Here is the circuit- Can someone please tell me why the lamp will always have a voltage of 20v across it no matter how high the voltage is? Doesn't the zener diode allow voltage through it after 20v? So then only the voltage after 20v has been dropped should go through the lamp right? For example a 35 volt power supply means 15v goes across the lamp. Apparently 20v goes across the lamp but I don't see why that is. Also what will the total voltage of the circuit be in this circuit if the power supply is 35v? If the power supplu was 20v surely that would mean the voltage across the lamp would be 0v?