Panel Mount Voltage Regulator / Knob

[Robin Hewitt]

I'd still go with the LM338T, it might run a tadge warm but it's so cheap and easy. Heck, just PM me a mailing address and I'll send you a freebie

[irving2008]

I'd still go with the LM338T, it might run a tadge warm but it's so cheap and easy. Heck, just PM me a mailing address and I'll send you a freebie

Robin, it'll run rather more than warm. On a decent 4degC/W heatsink the dissipation limit is 12.5W (at 25degC ambient) so at 20v input and 5V output the allowable output current before it goes into thermal shutdown is less than 800mA, rising to 2.5A at 15v output. Maximum output would be 18.8v at 5A.

Also there's no current limit, only short circuit protection, again not meeting expectations...

I'm not saying that you couldn't use this, and its cheap and simple, but Lee will have to downgrade his expectations and thats not making best use of his 160VA transformer when a 50VA would do.

regards,
Irving...

[Jeremy]

re: using an LM338.

I guess it depends as to whether or not current limiting is really needed and whether or not there is a need to supply a constant high current.

The power supply in that pic I posted is a good example. It uses a 5A linear voltage regulator IC rather similar to the LM 338, but only needs to deliver full current for short periods when charging up the 1.445 F, 20V capacitor bank. The same might be the case when driving steppers with PWM chopper control, for example, where the average current might actually be pretty low much of the time.

Jeremy

[irving2008]

I agree Jeremy. The point is that Lee wants a bench supply with a specific capability. If I was making a fixed supply then the LM338 is an ideal candidate. IF you want full output current to a given spec over a wide voltage range then linear regulators arent going to cut it. All depends on whats required. If Lee wants to reduce his expectations to 0 - 20v at .5A max guaranteed then the LM338 will do the job without overheating across the range. If he wants >1A output guaranteed then the LM338 isnt the way to go.... as the heatsink will be more expensive than the rest of it all together....

There is a secondary issue - is the goal just to produce something that'll 'just about do' or also to learn more about how to design a proper supply?

Irving...