Re: gEDA-user: Dropping a few volts

[John Doty <jpd@xxxxxxxxxxxxx>]

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?

http://search.digikey.com/scripts/DkSearch/dksus.dll? 
Cat=2556290;keywords=lm317hv

>   So
> I need to drop that down by at least 5V or so.  The good news is that
> I'm only pulling around 70mA on this, so whatever drops it doesn't
> have to dissipate much power.  It occured to me to use a normal shunt
> zener, but that will end up dissipating extra power through the zener,
> which is just wasteful.  It also occured to me to put a bunch of
> diodes in series and use their Vf drops, but that would require quite
> a few diodes.  So why not use a 5V zener (reverse biased) in *series*
> with the load?  It will never dissipate extra power, only what is
> required by the load.
>
> Except then it occured to me that zeners have some sort of minimum
> current to keep them stable, which seems to be around 0.25mA.  What
> would happen if my load started drawing <1mA, or was just disconnected
> entirely (leaving only the quiescent current of the linear regulator)?
> I *think*, from looking at the V vs I charts, that the zener would
> start leaking current with very little voltage drop, thereby
> subjecting the linear regulator to higher than its rated voltage.
> Though this would be only at a very small current, so maybe it
> wouldn't hurt?  I dunno, but I suspect that operating it in that range
> 24/7 for a while would do some damage no matter what.
>
> So the idea I had is to put a resistor in parallel with the load sized
> to draw the zener's minimum current in all cases.  That gets me back
> to wasting current -- but only ~0.25mA, which is orders of magnitude
> less than I'd waste in a shunt arrangement.  (That works out to ~9mW).
> In terms of total power dissipation, this would be roughly the same as
> a zener plus an emitter follower (zener carries minimum zener current,
> transistor carries load current), but without the extra parts.
>
> Any thoughts?  Am I crazy?

That'll work. 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.

But I'd just go with the LM317HV here, I think.

>
> TIA,
>
> -- 
> Randall
>
>
> _______________________________________________
> geda-user mailing list
> geda-user@xxxxxxxxxxxxxx
> http://www.seul.org/cgi-bin/mailman/listinfo/geda-user

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




_______________________________________________
geda-user mailing list
geda-user@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user