ZENER DIODES IN LTSPICE - Page 1
Forum section: electronics / beginners

author: neko efecktz
author_rank: Regular Contributor
timestamp: November 03, 2016, 12:08:37 am
content: Hello again.I'm having another bit of trouble with LTSpice ver 4.23.I need to use a 3.3volt zener diode.There is a generic Zener Diode in the library but this does not allow for editing.There is an option within the properties of this Zener Diode to select another diode.There are quite a few diodes listed. Leds, Schottky, TVS, Rectifier, Silicon, and Zener, howeverthere does not seem to be any Zener with a breakdown voltage less than 4.7volts.Also, none of the diodes are editable.Even selecting LED in the generic components does not allow editing.How many other components have this same problem.I may just select a screen full of components and try them all.Any one have any suggestions? Please keep it clean.Going to meditate for a while.Thank youBILL.

author: danadak
author_rank: Super Contributor
timestamp: November 03, 2016, 01:46:38 am
content: This might help (import a model) -http://www.linear.com/solutions/4678http://www.linear.com/solutions/1083Regards, Dana.

author: Ian.M
author_rank: Super Contributor
timestamp: November 03, 2016, 03:16:09 am
content: v4.23 should have the BZX84C3V3L 3.3V Zener. However if you need to go lower, the quick & dirty method is to alias an existing model and override the breakdown voltage. You then right click the model name and type in the new one manually as you wont be able to pick it from the diode list.Code:.model BZX84C2V7L ako: BZX84C3V3L bv=2.7

author: StillTrying
author_rank: Super Contributor
timestamp: November 03, 2016, 03:17:57 am
content: There's a BZX84C3V3 in this standard.diohttp://ltwiki.org/?title=Components_Library#DiodesJust carefully copy the whole line and paste it on to the end of your \lib\comp\standard.dio file.

author: neko efecktz
author_rank: Regular Contributor
timestamp: November 03, 2016, 10:23:50 pm
content: Good morning to everyone.The version of LTSpice I am using is 4.23I seeCAPTURE-1I have had a look through the list of diodes and am unable to find BZX84C3V3L 3.3V Zener Diode you both mentioned.I right clicked on the diode and then thePICK NEW DIODEbutton.It doesn't show up in the list.I may have to add it as Posted by: StillTrying. I have saved a link to the web page and will try a few things in a few days.After doing a backup of the libraries first.I was able to string 5 diodes together to give me a 3.5V Zener Diode.I am unable to try Ians suggestion as I am unable to edit any of the diode properties unless there is another way of doing it. see CAPTURE-3Any idea on that problem?Speaking of the diode list.I found something you may not realize.By clicking on any of the column names eg.Part No. Mfg. Typeetc,you can sort the list in any direction you want.seeCAPTURE-2Once again,Thank you for your help and suggestionsBILL.

author: Ian.M
author_rank: Super Contributor
timestamp: November 03, 2016, 11:37:35 pm
content: To change a diode to use one defined in a .model statement, simply right click on the text of the diode's type on the schematic and edit it to the new model name. Alternatively you can access the generic SPICE component properties dialog, bypassing the diode selector by ctrl-right clicking the diode.If you've never updated LTSPICE, you should run 'Sync Release' from its Tools menu, which *should* bring all standard libraries up to date (unless your computer previously had the clock set way into the future) or something else is blocking updates. IIRC you may need to run as administrator to do the update as it needs to write to the program directory.As a last resort, I've attached the LTSPICE standard.dio file as delivered by the most recent time I updated, today. Back up the one in your lib\cmp folder and replace it with this one, renamed to remove the .txt.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: November 04, 2016, 12:42:55 am
content: Quote from: neko efecktz on November 03, 2016, 12:08:37 amHi Bill,There are several ways to do this. This is the way that I like to use. It puts the model for the Zener right on the schematic so the LTspice file is not dependent on having any other files. This is important if you want to share your model with others.I did a Google search for 'BZX84C3V3 SPICE MODEL' and I found a Diode Inc model.The Diodes Inc model was included in a large file with all their Zener diodes. I cut out the one that was needed and pasted it into a SPICE directive.I drew the schematic in LTspice the normal way and includedanyzener diode from the library of parts. You do this to get the right symbol.A diode can be defined as an intrinsic diode which has the SPICE prefix D. In the case the diode was defined by a .SUBCIRCUIT statement, so the SPICE prefix has to be changed from 'D' to 'X'.Right clickon the diode and change thePrefix to X. ChangeValue to match the name of the subcircuit.These pictures will help:Here is the modelThis is a DC sweep. You can plot I versus V.And here is the result.Like most low voltage zeners the BZX84C3V3 has a soft knee.I have attached the model for those playing along at home.You can cut and paste from this model into your model.Regards,Jay_Diddy_B

author: StillTrying
author_rank: Super Contributor
timestamp: November 04, 2016, 01:34:39 am
content: Yes, but if you paste this on to the end of your \lib\comp\standard.dio file, then a 'BZX84C3V3 Zenner conveniently appears in your "Pick new diode" list.Code:.MODEL PH_BZX84C3V3 D ( IS=7.035E-16 N=0.9809 BV=3.3 IBV=0.005 RS=0.3487 CJO=3.162E-10 VJ=0.6687 M=0.3164 FC=0.5 mfg=Philips type=zener )I've not looked to see if there's one in Ian.M's new standard.dio.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: November 04, 2016, 02:14:02 am
content: Quote from: StillTrying on November 04, 2016, 01:34:39 amIf you do this, and send your .asc file to somebody else. You will get an error like 'model not found'.If you include the model on the LTspice schematic like this:And you send the file to somebody they will have the models too. If the model also exists on the host computer, the one on the schematic is the one that is used.Because the Philips model is an intrinsic diode model the Prefix is D.These are the results from the Philips model:This is a little different to the Diodes Inc. model. It is definitely I good idea to run the models in a test circuit.I have attached the model.Regards,Jay_Diddy_B

author: StillTrying
author_rank: Super Contributor
timestamp: November 04, 2016, 02:41:12 am
content: Quote from: Jay_Diddy_B on November 04, 2016, 02:14:02 amNo, you'll be fine, but they wont.

author: neko efecktz
author_rank: Regular Contributor
timestamp: November 04, 2016, 03:49:31 am
content: Thanks IAN.That did the trick.I first tried the sync to update all files and was informed that I have the latest versions.I then renamed STANDARD.DIO TO STANDARD.DIO.REPLACED then copied your version over and renamed it as you suggested.It now contains the 3.3v zener plus maybe a few more various Diodes.Something I noticed is that the file I had was one long string of text, where the one you sent was one line for each Diode.Well that's solved that problem, but I still want to be able to print to my laser printer without first saving to a .PDF file.Is the problem I am having there common?See My Previous post. I think I called it PRINTING WITH LTSPICEThank you for your help,BILL.

author: flair4ever
author_rank: Newbie
timestamp: January 29, 2018, 05:06:13 am
content: hello, JayThanks for your input. I tried to duplicate your example but failed. I right click the model but unfortunately, I cannot change the prefix of a generic diode mode.I ran your model and it works great. I am wondering how did you make a diode model's prefix as "X".Thank you!

author: flair4ever
author_rank: Newbie
timestamp: January 29, 2018, 05:09:46 am
content: I just found the solution: ctrl+right clickthank you all the sameHope it helps to others

author: TW
author_rank: Newbie
timestamp: October 26, 2019, 03:36:42 am
content: Hey,Apologies for necro-posting. It is almost Halloween 2019 as I type this. So, maybe it's seasonally appropriate raise a dead thread. At any rate this thread is a top google hit for zener in ltspice and that brought me here.The thread has been very helpful, but also quite confusing to me. Almost certainly because of my lack of skill. Both electronics and spice are all unknown unknowns to me.I ran both of the models listed here and looked at a datasheet and as nearly as I can make out they don't have a lot in common.The .model version seems very unlike a zener and the subckt only seems to fit at very low currents. I hacked a spice resistor to fit the Zz for the points listed in the datasheet. The .model version needs some series resistance to be in the ball park so I added that. I've attached a test file.variableR.asc(2.85 kB - downloaded 1526 times.)Questions:1) Is this a very silly way of looking at the problem?2) Have I made any obvious blunders?3) how should one use a datasheet to test models or make simple models for testing?TIA,Tom.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 26, 2019, 04:02:44 am
content: Tom,I have rearranged your models a little. I have changed the models to measure the Zener voltage while sweeping the current in a logarithmic manner.The modelling result is:I will try and measure a 3v3 Zener on a curve tracer.I have attached my modelRegards,Jay_Diddy_Bzener models.asc(3.62 kB - downloaded 519 times.)

author: TW
author_rank: Newbie
timestamp: October 26, 2019, 04:19:46 am
content: Jay,Thank you for that!Tom

author: TW
author_rank: Newbie
timestamp: October 27, 2019, 02:26:57 am
content: I found a model for the 1N4728A. It's a less obscure part than the BZX84C3V3.http://ltwiki.org/files/LTspiceIV/Vendor%20List/Diodes%20Incorporated/Spice/spicemodels_zener_diodes.txtCode:*SRC=1N4728A;DI_1N4728A;Diodes;Zener <=10V; 3.30V 1.00W Diodes Inc.*Zener*SYM=HZEN.SUBCKT DI_1N4728A 1 2* Terminals A KD1 1 2 DFDZ 3 1 DRVZ 2 3 0.972.MODEL DF D ( IS=125p RS=0.620 N=1.10+ CJO=364p VJ=0.750 M=0.330 TT=50.1n ).MODEL DR D ( IS=25.0f RS=1.24 N=3.00 ).ENDSI happened to have one of of those to hand. Its provenance is dubious and my test methods are poor. FWIW, as nearly as I could make out the part did this for me:Code:reverse VmAForward VmA1.680.50.650.51.810.6612.350.74102.5100.76202.8200.79403.0400.82803.3800.83 1003.38100Tom.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 27, 2019, 02:09:03 pm
content: Hi Tom and the group,I had a few minutes to put a couple of Zener diodes on my HP 4155B Semiconductor Parameter Analyzer.This is Zener I tested:These are the specifications, it is supposed to be 3.3V at a test current of 76mAIf I plot the VI curve I get:Voltage 0.5V/div on the X axisCurrent 5mA/div on the Y axis.If I plot V versus Log current, I see:This shows the voltage with currents from 1uA to 100mAThis Zener has very soft curve.By contrast a 6.2V Zener looks like this:'Zener' diodes above about 5.6V are not Zener diodes, they are avalanche diodes and have a much sharper knee.Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 27, 2019, 02:25:21 pm
content: Hi,If I compare the Diodes Inc Model versus the measured VI curve.There is a large error for low currents.I have never found low voltage Zener diodes to be really useful due to these very soft curves.Regards,Jay_Diddy_B

author: iMo
author_rank: Super Contributor
timestamp: October 27, 2019, 03:34:50 pm
content: https://www.onsemi.cn/pub/Collateral/AND8250-D.PDFhttps://people.eecs.berkeley.edu/~hu/PUBLICATIONS/Hu_papers/Hu_Melvyl/Hu_Melvyl_91_01.pdfhttps://prod-ng.sandia.gov/techlib-noauth/access-control.cgi/2017/1713790.pdfhttps://www.aeng.com/articles/Zener.pdf

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 28, 2019, 01:01:09 am
content: @imoThose are some interesting references that you have provided.https://www.onsemi.cn/pub/Collateral/AND8250-D.PDFtalks about the creation of a model for MMBZ5232. This is a 5.6V with a fairly well defined knee. The Zzt= 11https://people.eecs.berkeley.edu/~hu/PUBLICATIONS/Hu_papers/Hu_Melvyl/Hu_Melvyl_91_01.pdfdescribes the construction of a model for 1n5236 This is a 7.5V with a Zzt=6Again this has a sharp knee.https://prod-ng.sandia.gov/techlib-noauth/access-control.cgi/2017/1713790.pdfIt is not clear what Zener diode they are modeling in this paper.The text indicates:Figure 3-1: Measurement of a MMSZ5221BT1 Zener Diode. Swept ranges shown in (a-d)focus on capturing data for specific model parameters.Figure 3-1 shows breakdown in the 8.8 to 9V region.The MMSZ5221BT1 is supposed to be 2.4Vhttps://www.aeng.com/articles/Zener.pdfis about modeling a 1N4623 this is a 4.3V Zener. This is probably the most relevant because it has a soft knee.This is illustrated clearly on the Microsemi datasheet:Regards,Jay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 28, 2019, 01:13:29 am
content: Hi group,This is my attempt at modeling the behavior of the 1N4728A.and these are the modeling results:This is a reasonable good fit to the measured result:Regards,Jay_Diddy_B

author: iMo
author_rank: Super Contributor
timestamp: October 28, 2019, 12:01:10 pm
content: There are more params in the ltspice diode model as the usually used in the component libs.For example, in a different thread where we messed with TC of a zener I had to add Trs1 and Trs2 parameters, and, indeed, the resulting TC was pretty different afterwards..http://ltwiki.org/LTspiceHelp/LTspiceHelp/D_Diode.htm

author: TW
author_rank: Newbie
timestamp: October 28, 2019, 06:27:38 pm
content: Thanks for all the replies.I'm in way over my head but feel like I'm learning a bit even if just from propinquity. For instance Jay's use of "B" sources was a revelation to me. Also, the soft knee of the 1n4728a was exactly the symptom I was seeing in my circuit that I didn't understand. Good stuff. Much appreciated!With the caution that I don't know what I'm doing and suspect this approach will cause problems with some simulations I think these resistor value functions give a decent fit for my 1n4728a up to 100mA:reverse: R={4.48*(I(V1)**-0.878)}forward: R={0.885*(I(V1)**-0.972)}Where V1 is a zero voltage just used to get the current.It might be possible to wrap those up in some logic for use in some simulations. I suspect it isn't a great idea and it's not obvious to me how to construct it.Cheers!Tom.

author: Ian.M
author_rank: Super Contributor
timestamp: October 28, 2019, 07:18:23 pm
content: Put each of your behavioral resistors in series with a (near) ideal diode. e.g:Code:.model Dideal D(Ron=10u Roff=100Meg Vfwd=0)then put the forward and backwards behavioral R + ideal D in anti-parallel with V1 in series with the whole combo.However you haven't modeled the junction capacitance, nor any snap recovery effects, so it isn't a lot better than a properly configured standard 'native' (Berkeley SPICE) diode model which is likely to be considerably faster and more stable in the simulation.N.B. LTspice sims can get very slow if you approach 12 orders of magnitude between Ron and Roff, hence the values chosen for the ideal diode model.

author: TW
author_rank: Newbie
timestamp: October 28, 2019, 10:21:57 pm
content: QuoteUnderstood. However, it seems hard to get the standard model to look much like a zener in both forward and reverse. The subckt examples posted above go to some lengths and still aren't all that convincing. Is it possible to make a .model work? I suspect most of the time it doesn't matter but it was news to me and it might be worth keeping in mind.I've attached a file with my curve fitting and Jay's .model just to show what I'm looking at.Tom.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 28, 2019, 11:39:42 pm
content: Quote from: imo on October 28, 2019, 12:01:10 pmThe help file on diodes is useful in that it gives us the names and descriptions of the parameters within the diode model that can be adjusted.Another useful feature of LTspice is the directiveAKO:AKO means 'A Kind Of'.model my_transistor ako: 2n2222A (BF=400)This describes a parts with all the properties of a standard 2N2222A but with a beta= 400We can do this with Zener diodes..model 1N4728A AKO: 1N750 (Rs=3.5 bv=3.3 ibv=76m ibvl=76m Nbvl=9.5)This describes a 1N4728A asA Kind Of1N750 with 5 parameters modified.We can build models with the parameters stepped to explore what each of the parameters does:Result for stepping RSResult for stepping BVResult for stepping IBVResult for stepping IBVLResult for stepping NVBLWe can see how each of these 5 parameters changes the VI curve.Regards,Jay_Diddy_B

author: TW
author_rank: Newbie
timestamp: October 29, 2019, 01:45:13 am
content: I'm still messing about with the resistor model. I'm sure it isn't a good general purpose solution. It may not be any good at all. I dunno, but think it might work for the circuit I'm working on. It is low voltage, low current DC.Code:.SUBCKT 1N4728A 1 2* Node 1 -> A* Node 2 -> K* Curve fit to a single, used sample. YMMV* sampled 0.5mA - 100mA. Very likely bogus outside that range.* y=a*I**b***************************************VS 1 3 0 ;shuntRZ 3 2 R ={ if( I(VS) > 0, 0.885*abs(I(VS))**-0.972 , 4.48*abs(I(VS))**-0.878) }.ENDSTom.

author: Zero999
author_rank: Super Contributor
timestamp: January 11, 2025, 06:46:28 pm
content: I wanted a basic model which would allow you to simply put the zener voltage on the schematic, so I decided to make one.I focused on simplicity, rather than accuracy. It's roughly based on the BZX84C & BZX384B in the Standard.dio file linked below.https://ltwiki.org/index.php?title=Standard.dioV = the zener's breakdown voltage. The default is 5.6V.Iz = the zener current at which the voltage is equal to the specified breakdown voltage. This defaults to 1mA.