From: Winfield Hill
Subject: Re: Simple adjustable 0 - 400V regulator, with flaw
Date: 18 Jan 2003 10:37:01 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
> Thomas wrote...
>> Winfield Hill wrote:
>>>. see below
>>> ... 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. ...
>> If the switch is flipped, and the pot is in zero position, the
>> 1.5 K resistor gets to see 300 volts, maybe even 470+ volts if
>> the switch is open for a moment.
>> That's 60 watts, and nearly half a joule. Probably too much
>> for whatever little R is there.
> Actually, the 1500-ohm resistor has never minded the short power
> surge. Amazingly, neither did the '5600. However a 5.1V zener
> was placed across this resistor in a design refinement; I should
> have shown it.
>> And the 200 mA will be too much for the pot...
> BINGO, that's part of the answer. If the pot is turned down
> when the switch is thrown, a small part of the pot's resistance
> path has to absorb all the 10uF capacitor's energy. Bamm!
> OK, what's the solution?
Frank and Gibbo both suggested a capacitor to ground at the
ADJ pin, as shown below.
>>>. +480V ___ HIP5600
>>>. ---| |---+---+-----------(O) HV out
>>>. |___| | |
>>>. | | 1.5k
>>>. | | | adjust, 0 - 400V
>>>. '---- | --+-+--o->o--------,
>>>. | | o--, |
>>>. 10uF === | 300V | 500k
>>>. 450V | === fixed | pot
>>>. | C | 380k |
Yes, one hopes to prevent the pin voltage from increasing much
during the time the switch's wiper moves between the contacts.
For example, a 0.1uF will restrict the increase to 40V if the
switch is open for a maximum of 5ms. However, we now know the
failure mode is a blown-out / fried pot, under the condition
it's at a low setting when the switch is thrown. Clearly the
added capacitor doesn't solve this particular problem, although
it may slighty reduce the discharge energy (if the capacitor
is too large, it'll actually increase the discharge energy).
There are in fact two good solutions that I've identified so
far, maybe more, does anyone see them?