Voltage limiter

I have concerns about testing the gauges with 3 batteries in series. Testing I have done using 5VDC power supply has the gauge at FS (full scale) with 1.3 V at sensor, so 3.7 V would be across the gauge. I don’t know what would happen with 4.5 V across the gauge, but unfortunately, I don’t have a gauge I want to sacrifice for the sake of experimental collateral damage!

The RTE device outputs a square wave 10 V P-P (peak-peak). At 180, the reading was too low and not adjustable. However, my mods require a DC gauge power, so I will use an adjustable unit so the readings can be set to a better position.