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Subtract 2-4VDC at 0.1-10A+ from a DC line

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digsys:
Got an unusual one - I have a lot of "stuff" out there designed for 48VDC. It can tolerate 51VDC continuous, and will crowbar at 52V (app 1 sec) - all fine.
So everyone I deal with is moving to a SYSTEM 48V BUS and doing away with converters, both 12V etc up and 240VAC down. Some of these BUSes are 1,000s Amps. They are also directly connecting to 48V vehicles - with 4x12V batteries - with flat > Full alternator charging.  PoE cameras, switches etc etc. This now sees anything from 48-56VDC on the BUS. Re-designing 100s devices is not an option.
I could just "Insert" a string of high current diodes in the DC line (in a nice 2 terminal Al case), for 6-7V drop, and call it quits, but that is a lot of often UN-necessary heat and loss, especially for installs that are running at 48-51V.
I need to have that much higher Vloss, as damn diodes Vf vary from 0.5+V at 0.1A > 2.0V at 10+A. When the systems are in idle, they draw very little current, and once the crowbar fires, they'd have to restart the machinery, some of it is huge :-)
I had planned on a series of diodes with "taps", so the Installer could check the system voltage and select the appropriate "drop-out voltage" required ... perfect
Anyone know of a device with a junction that has ~ 0.6-2.0V loss at all currents? Even a zener assisted PNP varies too much .. or other ideas? Has to be DC line only and simple.

Zero999:
What's the minimum voltage it will work from?

How about using a buck converter to get 40V? It will have much lower losses.

digsys:
These are mostly existing installations that "basic" electricians will go out to and I have no access to the 0V rail. I can easily afford to waste 0-4V, but don't want to do a blanket install, as many installations will be fine, and need nothing. A diode would be perfect, except for it's damn variation with current.

nali:
I needed something similar albeit much lower power, I used a simple NPN emitter follower with a zener reference which worked fine.

Mazo:
A TL431 style reference+the amount of transistors needed to dissipate the power?All you will need to ensure is that the minimum current on the line is >quiescent current of the "TL431"

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