I connected a potentiometer to my DC power supply (max 1A, 12V), and then connected a motor to the variable voltage side of potentiometer as well.

The potentiometer is just 1K.

When I tried using some China-made motor (RS-545) there appeared sparks and red light and subsequently burning smell coming from potentiometer as it near the 12V range. Why is it so?

Is it caused by back EMF?

  • \$\begingroup\$ Is it a big beefy pot or something small? (part number?) My guess is that you are putting too much current through the wiper of the pot. At the high end is the motor taking the full 1 amp of the supply? \$\endgroup\$ – George Herold Aug 22 '14 at 17:48
  • \$\begingroup\$ You might want to use a rheostat instead. Related: electronics.stackexchange.com/q/11048/2028 \$\endgroup\$ – JYelton Aug 22 '14 at 18:05

If the pot is rated at 1 watt (end to end of course) it will be rated at 0.1 watts for 10 % of its travel (100 ohms) or 0.01 watts at 1% of its travel (10 ohms).

So, say you have the wiper positioned to provide 10 ohms and this is feeding a stalled motor from a 12 volt supply, there might be up to 1 amp trying to flow thru the 10 ohm part of the pot.

This is a power dissipation (in that 10 ohm part of your pot) of 10 watts and if the pot was only generally rated at 1 watt (end-to-end) it will fry (in an instant) over that small section of the pot that was producing 10 ohms between wiper and one end.

Think about it another way - it's a 1k pot and say it's rated at a sizable 10 watts - power = current-squared multiplied by resistance - so : -

10 watts = \$I^2\times 1000\$ or \$I^2 = 0.01\$ or \$I = 100mA\$

Any current greater than 100mA thru a 1k 10 watt pot will exceed its rating. If it's a 1 watt pot then any current thru the 1k pot greater than 31.6 mA is exceeding its rating.

  • \$\begingroup\$ +1, Dang I never thought of that before. (I almost always buffer pots w/opamps.) \$\endgroup\$ – George Herold Aug 23 '14 at 0:17
  • \$\begingroup\$ this is great knowledge, thanks for the elaborate explanation. \$\endgroup\$ – Peter Teoh Aug 24 '14 at 15:51

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