Looking for a constant voltage drop circuit

[RoGeorge]

The circuit has to:
- act as a two terminals load and able to dissipate 10-50W or so
- keep a constant voltage drop in a range from mA to a few Amps
- preferably have a continuously adjustable voltage drop set (2V ... 15V), but it can eventually have only a fixed voltage
- preferably bidirectional

Briefly, the circuit has to act like a power Zener diode, preferably symmetric, as if it were two anti-series Zener diodes.

I need such a circuit to put it in series with a CC-CV power supply that generates constant current, to discharge/measure rechargeable batteries without the need of a dedicated active load.  Something similar with the attached schematic, but that can keep constant voltage in reverse, too.

[Ian.M]

While you *CAN* build a shunt regulator capable of 50W dissipation, (e.g. TLVH431 driving a PNP input Sziklai pair to boost the current), at that point you might as well build an active load . . .

[langwadt]

50W is a lot. Anyways, stick you favorite "power zener" inside a diode bridge?

[RoGeorge]

you might as well build an active load . . .

Fully agree with that, just that it seems so much easier to solder 5-10 parts in air, on a TO3 with a radiator.

The main advantage of using a series drop load + a power supply (instead of building a complete active load) would be that the Rigol DP832 power supply I already have can be controlled remotely, and it can log the voltage and the current while (dis)charging the batteries on all its 3 channels.

50W is a lot. Anyways, stick you favorite "power zener" inside a diode bridge?

The bridge idea fits very well, thank you! 
Especially that I might have a few unused 30A bridges so big they might serve as a radiator, too, if I lower the 50W request.

Another advantage of the bridge is that it can turn off the load, which would be great.  It would be possible to sense the direction of the current flow, and turn off the load to protect the power supply in case of a mains power surge (DP832 can only source current, it can not sink).