My guess is they have a few secondary windings from the mains transformer
that get rectified to a few different DC voltages, and going between output
ranges switches between the input signals. That way, you the output
regulator doesn't have to deal with dissipating I x 29V when you want a 1V
output, but instead just up to the next stage.
- Marcel

On Wed, Jul 16, 2008 at 5:47 PM, Jinx <joecolquitt@clear.net.nz> wrote:

> > I've had two commercial "lab" linear power supplies, and they
> > both seem to engage relays (audible clicks) at set voltage points
> > along their range.  The 50V supply that's working now clicks at
> > about 15 and 30V...
>
> What do you think they would be switching ?
>
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