I'm trying to use a Vishay 3.3V Zener for a reference voltage in an amp circuit. The problem is that it's not maintaining 3.3VDC. I'm working with a 20VDC supply, and using a 10K resistor in series with the Zener. As I raise the power supply to 20V, the Zener's voltage goes up and up and up - never stopping at 3.3V. I've got it connected properly, so what am I doing wrong? Thanks!