lm317zener
Since the zener breakdown voltage is set to 5v, i actually thought that the voltage would be 0, since it goes directly to ground, I just wanted to try a "simple overvoltage protection". How is it that the voltage is 5v instead of 0? Without the diode, the voltage is around 22 volt or so.
Can anyone explain why this is and maybe give a few examples on how to fix it?
(This is my first time working with Zener Diodes)
Thanks!
You need a resistor between the switch and zener, and a resistor between the Vout point and ground, like so:
simulate this circuit – Schematic created using CircuitLab
With the switch up, the Zener will limit Vout to 4 volts.
With the switch down, the Zener will not conduct, and Vout will be 3 volts (less a bit, due to the current drawn by R2)
Without R1, when the switch is up, infinite current would flow through the Zener diode as V1 tries to deliver 5 volts, and D1 tries to hold the voltage to 4 volts.
Spice models generally do not include noise in transient simulations. The "noise" model in spice is used only in AC sweeps, where the noise power is calculated as a function of frequency. While resistor (Johnson–Nyquist) noise is in the model, semiconductor models often do not have accurate noise models. The spice diode model does include flicker noise, but not other noise sources.
For your purposes, AC analysis may be sufficient assuming that your diode has a proper model, since what you want is to see if the noise power density is flat. But, I doubt that the Zener model includes accurate noise parameters. The spice model of the diode mentioned in this question (EDZV24B) does not include any noise parameters (which are the AF, KF, and FFE parameters).
Another option (for transient simulations) is to include a voltage sources controlled by a random number. For a description of using this approach to noise modeling, this website from Giorgio Vazzana has good information. But, to follow this approach, you have to know how much noise to expect. Also, the transient simulation would not normally include noise added by the transistor.
An example noise voltage source (from the above mentioned website) is:
Vn 1 n1 dc 0V ac 1mV trrandom (1 5us 0s 125m 0m)
Best Answer
You seem to be confusing the Zener action with a 'crowbar' circuit. A Zener will limit the voltage whereas a crowbar circuit will short out the supply in the event of an over-voltage. The crowbar short can usually only be removed by cycling the power.
The Zener diode will start to conduct in the reverse direction when the voltage across it reaches about 5 V. From then on very small increases in voltage will cause it to pass an exponentially greater current. At the same time the voltage will increase slightly above 5 V.
The problem with your arrangement is that if the regulator can pass, say 1 A, before going into limiting then your Zener has to handle all that power. \$P = V \cdot I = 5 \cdot 1 = 5~W\$. It will get hot.
Crowbar circuit
simulate this circuit – Schematic created using CircuitLab
Figure 1. Crowbar circuit.
This is a basic crowbar circuit. When the supply voltage rises above 4.7 V the Zener starts to conduct. When the voltage across R1 gets high enough SCR1 will be triggered and pretty-well short out the supply. The thyristor has to pass all the current (1 A in our example above) but this time the voltage across the device is only about 0.5 V so \$P = V \cdot I = 0.5 \cdot 1 = 0.5~W\$. Cool!
The thyristor will stay on until the current though it is reduced to zero by interrupting the supply.
History lesson: the term crowbar protection comes from the railway third-rail electrification. If the lineman saw a problem and needed an emergency isolation of the track power supply he would throw his crowbar to short the third rail to one of the running rails and trip out the breaker.