From: Spehro Pefhany
Subject: Re: Simple adjustable 0 - 400V regulator, with flaw
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Date: Sat, 18 Jan 2003 20:09:11 GMT
NNTP-Posting-Date: Sat, 18 Jan 2003 15:09:11 EST
On 18 Jan 2003 09:23:24 -0800, the renowned Winfield Hill
>> On 18 Jan 2003 the renowned Winfield Hill wrote:
>>> The switch allows one to select a fixed 300V output, or to
>>> adjust the voltage to any value between 1.2 and 400 volts.
>>> But this circuit has one potential problem that can cause
>>> a component failure. As a design quiz, tell us what the
>>> problem is, and find a solution that doesn't significantly
>>> affect any of the circuit's functionality. Hint, the '5600
>>> isn't the vulnerable part.
>> A couple of 1N5281Bs in series.
> Couple of 200V zeners? Where? To insure zero output?
I was thinking from ADJ to gnd to prevent overvoltage, but
you say that's not the problem.
> ROFLOL. At least it'll pass the smoke test!
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.
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.
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.
BTW, that part is marked "obsolete" or missing from Harris and
Intersil sites, from what I could see, does anyone still make it?
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