problem replacing switch with potentiometer

[jeramiez]

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

 

[Nigel Goodwin]

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.

 

[stevez]

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.

 

[Nigel Goodwin]

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.

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!.

 

[stevez]

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?