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eranrund 23rd September 2011 09:25 AM

Very basic Zener Diode/LTSpice simulation question
 
1 Attachment(s)
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 23rd September 2011 09:38 AM

Quote:

Originally Posted by eranrund (Post 2721260)
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 23rd September 2011 09:51 AM

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 23rd September 2011 09:57 AM

Quote:

Originally Posted by eranrund (Post 2721273)
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 24th September 2011 01:05 PM

Does the model include temperature?

bill_a 24th September 2011 01:06 PM

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 24th September 2011 01:39 PM

Quote:

Originally Posted by bill_a (Post 2722413)
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 25th September 2011 10:14 PM

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 26th September 2011 08:43 AM

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 26th September 2011 11:34 AM

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

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


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