the midpoint of the element, however, there will be 5000 ohms above the wiper and 5000 ohms below the wiper, forming a voltage divider:

enter image description here

Knowing E1, E3, R1, and R2, we can solve for E2, as shown earlier, like this:

$$V2 = \frac{(V1-V3)\times R2}{R1 + R2} +V3 \text { volts} = \frac{(10V-0V)\times 5000 \Omega}{5000 \Omega + 5000 \Omega} +0V = 5 \text {volts} $$

Since there's zero volts at one end of the wiper, 10 volts on the other, and 5 volts when the wiper is midway between the ends of the element, there must be a voltage gradient along the element, from one end to the other.

Therefore, since the slider can be positioned anywhere along the element by rotating the shaft, any voltage between zero volts and 10 volts (in this instance) can be taken from terminal 2.

  • \$\begingroup\$ Good so far. This all makes sense. I just can't figure out what would ever be the good of attaching a third wire. Or how variable resistors and voltage dividers are different. \$\endgroup\$ – Seph Reed Sep 27 '15 at 6:22
  • \$\begingroup\$ I'll edit my answer to try and clear up any confusion. \$\endgroup\$ – EM Fields Sep 30 '15 at 21:04

A potentiometer has 3 terminals. Let's call them A, B, and Wiper. They are constructed like this.

There are two resistance values of interest: Ra is the resistance between terminal A and the wiper. Rb is the resistance between terminal B and the wiper. The resistance between A and B is always the same no matter where the knob is.

When the knob is all the way toward one side, Ra is zero and Rb is the full resistance of the potentiometer. When the knob is all the way to the other side, Rb is zero and Ra is the full resistance of the potentiometer.

The reason they are arranged this way is so that Ra and Rb can be used to form a voltage divider whose ratio changes linearly with the position of the knob. "Voltage divider" refers to the circuit arrangement of two resistors. "Potentiometer" refers to the physical component.

I recommend you get a potentiometer and an ohmmeter and play around. It will become clear.


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