Very basic Zener Diode/LTSpice simulation question

[eranrund]

Hello.

I have set a very simple circuit in LTSpice: Voltage source of 12v connected to a resistor which is then connected to a 8.2v Zener diode.
When measuring the voltage between the resistor and the Zener I notice two things I cannot explain:
1) The measured voltage is around 8.7v (i was expecting 8.2)
2) From 0ms to ~1.5ms I see the voltage slightly dropping. Why is that?

Attached is a screenshot.

Many thanks

 

[jan.didden]

Hello.

I have set a very simple circuit in LTSpice: Voltage source of 12v connected to a resistor which is then connected to a 8.2v Zener diode.
When measuring the voltage between the resistor and the Zener I notice two things I cannot explain:
1) The measured voltage is around 8.7v (i was expecting 8.2)
2) From 0ms to ~1.5ms I see the voltage slightly dropping. Why is that?

Attached is a screenshot.

Many thanks

Hi,

The nominal zener voltage of 8.2V is specified with a specific zener current, often 10mA, which should also be in the data sheet.
Your circuit has much larger zener current and that, with the non-zero differential zener impedance, gives a higher zener voltage.

Can't explain the 40uV (!) initial drop - possibly some numerical initialisation in LTspice?

jan didden

 

[eranrund]

Hello,

I have changed the resistor value to 730ohm in order to have a current of 5.03ma and not fry the Zener and now I measure 8.32v.
The datasheet specifies a minimal value of 7.7v and a maximal value of 8.7v, nominal being 8.2v for 5ma current.

How does the Zener's model come up with 8.32 instead of, for example, any other close value?

 

[jan.didden]

Hello,

I have changed the resistor value to 730ohm in order to have a current of 5.03ma and not fry the Zener and now I measure 8.32v.
The datasheet specifies a minimal value of 7.7v and a maximal value of 8.7v, nominal being 8.2v for 5ma current.

How does the Zener's model come up with 8.32 instead of, for example, any other close value?

I would check the model text; you'll probably find a small series R.
Possibly there is also a current dependent voltage source.

jan

 

[DF96]

Does the model include temperature?

 

[bill_a]

Its hard to read your screen capture, but I just duplicated your description, with a 720 Ohm resistor and it works fine. Zener voltage of 8.269 @ 2.4mA. That's about the best you can hope for with a Zener - they're just not that good a regulator.

The 'droop' you see is due to the junction capacitance charging up - you'd see something similar in real life if you could set up a good enough experiment. If it bugs you, change your simulation time to say 1mS - you hardly notice it at all....

Bill

 

[sbrads]

Its hard to read your screen capture, but I just duplicated your description, with a 720 Ohm resistor and it works fine. Zener voltage of 8.269 @ 2.4mA. That's about the best you can hope for with a Zener - they're just not that good a regulator.

The 'droop' you see is due to the junction capacitance charging up - you'd see something similar in real life if you could set up a good enough experiment. If it bugs you, change your simulation time to say 1mS - you hardly notice it at all....

Bill

Junction capacitance would make the voltage rise slightly if anything, not droop. Zeners over 6v go up in voltage with a rise in temperature so it can't be temperature either. It must be a bug in LTSpice.

 

[jcx]

you may need to adjust/inform your expectations of simulation, modeling, real world device tolerance/production spread

Spice DC solvers try to find the "steady state" solution before running the .tran simulation

these 2 different stages of simulation use different algorithms, can come to different conclusions - sometimes at very gross levels - much less the 10ppm you seem concerned about


a big weakness of Spice for matching "real world" results is that there is no "live" modeling of device thermal effects - the standard Spice transisitor, diode equations can be assigned a junction temperature -but is is fixed for the duration of the sim

 

[eranrund]

I see.

I was hoping to use LTSpice to experiment with basic stuff (such as the Zener regulation) I am trying to learn.
Guess I'm gonna have to just take its result with a grain of salt

Thanks again

 

[ingenieus]

LTspice is pretty good, especially when you consider the price being charged for it.

It is not perfect. Better models do give better results. A good start in this regard is Bob Cordell's website, which has some improved models (including Zeners if I remember correctly).

You can also have a look at this thread.

Things you should know about LTSpice

 

[jan.didden]

I see.

I was hoping to use LTSpice to experiment with basic stuff (such as the Zener regulation) I am trying to learn.
Guess I'm gonna have to just take its result with a grain of salt

Thanks again

LTspice or any spice is VERY good for the use you want it for. You can build a regulator, experiment with current values, 'what if' questions, that sort of thing.
For instance, you can verify how the ripple rejection changes when you vary the series resistance to the zener. That is ideal to play with your circuit before putting it in hardware.
You don't get a 0.0001% correct value answer, but almost always you get a correct 'trend' answer, and that's very important.
Also in hardware you wouldn't get 0.0001% correct values.

And let's face it, it is totally irrelevant whether your opamp supply gives out 14.8 or 15.123 or something in that area. 15V supply is just a design value.

jan didden