Very basic Zener Diode/LTSpice simulation question - diyAudio
Go Back   Home > Forums > Design & Build > Software Tools

Software Tools SPICE, PCB CAD, speaker design and measurement software, calculators

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 23rd September 2011, 10:25 AM   #1
diyAudio Member
 
Join Date: Jun 2003
Location: Haifa, Israel
Send a message via ICQ to eranrund
Default Very basic Zener Diode/LTSpice simulation question

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
Attached Images
File Type: png zener.png (54.5 KB, 84 views)
  Reply With Quote
Old 23rd September 2011, 10:38 AM   #2
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Quote:
Originally Posted by eranrund View Post
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
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news? - W. S. Maugham
Check out Linear Audio!
  Reply With Quote
Old 23rd September 2011, 10:51 AM   #3
diyAudio Member
 
Join Date: Jun 2003
Location: Haifa, Israel
Send a message via ICQ to 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?
  Reply With Quote
Old 23rd September 2011, 10:57 AM   #4
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Quote:
Originally Posted by eranrund View Post
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
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news? - W. S. Maugham
Check out Linear Audio!
  Reply With Quote
Old 24th September 2011, 02:05 PM   #5
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Does the model include temperature?
  Reply With Quote
Old 24th September 2011, 02:06 PM   #6
bill_a is offline bill_a  United States
diyAudio Member
 
Join Date: Aug 2009
Location: Seattle
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
  Reply With Quote
Old 24th September 2011, 02:39 PM   #7
sbrads is offline sbrads  United Kingdom
diyAudio Member
 
Join Date: Jun 2005
Location: Kent, UK
Quote:
Originally Posted by bill_a View Post
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.
  Reply With Quote
Old 25th September 2011, 11:14 PM   #8
jcx is offline jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
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

Last edited by jcx; 25th September 2011 at 11:17 PM.
  Reply With Quote
Old 26th September 2011, 09:43 AM   #9
diyAudio Member
 
Join Date: Jun 2003
Location: Haifa, Israel
Send a message via ICQ to 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
  Reply With Quote
Old 26th September 2011, 12:34 PM   #10
diyAudio Member
 
Join Date: Apr 2011
Location: Pretoria
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
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Distortion simulation with LTspice tiagor Solid State 32 22nd December 2012 10:33 PM
zener LTSpice model Mosquito Tubes / Valves 4 2nd January 2011 09:53 AM
About LTSpice Simulation. nicholas1113 Solid State 2 15th November 2009 08:52 AM
Zener Diode Question Captn Dave Tubes / Valves 12 17th June 2009 09:16 AM
zener diode question mike49504 Car Audio 2 2nd September 2007 07:17 AM


New To Site? Need Help?

All times are GMT. The time now is 12:11 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2