diodes
Through the fantastic help of one viewer, I was able to set up a diode chain with LTspice and view it's I-V curve. However this led to 2 very interesting issues.
What I need to know is:
1) Does this I-V curve look reasonably correct? As I wanted to know how a mixture of reversed and forward biased diode would react, I probed in the direction of the diode chain's ground.
2) I cannot locate the spice models of the actual diodes that I need to simulate. How can I manually generate one?
It's a 1.5 watt zener so to generate 1.5 watts at 47 volts requires a current of 31.9 mA. This is the maximum allowable zener (EDITED) current but, there's a little sting in the tail. If the device is actually a 50 volt zener then this current is of course a little less. Ditto if the tolerance forces it to be a 43 volt zener but this time the current is slightly higher.
This is largely confirmed by the term "Admis. Zener Current" being 30mA. A 1.5 watt 50V zener can only be driven with 30mA. I'm assuming "Admis." means admissible meaning "acceptable" or "valid".
Zeners don't switch off immediately below their zener voltage. In the data sheet that I found (because your link doesn't work for me), it quotes 24V as being the point where 1uA of reverse current flows. Above that voltage, current will increase and below that voltage current will get less.
It's also worth pointing out that if the zener self-heats and takes the local temperature above 60degC then the zener power rating reduces. Here are two graphs, the left graph shows the non-instantanous rise in current as the zener voltage is approached. The right graph shows the maximum power curve just mentioned and ultimately also governs current flow into the device i.e. if you are putting 30mA in and the temperature rises above 60 then you are exceeding the power rating of the device: -
The purpose of the resistors across the diodes is a balance the magnitude of the reverse voltage across each individual diode.
A quick way to estimate the resistor value is to check the diode datasheet for the maximum reverse current at the maximum voltage you will be applying across the diode. Then choose a resistor value that gives about 10 times that current, again at the maximum voltage across the diode.
You also need to ensure that the voltage rating of the resistor is adequate. That is: the maximum voltage across the resistor must be less than what the datasheet for the resistor says.
Best Answer
Forget about spice models and simulation and think about what's happening instead. First, the curve shows significant current changes below 1V even though you have 6 zeners and one forward diode in series. Second, the zener chain should eventually look roughly like a voltage source although your plot shows it is acting like a current source.
You didn't say what voltage the zeners are, but I suspect that the sum of their zener voltages is well above the 10V your plot ends at. That means your plot is showing the leakage current. That also explains why it's showing a current source because diode leakage current can look largely like a current source in that part of its operating range. The leakage current is a function of temperature, and light hitting the junction if the junction is open to illumination.
What you should do is run a plot with a current source, not a voltage source. In other words, control the current up to maybe 10 mA and see what voltage you get. That will show you how these things are normally supposed to operate. If you want to understand how a zener diode works, it would be better to just use a single one without adding the confusion of additional zeners and a ordinary diode in the mix.