From: Kevin McMurtrie
Subject: Re: Simple adjustable 0 - 400V regulator, with flaw
User-Agent: MT-NewsWatcher/3.3b1 (PPC Mac OS X)
Date: Sat, 18 Jan 2003 18:49:42 GMT
NNTP-Posting-Date: Sat, 18 Jan 2003 10:49:42 PST
In article ,
Winfield Hill wrote:
> The Intersil HIP5600 is an adjustable three-terminal regulator
> that can be thought of as a 500V version of the LM317. I've
> used it in many of my lab instrument designs and found it to
> be reliable and, like the LM317, an elegant design simplifier.
> For example, here's a simple adjustable 0 to 400V regulator,
> with a fixed-voltage feature.
>. +480V ___ HIP5600
>. ---| |---+---+-----------(O) HV out
>. |___| | |
>. | | 1.5k
>. | | | adjust, 0 - 400V
>. '---- | --+----o->o--------,
>. | o--, |
>. 10uF === 300V | 500k
>. 450V | fixed | pot
>. | 380k |
> 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.
> - Win
As others have mentioned, the switching from 300V to O.C. to variable
can smoke, um... burn out, the pot.
You've made a 0.8mA (1.2V / 1500 Ohm) constant current source. You
could put a constant current sink between the pot and the ground that
sinks a bit more current but doesn't burn out the pot when the output is
higher than it should be. (I hope that's a 1/2W pot.) It costs a few
mV in output stability but just tapping the pot knob probably causes 5V