Best way to pick 5V off a 20-80V DC line.

[rnorman3]

Hi, I have a line voltage that fluctuates from 20-80V...I want to put a 5V regulator on there...most only accept voltages up to 12V. How can I get the regulator to work on that line? I was thinking to charge a cap off the line though a diode, but it could still drop out if the voltage is low enough for long enough...maybe there's an OP AMP answer?
Thanks!

 

[ronsimpson]

How much current?

 

[rnorman3]

The small regulator would only have to supply about 200mA perhaps to run a PIC. The main 20-80V line carries up to 140A from a welder.

 

[ronsimpson]

The TL783 linear regulator will handle input voltages to 140V. BUT 200mA and 80V will cause about 15 watts of power loss. HOT HEATSINK!!

I would use the LM5008, LM5017, LM5018. Input voltage to 95 volts. Can work with the input as low at 9 volts. The power loss will be about 0.1 watt.

 

[rnorman3]

Thanks ron, these seem like great options!

 

[JimB]

The small regulator would only have to supply about 200mA perhaps to run a PIC. The main 20-80V line carries up to 140A from a welder.

My first reaction is "why would you want to do this?"

I do not have the "big picture" of what you are trying to do, but it sounds like you are doing it the hard way.
I would investigate the possibility of using a separate supply.
The welding supply is going to be electromagnetically "dirty", which will give many possibilities for the PIC to not work correctly.

JimB

 

[MikeMl]

I second JimB. I have boxes of discarded wall-warts (plug-packs) just for projects like this...