PSU too much current?

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....

Thanks in advance.

Jason

How does the load current have any effect on the trim-pot current?

You better tell us the trim network and values you are using.

For 317, I thought it was conventional to use 120 ohms from OUT to ADJ, and (V/10)K or ~2.3K from ADJ to ground. There may be 20% accumulated errors in chip and resistors, so you want 2.3K +/-20%. That could be 1.8K fixed plus 1K trimmer.

> lm317/337

Pardon me beating the dead horse, but you do recall that the 337's pinout is different from the 317's pinout? There is good thermal reason for this, though sometimes it seems they do it just to confuse us and sell more chips.

Thanks for the response prr, I used 120 ohms from out to adjust, and a 5k trimpot. I get 37v at the output with a 500 ohm load resistor, and am able to adjust down to about 34v before i smell smoke. This happens on both + and -. I have double checked my wiring (it is point to point), and will do so again... I have not tried fixed resistor yet because i want to understand whats happening here.

And thanks for the reminder, but I did check and the 337 wiring carefully.


Thanks alot

[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

Thanks alot paul, I did not understand that the 120ohm resistor from out to adjust terminal was setting the current for the adjust resistor... I think I have a much better understanding of whats going on now...

You might put a trim in the 100 ohm side. That way if it fails open the supply shuts off, not to the highest value. Like paul said, you may need a minimum load this way.