# Pre-regulator sizing

I want to make a voltage regulator for a symmetric power rail +30/-30V. The idea is to use an LM317 and an LM337.

I want to be able to regulate with a Vin that can go up to +60V (and -60 and negative rail).) The LM317 can only have a voltage drop between IN and OUT of 40V. In case of a short that value would be exceeded. After some digging I found that I can do a pre-regulator as in the picture below.

Once the voltage VIN is above about 36V, the Zener diode would kick in and the BJT will do it's thing.

My issue is tgat I don't know how to size that pre-regulator.

More specifically, how do I size R9?

I think it's R9= (Vzener-Bbe)/(Izener+Ibase), but Izener depends on temprature and voltage accross the Zener diode. I'm not sure about the Ibase.

Important note (maybe) my load is around 25mA. D1 is just there for reverse polarity protection.

EDIT 19/04. As Vangelo pointed it out, with the current disign there is a issu in case of a short on Vout. So I added this simple circutry and it seems to be working. I found this here https://jeangaillat.wordpress.com/2018/03/29/je-suis-fou-des-alimentations-lectroniques/amp/ the 1200 resistor is to simulate my charge at 25mA.

Normal operation:

Short:

And for the negative rail:

• Think about what happens when R9's value is too large: there will not be enough current through the zener (see zener's datasheet what (knee) current is needed). Also, if R9 is too large, Q1 might not have enough base current. You know the supply current so you can derive Ib from that. What if R9 has a value that is too low? Then too much current will flow through the zener, it will get hot. Apr 16, 2021 at 9:38
• Unless you're planning to draw only single, maybe two digits of milliamperes: almost certainly the approach of using a linear regulator here is a wrong way to start, unless you have plenty of heatsink to spare, anyway.
– mmmm
Apr 16, 2021 at 9:40
• Important note (maybe) my load is around 25mA. That is not a "maybe" the maximum load current can make or break this design. At 25 mA you're probably not going to dissipate that much heat in the components but do a calculation of that anyway! 36 V at 25 mA is 1 Watt so enough to fry a small component. Of course, some power goes into the load but consider where the rest will go. Apr 16, 2021 at 9:42