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Basic tips for using rheostats/pots?

M

MasterOfTheHat

Weaksauce
Joined
Apr 21, 2006
Messages
66
Ok, I've searched this forum and found some posts on the subject, but I don't think I found anything simple enough for me to grasp... :)

Just because I want to play and because it's sort of useful, I want to build a LED light bar, or combo of LED clusters, or whatever to light up my keyboard at night. I built a really simple baybus using LEDs, resistors, and SPST switches, so I could wire up the LED bar for basic on/off operation. But I want a dimmer in the circuit, too. All that said, I have two questions:
1) What is the basic circuit for using a rheostat?
2) How do I calculate what size rheostat I need for the application?

As I quasi-understand it, the basic layout would be something like:
source -> switch -> rheostat -> resistor -> load
where the resistor is the size needed for the load if the switch and rheostat were not in the picture. Is that correct? This is what I have in mind:
RheostatLEDs.jpg


And as far as calculating the size pot i neeed, I'm completely lost...
 
Have you checked out this link?

Rheostats work well for low power circuits, but once you get upwards of a couple led's it's usually better to start looking for another solution like variable regulators or pwm. Rheostats with larger power capabilites are available, but they tend to be large and expensive.

[edit]

The above links discuss fans, but any other circuit load such as led's can be substitued. You'll just need to change current values to whatever you need for the led's.
 
I'll second the suggestion for a non-potentiometer solution. There are lots of ways to do it, from a linear voltage regulator to a PWM circuit. Using a linear regulator is probably the simplest way to do it.
 
Heh, well, so much for the really simple stuff... :)

I've done some looking into variable regulators and am still trying to grasp the concept. I haven't looked into PWM yet, but I will. Why is one better than the other? Oh, and which is going to be cheaper/easier to implement?

EDIT: Also, at first glance it looks like PWM isn't a constant power source; it goes up and down. Wouldn't that cause the LEDs to 'blink'? With the fans, you have inertia to keep them going and you wouldn't notice the drops, but with a light, I would think you would see every one.
 
Adjustable regulators are easier to implement by a fair margin. However, the regulator wil be dissipating a not-insignificant amount of power, so you need to make sure you keep that in mind. PWM is more complex, but more efficient.
 
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