I have a 10k poti. I use it as a variable resistor by soldering two of the three terminals. It is connected to a 1/4 watt 220 ohm resistor in series(this is the load). The voltage across this circuit is 12V DC.
Is the poti safe even it is set to zero ohm? Would 220 ohm prevent it to burn in this case? If so, what should be the quantative method to calculate the minimum value of series resistor which would prevent this poti?
Answer
If you know the power rating for your potentiometer, you can start with that as a guide as to how much current you can pass through it. Little trimpots adjusted by screwdriver or mini ones that you turn with your fingers are often 0.5 watt, but may not be so you'd best check.
The power rating for a potentiometer tells you how much power it is rated to dissipate if the current passes along the entire resistance. For example, say the power rating is 0.5 Watts for your 10k pot. Then, the current that would flow to generate that power:
$$ P = I^2R $$ Rearranging... $$ I = \sqrt{P/R} = \sqrt{0.5 / 10000} = 0.0071 A $$
This is the most current that should be allowed to flow through the 10k 0.5 watt potentiometer at any position.
Let's go to ohm's law and see what series resistor can prevent that:
$$ R = V/I = 12 / 0.0071 = 1690 \Omega $$
Substitute the actual power of your potentiometer, and maybe use one a bit bigger to be safe, and note that it could heat up a fair amount if used at the rated power.
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