XHDZ-FZ-2G electronic load comparator function?

[ledtester]

I've been studying the XHDZ-FZ-2G electronic load board. An earlier revision was used in Voltlog #82 to create an analog electronic load. That used the XHDZ-FZ-2E version.

I've attached the schematic and a pic of the -2G version.

My question is what's going on with the comparator and the diodes D2 and D3.

I'm not sure exactly what kind of diodes D2 and D3 are. They very will may be Zeners. They both have a forward voltage of 0.6V. Using my multimeter's diode test function the voltage in the reverse direction is 1.3V for D2 and 2.1V for D3.

From comments about the -2E version of the board, it seems that the role of the LM393 was to bring the non-inverting input of the LM258 low when there was "no load between V+ and V-".

V+ / V-: power supply under test connections
VCC / GND: isolated 12V power supply

A 1N4007 diode is used to make GND 0.6V lower than V-.

- Voltlog #82: https://goo.gl/JN46Yt
- XHDZ-FZ-2E schematic: https://goo.gl/Olr6k2
- Comment about the LM393 in the -2E board: https://goo.gl/x3XJrX