lm338 regulator programming resistors

You could place a resistor in parallel with the 5k pot to bring it down to 2.7k too. Always more than one solution.

The 120 Ohm set resistor has the advantage of soaking up the bias current from the regulator. You can up that, provided you have a minimum load for regulation. Add an output LED or something to eat the bias or quiescent current.

Grant.

I thought about that, but I ran the numbers through a parallel resistance calculator, and it looks like it messed up the linearity of the pot. Possibly not bad enough to make it unusable for a power supply, so that may be an option.

Thanks for the tip on the LED; that might just do it.

If you add an op amp, you can make the resistances anything convenient. Alternatively, if you don't mind a bit of nonlinearity, you can load down the pot with a 5.90k resistor in parallel. Or, of course, use a

Cheers,

Phil Hobbs

Or 91 + 1800 in E24 series :)

Grant.

You're no fun anymore. ;)

Cheers

Phil Hobbs

So, replace the 120 ohm resistor with a smaller one to make the 2k pot work. The pot is unlikely to be high precision, you'll want to wire it up then use a resistor-substitution box in parallel with the 120 ohm starting-point resistor to find the best R value.

Change r1 to 82 ohms - the only downside is a slightly higher biasing current - a milliamp or so, which is totally irrelevant.

A serious question. You didn't suggest changing R1. Is there a consideration I am missing?

This approach has the disadvantage that as you rotate the pot you they will often have momentary disconnects that will result in the max voltage (>30V in your case) at the output.

It is much better to have an arrangement where the wiper is not carrying significant current and where any disconnects will cause the output to fall to a low voltage.

I would do it with the pot being used as a divider from a reference and feeding the non-inv input of an opamp whose output feeds the LM338 adj pin.

Feedback from the output to the inv input of the opamp through a fixed divider will give the appropriate voltage range (together with a compensation capacitor). The LM338 will then act as a protected output stage (overcurrent and thermal protection). A high value resistor and a medium value cap from the non-inv opamp input will ensure that any disconnects as the pot rotates will not cause any disturbances at the output and if they do occur they will be in a safe direction.

kevin

R1 is from output to feedback--change it to 86.6 ohms and use a 2k pot. It'll waste a bit more power, but it'll give you the right range.

Cheers

Phil Hobbs

You could put a cap from adj to ground to mask that? I've not noticed the open wiper effect, but I'm usually running fixed divider or a restricted, say 5 or 10%, adjustment range.

Add a small negative supply and you get adjustment down to 0V too.

Grant.