From: Winfield Hill
Subject: Re: Simple adjustable 0 - 400V regulator, with flaw
Date: 18 Jan 2003 13:35:15 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
> I think you've got two more problems with this (maybe a
> candidate for your next bad circuits section)-the pot is
> one, but the absolute maximum voltage from OUTPUT to ADJ
> is +/-5V. If that switch slams ADJ to ground it could
> see hundreds of volts.
> The 1K5 is probably never seeing much voltage because
> something in the regulator is breaking down, and that
> virtually unlimited current is helping to fry that
> little pot element end.
All true, and something I didn't consider at first. As it
happens the '5600 seems OK with this mistreatment, but as
soon as I noticed I added a 5V zener acorss the terminals.
However, I think a 1/4-watt resistor can handle all the
discharge energy, this is an issue worth exploring.
> If that's right, then the obvious mechanical solution of
> using a 3-position toggle with the middle connection grounded
> is not a good one.
It's OK with the zener, but a current source is better.
> Maybe an N-channel 500V mosfet from the pot element to ground,
> and a few volts on the gate. When the switch is switched,
> it can go into constant current.. and a low voltage zener
> from OUTPUT to ADJ. Could probably use a depletion mode FET
> more simply, but I don't like oddball parts much.
Well, there's one part so good that we should start using it
and help keep it alive, the LND150 mentioned elsewhere in this
> BTW, that part is marked "obsolete" or missing from Harris
> and Intersil sites, from what I could see, does anyone still
> make it?
No sadly it's gone, Intersil / Harris recently killed their
entire 600V fab line all at once, from what I can tell. This
part is so very useful I purchased a lifetime supply. :>)
But I think it's still available from a surplus distributor.