OK, I know this is a little off topic from this thread and I’m not trying to belabor this discussion or nitpick anyone’s comments, but I’ve been grinding my gears about the pot burning out issue. I am not a EE by education so I feel like I’m at a little bit of a disadvantage here sometimes and I have a few questions that might be really basic.
To help me understand the problems with using a pot the way I proposed, I came up with some equations and graphed the results. If I’ve made any errors in any of the steps please let me know. Here’s what I did.
I took the OP’s circuit and replaced it with an equivalent circuit just to make the analysis easier:
Then I wrote the equations for current through the circuit, voltage drop across the pot, power dissipated by the pot, and lastly the power dissipation capability of the pot all with respect to its resistance setting.
From an earlier post I got the impression that the power dissipation capability was linearly proportional to the resistance setting. When the pot is at its full resistance, the power dissipation capability is whatever it’s rated for, when it is at or near zero resistance, its power dissipation capability is at or near zero. Here are the equations:
Next, I plugged the equations into an excel spreadsheet so I could see what was going on. In viewing the graphs the power dissipated by the pot never even gets up to 200 mW but there is a region from about 0 - 11Ω where the pot is set to low resistance that the power being dissipated exceeds its power dissipation capability. That I assume is what adiouguru referring to.
I can see this now and it makes sense. This region exists because the power dissipation capability increases linearly while the power being dissipated increases exponentially. As a result, there will always be a region where the pot resistance setting is low that you have insufficient power dissipation capability.
The higher the power rating of the pot and the lower the initial current, the smaller that region will be but it will always be there. Here is another graph with a line corresponding to the power dissipation capability of a 1/2W pot and one for a 5W pot. It's zoomed in to show the effect but even with the 5W pot the region still exists.
And, if I understand all this correctly, that region will exist anytime you use a pot regardless of the specific circuit, voltages, or currents involved.
So my questions are what’s the breaking point? How do you know when this region will be problematic and when won’t it? I have used pots in many circuit and have never considered this and so far to my knowledge it hasn’t bit me in the you know what and I don’t want it to. How do I know when this effect will be a problem and when it will not?
Thank you in advance for the explainations.