im simulating the power input section of this circuit in multisim:

http://www.maximintegrated.com/app-notes/index.mvp/id/1923

the zener im using is 1N5921BRLG. this is a 6.8v 3 watt zener. im simulating both 50vdc and 90vac to simulate a phone line. now i would expect a voltage drop of 50-6.8 and 90-6.8. but i am getting way different results. it is supposed to allow 6.8 volts to power the circuit but i am getting very different results. am i doing something wrong? here is a screen shot and the multisim file attached. thank you

 

I admittedly am not an expert, but in all cases where I have seen zener diodes used as regulators the supply line has a resistor on it to limit the current input. Basically the zener then works to regulate the voltage drop across the resistor which provides the stable voltage to your circuit. I think that circuit relies on the phone wiring to the device having a certain amount of resistance.

 

can anyone explain these unusual measurements?

 

can anyone explain these unusual measurements?

ok let me try,

The first one:

Multisim see the zener in parallel with the power bridge rectifier, that's why it read 47V, 3v drop is mainly caused by the voltage of the bridge rectifier.

In reality, your zener will cook, as zero10 suggested, you need a resistor in series with the zener.

The circuit from Maxim is different, it's more complicated, it properly has small current from the source, I will let someone else to explain this one. (aka I don't know much about it)

so the second one is about the same, except the source is AC, so you need to convert the RMS value.

note: you also need to consider the load current and ripple as well, which I am not going to go into details.