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 > [email protected] > http://www.seul.org/cgi-bin/mailman/listinfo/geda-user John Doty Noqsi Aerospace, Ltd. http://www.noqsi.com/ [EMAIL PROTECTED] _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
