Re: gEDA-user: Dropping a few volts

[Randall Nortman <geda-lists@xxxxxxxxxxxxxxx>]

On Fri, Oct 12, 2007 at 09:48:12AM -0600, John Doty wrote:
> 
> On Oct 12, 2007, at 7:20 AM, Randall Nortman wrote:
> 
> > So I have a 42VDC supply that I want to feed into a linear regulator.
> > But almost all linear regulators want 40V absolute maximum, and the
> > ones that have higher maximums are not reliably stocked anywhere.
> 
> What's wrong with an LM317HV?

Nothing at all.  Great minds think alike, even if some of them are a
bit slow -- I received your response while composing my previous
message, in which I came to exactly the same conclusion.  (And it only
took me all morning to get there!  Oh wait, I went to bed thinking
about this problem -- how pitiful is that?)  I don't even think I need
the HV version, since my Vin-Vout is quite a bit below 40V.

> Another trick is to notice that the three terminal  
> adjustable types aren't grounded: what matters in input-output  
> differential. So, limit that with something like a 33V zener between  
> input and output (to handle power on), and all will be fine.

Oh, now there's an important tip.  I hadn't thought about the startup
voltages.  Can I get away with putting a low-value resistor in front
of the input capacitor, which would limit inrush current and cause Vin
to ramp up slowly when power is connected?  That might be cheaper and
smaller than the zener.  Plus, even zeners have a non-zero response
time -- might the regulator be damaged during those microseconds?
(nanoseconds?)

-- 
Randall


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