Hi there, Jason. Can you post a link to a bare-bones schematic of what you are trying to do? I ask this because I'm confused. A TL431 is a shunt regulator and is=20 normally used with a resistor from the incoming supply rail to the=20 TL431. This makes the circuit extremely resistant to voltage spikes=20 and surges. Pretty much the only failure mechanism is that the=20 series resistor might overheat. I'm don't know why you would need a pre-regulator in front of the=20 TL431 circuit. dwayne At 01:34 PM 4/29/2019, Jason White wrote: >The zener supply exists, in part, to protect a TL431 based supply from hig= h >voltage transients. > >On Monday, April 29, 2019, Bob Blick wrote: > > > My rule of thumb is to keep a zener between 25% and 50% of its rated > > dissipation. I know that seems like a lot, but I believe that most zene= rs > > are at their spec voltage at 50%. > > > > Zener diodes are why there are things like the LM385 and TL431 :) > > > > Another thing I'll do instead of a low voltage zener is use a few LEDs = in > > series. Reds are pretty solid. > > > > Cheerful regards, Bob > > ________________________________________ > > From: piclist-bounces@mit.edu on behalf of > > Jason White > > Sent: Monday, April 29, 2019 11:14 AM > > To: Microcontroller discussion list - Public. > > Subject: Re: [EE] Stability of Zener+Darlington BJT linear regulator? > > > > Thank you Dwayne, Bob, and Spehro for your input. I think I will opt > > for a base resistor in the 1k to 10k range to suppress oscillation. > > > > Any recommendations for the minimum correct zener current over a -40 > > to 85C operating range? I imagine that the minimum zener current is > > dependent on temperature. From the attached graph at 25C it looks like > > 500uA is as low as I could go. > > > > -- > > Jason White --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA 780-489-3199 voice 780-487-6397 fax 888-489-3199 Toll Free www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .