[quote author="JCMaudio"]Hey gys, I have a simple bipolar lm317/337 psu trying to get +/-24v... My transformer is a 24-0-24 @ .5A. It seems that I have too much current, for the rather light load (.050A) that I need out of it, and I am burning up my trimpots trying to set my voltage. Whats the best way to deal with this? Can I simply load down the outputs with appropriate value resistors to draw down the current? Are there drawbacks to this? I feel like I am missing something here....[/quote]
What you're missing, I think, is that the transformer only supplies as much current as the circuit draws. It's not a pump that pushes current willy-nilly into the circuit; think of it instead as a reservoir from which the circuit pulls current. If the circuit pulls only 0.05A and the transformer is rated at 0.5A then the transformer is loafing along. It's overdesigned, but it won't burn out anything that's wired right.
So...what's the adjustment circuit on the LM317? Same on the LM337? Typically there's a 120 ohm resistor from the output to the adjust terminal, which sets the current through the adjust resistor at 10mA. (That means, by the way, that you're really drawing 0.06A from the supply -- 0.05A for the circuit itself, and 0.01 for the regulator. But I digress; the transformer is still loafing.) If the adjust resistor has 10mA flowing through it, and you're wanting to get an output voltage of 24V...the adjust terminal should be at 22.8V, which means that the resistor should be 22.8V / 0.01A = 2280 ohms. And it will be dissipating 22.8V x 0.01A = 0.228W, just under a quarter watt. If you're using quarter-watt trimpots you're really pushing your luck. I'd use 1W.
Better still, I'd use a larger resistor between output and adjust. If you use 1.2k, then you're only running 1mA through the adjust resistor. 22.8V / .001A = 22800 ohms (so you'd use a trimpot 10x as big), and 22.8V x .001A = .0228W, which is a lot easier on the trimpot.
Why do people usually use the smaller resistors which create a 10mA load? Because these regulators don't work so well with a total current draw of less than 10mA, so you make sure they're always getting that minimum preload by making it run through the adjust resistor. But if your actual audio circuit will always be connected to the regulators, and its idle current is 50mA (0.05A) and it never dips below 10mA under any conceivable circumstances, that's all the preloading you need, and you can use the 1.2k resistor between output and adjust with impunity.
Peace,
Paul