From: Winfield Hill
Subject: Re: Simple adjustable 0 - 400V regulator, with flaw
Date: 18 Jan 2003 12:36:19 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
At Sat, 18 Jan 2003 18:49:42 GMT Kevin wrote...
> 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.
> As others have mentioned, the switching from 300V to O.C. to
> variable can smoke, um... burn out, the pot.
Yes, that's the fault. Excellent.
> 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. ...
Very good, that's the basis of the 2nd improved solution I have
in mind. Furthermore, it appears that you beat nospam by about
an hour, he posted at 19:46:31, right? However, he did give a
better answer by naming the exact part I had in mind. Amazing.
It's the Supertex LND150 depletion-mode MOSFET, which with G-S
connected, is a 1k resistor that morphs into a 2-terminal 500V
roughly 1.5mA current sink or source. Perfect for the job.
. +480V ___ HIP5600
. ---| |---+---+-----+-------------(O) HV out
. |___| | \_|_ |
. | | /_\ 1.5k
. | | | | adjust, 0 - 400V
. '---- | --+-----+--(->)--o->o--------,
. | 5.1V LND150 o--, |
. 10uF === zener 300V | 500k
. 450V | fixed | pot
. | 380k |
Can anybody describe the other lower-tech compromise solution
I had in mind? What if the solution budget was one low-cost
part and the LND150 was discontinued / not available?