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DISCLAIMER: I am just tinkering with electrical engineering and can barely do the basics yet.

Background

I have done a little work on a breadboard connected to my Raspberry Pi, but today I wanted to put my new battery pack to use. I set up a simple circuit to control the brightness of an LED with a potentiometer and it worked just fine. Then in my usual how-much-can-I-change-before-it-breaks fashion, I removed the resistor to see how bright I could make the LED before it burned out (yes I know it was a dumb idea). I got the LED up until it turned red (it was a green led) and then started smelling something burning. The LED was fine but the potentimeter was what was making the smell. Yes, I unhooked everything as soon as I stared smelling burning.

Question

What did I do? I looked online and didn't find anything about burning out a potentimeter.

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  • \$\begingroup\$ Pots are a dime a dozen. I wouldn't sweat it. Measure the resistance of it and if it's not right , replace it. Which it probably does need to be. \$\endgroup\$ – Tim Spriggs Mar 28 '16 at 22:55
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    \$\begingroup\$ Pots are just adjustable resistors, and, like resistors, have a maximum power rating. If you set the pot near one end, so you only use a small part of the resistance element, the usable power rating will be much less than the pot's advertised rating, as all the power will be dissipated in a small part of the resistance element. \$\endgroup\$ – Peter Bennett Mar 28 '16 at 23:01
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    \$\begingroup\$ "The LED was fine" - No, it likely isn't. It is probably damaged, and may well be dimmer, or fail inexplicably later. \$\endgroup\$ – marcelm Mar 29 '16 at 8:13
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. LED and pot smoker.

You forgot to tell us the voltage of the battery and the value of the pot. I'm going to assume 6 V and 1k.

A typical potentiometer has a power rating of about 1/8 W. That's the amount of power it can safely dissipate along the whole resistance track without overheating. We can now work out the maximum current that can safely be run through the pot. The relationship between power, current and resistance is given by the formula \$ P = I^2R \$ so \$ I_{MAX} = \sqrt {\frac {P}{R}} \$. For a 1k, 0.125 W pot that works out at \$ \sqrt {\frac {0.125}{1000}} = 0.011~A = 11~mA \$.

enter image description here

Figure 2. LED current vs voltage curves. Source: Electronic Tutorials.

Next we turn to the LED. From the graph in Figure 2 we can see that once the LED turns on a small increase in applied voltage results in a large change in current. This is why LEDs are always powered from current limited supplies rather than constant voltage.

What did I do?

You decreased the pot resistance allowing more and more current to flow through the LED. At 20 mA the LED was probably at full brightness and the pot was already at twice its rated current. By the sound of things you kept on going so you probably had 50 to 100 mA in your circuit which is above the rating for both components. Smoke is the typical result.

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  • \$\begingroup\$ I'd give another +1 for "pot smoker" if I could :) \$\endgroup\$ – bitsmack Mar 29 '16 at 0:07
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You need to understand how pots are built. What you have is a long(ish) resistor which is connected at each end to a terminal of some sort. Then you have the actual variable part, called a wiper. This slides along the surface of the resistor, and where it makes contact determines its resistance to the ends. That's why the symbol looks like

schematic

simulate this circuit – Schematic created using CircuitLab

where the arrow is the wiper. The problem with this approach is that the contact area of the wiper is pretty small, so the current density gets pretty high, and the contact area starts to get hot. And in your case, it got catastrophically hot.

Generally, you should not draw more than about 10 mA through a pot, and less is better. It's a mark of marginal engineering to draw more than that out of a regular pot, and I've come across equipment that had knobs with "dead spots". As you turned the knob the circuit would go dead at specific points, and these were burned spots on the resistor where the wiper could not make contact.

There are, in case you're wondering, "pots" that are designed to draw a lot of current through the wiper. They are ordinarily call rheostats, and among other things their large wiper area (plus the fact that they are almost always wire-wound) means that they tend not to be very precise. But they do have their uses.

So, how do you control large currents with an adjustable knob? The best way is to set up a pot with one end at some fixed voltage and the other end at ground or another voltage. Then look at the wiper with an amplifier which has a fairly high input impedance (the input doesn't draw much current), and drive the load with the output of the amplifier.

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You simply put too much current through the pot (and through the LED as well). All components have practical limits on voltage, current, power, etc. An LED is not like an incandescent bulb (like a flashlight bulb). Within the design limits, you can connect an incandescent bulb to a power source and it will self-limit to its nominal power.

But an LED always requires some way to limit the current or the LED will self-destruct. Typically, we use a small series resistor to limit the current. The value of the resistor is calculated from the expected power source and the rating of the LED. For large, high-power LEDs, active current-limiting (or constant-current) power supplies are used.

When you removed the resistor, you eliminated the "safety valve" and allowed a destructive current to flow through the LED (AND the pot) when you adjusted the pot down to a value lower than the resistor you removed.

A potentiometer can handle only so much power/current before it exceeds the design limits and starts burning up or melting. If you could find the manufacturer's specifications for the pot, you would see the power rating among other limits.

Experimenting like this is good learning when done at a small scale where you don't blow anything up or set anything on fire. However, you might want to avoid further experiments like this. Your power source would be the next to die, and that could be catastrophic depending on what you are using.

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