Re: gEDA-user: Dropping a few volts

[John Doty <jpd@xxxxxxxxxxxxx>]

On Oct 12, 2007, at 10:58 AM, Randall Nortman wrote:

> 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.

Well, the advantage of the HV version is that there's a big region of  
transient/fault space you don't have to worry about analyzing.

>
>> 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?)

Zeners respond a lot faster than any ordinary power supply comes up.  
And yes, with analysis, you can control the supply rise to keep Vin- 
Vout in bounds. Need to understand the load dynamics and the ability  
of the chip to provide the current to bring the load up in timely  
fashion if you want to try that.

And then there are fault situations: can an overload or short drag  
the output down enough to put overvoltage across the regulator? The  
built-in overload protection doesn't work in that case. This could  
blow my suggested zener bridge, too. Is such a fault a credible  
hazard in your application? Or do you just say, OK, that's unlikely,  
I'll take my chances? I don't know the answer: have to draw that line  
somewhere.

You can duck these tricky issues with the HV regulator.

John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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