In message , Ian
Liston-Smith writes

I'm looking to build a 3A 34V regulated linear power supply. The LM338
adjustable regulator looks just what I need, but it's upper limit is
32V.

Can a zenner be placed in the common lead somewhere as can be done to
fixed regulators? I've downloaded the spec sheet, but it doesn't give
specific examples of how it may be done, although says "...several
hundred volts can be regulated..."

Are there other freely available regulators that will do what I want?

Many thanks,

Ian.

I've just scanned through the data sheet
(http://www.national.com/pf/LM/LM338.html).

I can't immediately see what limits the voltage to 32V. It seems to need
a minimum of about 4V across it, and the lowest output is 1.2V. However,
there doesn't seem to be any way it knows where 0V is.

I suspect that you can do as you suggest, ie jack up the common lead to
34V minus the 1.2V reference. You will need 38V input minimum.

Maybe it potentially dies on switch-on, because the output will
momentarily be at 0V (especially if you have any capacitor between the
output and 0V). However, you'll probably get away with it! Don't forget
to connect a diode back from output to input (to kill the reverse
voltage when you switch off).

I made a 1A PSU using 7805, with the common terminal jacked with zeners
(switched in as required). It goes from 5 to 24V. I've had it for about
20 years.

Ian.


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