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Simulators >> Circuit Simulators >> Spice thermistor... basically a volt var source..
https://designers-guide.org/forum/YaBB.pl?num=1180031139

Message started by groundhog on May 24^th, 2007, 11:25am

Title: Spice thermistor... basically a volt var source..
Post by groundhog on May 24^th, 2007, 11:25am

I have a simple BJT circuit in LTspice. I sweep it over temperature and the Ic changes.

So, I want to insert a thermistor (temperature varying resistor) in the collector to counteract the variation in Ic.

So, time to put in a model of a thermistor in for one of my collector resistors.

But, the way to model a thermistor in Spice is to create a voltage controlled voltage source that creates a voltage that would be seen across the equivalent resistor that you are replacing. So if the 100 ohm resistor would have made a voltage drop of .1 volts, you just make it so that the voltage source drops .1 volt at the same current. The circuit doesn't know the diff.

THE PROBLEM:

Title: Re: Spice thermistor... basically a volt var sourc
Post by groundhog on May 24^th, 2007, 11:31am

.... (sorry, I hit the wrong key and posted the above without finishing)

THE PROBLEM:
Is that in the descriptions I have seen in the thermistor model, you do a sweep of the control voltage and not temperature. So, if you want to see what the thermistor does over a range of say 0 to 50 degrees, you sweep the control voltage from 0 to 50 volts. You plot and just label the x axis as temperature rather than voltage.

Well, with the BJT, you have to actually sweep real temperature. It's model has temperature varying elements in it.

So I am left with sweeping two different things, temperature and control voltage in a nested sweep statement.

I NEED A THERMISTOR MODEL WHERE YOU SWEEP TEMPERATURE NOT VOLTAGE TO MAKE IT CHANGE.

does anyone know of one?

Title: Re: Spice thermistor... basically a volt var sourc
Post by Geoffrey_Coram on May 25^th, 2007, 8:40am

Why don't you use the TC1 parameter of the resistor to make it temperature dependent?

Title: Re: Spice thermistor... basically a volt var source..
Post by Cryptonite on Mar 19^th, 2009, 7:15am

Hi, I know this is an old thread now but I had a similar need & found this board just this week while trying (and failing) to find a solution.

I've now solved it myself & thought I'd share my solution which is to use an Arbitrary Behavioural Voltage Source combined with a 0V Voltage source as a current sensor like this:

.subckt ntc 1 2 Params: * these provide sensible defaults
+ R0=10k,
+ Beta=7660,
+ Rpar=1T,
+ t0 = 25,
+ TK = 273.15

* Arbitrary Behavioural Voltage source is used to generate
* Voltage equal to the NTC resistance @ global temp (in °C)
* multiplied by the current passing through it
BTherm 1 3 V = I(Vsense) * R0 * exp (Beta/(temp + TK) - Beta/(t0 + TK))

* Current sensor
Vsense 3 2 0

Rparallel 1 2 R = Rpar

.ends

If you make this a library, you can then add commercially available Thermistors, e.g.

* Vishay Surface Mount NTC0805E4 series 2k2 @ 25°C
.subckt 2381_615_13222 1 2
x1 1 2 ntc Params: R0=2k2 Beta=3680
.ends

and predefined linearised sensors such as

*Oil Temperature Sensor
.subckt OilTemp 1 2
x1 1 2 ntc Params: R0=50k Beta=1780 Rpar=18k
.ends

or you can just use the select "ntc" as SpiceModel and define the parameters as you need in the LTSpice Component Attribute Editor e.g. Value as "R0=15k" , Value2 as "Beta=4660", SpiceLine as "T0=0", SpiceLine2 as "Rpar=1k5"

The model is obviously a simple one, with no account taken of frequency response, self-heating, etc. but it's much faster to run in LTSpice than the EPCOS ntc models available online.

Hope this is useful to somebody!

Title: Re: Spice thermistor... basically a volt var source..
Post by Frank Wiedmann on Mar 19^th, 2009, 8:57am

Are you aware of the LTspice users' group at http://tech.groups.yahoo.com/group/LTspice/ ? There you will find an answer to almost every LTspice-related question.

Title: Re: Spice thermistor... basically a volt var source..
Post by Cryptonite on Mar 19^th, 2009, 9:37am

Yes, I am aware of the group's existence but I'm not a member. Unfortunately, because of the wide range of available, non-work related groups, Google Groups are generally frowned upon by our corporate network "guardians" (that's the polite word).

Thanks for the suggestion, though!