# Help needed working out a zener diode circuit's total Voltage

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?

## Answers and Replies

Anyone?

gneill
Mentor
The zener diode begins to conduct when the potential across it reaches 20V. The voltage across the zener (at least an ideal zener) will not go higher than 20V. In this respect it behaves somewhat like a regular ideal diode in series with a 20V power supply, and will only begin to conduct current after the applied voltage surpasses this "internal" supply voltage and becomes forward biased.

Since the lamp is in parallel with the zener, it must share the same potential drop: no more than 20V. The zener acts like a "voltage clamp". It conducts however much current is required in order to maintain that 20V drop across itself. If the power supply voltage alters, the difference shows up in the potential drop across the series resistor R.

So does the voltage through the whole circuit drop down to 20v?

gneill
Mentor
So does the voltage through the whole circuit drop down to 20v?

No. The voltage across the zener and anything in parallel with it will be maintained at no more than 20V. Everything else has to adjust accordingly to make KVL work out.