problem replacing switch with potentiometer

hello.
i have a unit powered by a 12volt .5amp power supply that has a 3 position switch (off/low/high) rated 125VDC / 0.5A i would like to replace with something that worked like a house lighting dimmer switch. (off/variable power) it was suggested i use a potentiometer. the one i installed was rated 500VDC / .5W / 1Megaohm. it provided no power until the last 10 degrees then started smoking and burning.

where did i go wrong?

i checked the equations to determine the Watts and Ohms of the power supply ( I=V/R & P=VI ) and came up with the following rating for the power supply:

if I = .5A and V = 12V then P = 6 Watts and R = 24 ohms

everything seemed within range of the potentiometer except the Watts.
is this what burned it out? and if so, what is the rating for the one i need or can i use another and incorporated an in line resistor or something.

thanks in advance.

jeramie

You can't really just use a resistor, or a variable one, to do this - it provides an extremely poor performance, and would be expensive as well.

Sounds like you started with the pot at 1 meg and as you turned it the resistance droped toward zero. Initially you'd have what looked like a 1,000,024 ohm total resistance so very little current flowed with 12 volts applied. The I 'squared' R loss of the pot was trivial. As the pot was turned and the total series resistance dropped the current increased to a point where the pot couldn't handle it. At about 200 ohms the pot is somewhat past it's rated dissapation - but all of that dissapation is in a very small segment - I don't know how pots behave in that situation.

stevez said:

At about 200 ohms the pot is somewhat past it's rated dissapation - but all of that dissapation is in a very small segment - I don't know how pots behave in that situation.

Click to expand...

It's very simple, the pot will burn out very rapidly (as happened), the wattage rating of the pot is for the entire track - so concentrating more than the total pot is rated on one tiny section will result in distruction of the track section.

If you REALLY wanted to use a pot in this way (and it will give a really poor performance), you need a massive great wire wound one - think the old Frankenstein movies :lol:

When I was at school the lighting for the stage had massive open wirewound light dimmers - probably two feet long, and totally suicidal by todays legislation!.

How to proceed is quite dependent on what the load is and what the present switch arrangement looks like. Does it switch in another tap on a heating element or a diode?