Voltage-variable capacitor doesn't work in LTSpice

I don't have time to play with this just now, but one quick question:

Given that C is a variable capacitor, set C to 1F and charge it to 1 volt. Now change C to 0.5F. What is the new voltage?

How many 1 farad capacitors are you aware of?

Oh and would not a 1F charged cap dump into a 0.5F cap and fully charge it? If that answer is yes, then the voltage would be the same... slightly less even, all elements considered.

You thought it would morph the EMF into something else?

In LT Spice, you can use any C value. 1F is the generic capacitor. I usually normalize theoretical circuits to 1F, 1H, 1 ohm. [1]

In real life, Digikey will sell you 1F caps. Or 5000F caps. But not variable 1F caps.

I'd like to know if Jim's model conserves charge, or conserves energy, or conserves voltage. Or whatever it does.

I haven't tried that. Since it _is_ a capacitor, it'll depend on whether a simulator treats capacitors as charge-controlled devices or otherwise. ...Jim Thompson

Voltage-Controlled Resistor Spice Model Updated

See VVR.zip on the Device Models & Subcircuits Page of my website

Includes Library File (.LIB), LTspice Symbol (.ASY), PSpice Symbol (.SYM)and a JPEG showing how it was developed. ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

This is what I did for a time-varying capacitance. In the component 'Value' field, I put Q=(4p/(0.25 +(time*5)))*x which swept the capacitance from 16pF downwards controlled by the internal variable 'time'.

I can't remember why you need the Q and the x, but you do, and it took a good while to find out - it seems you can't just vary the capacitance directly. Replace time with a voltage and the appropriate scaling and you should be good to go.

Cheers

Tried it and that completely bungled the linearity when the cap is inside a resonant circuit. At least no more error messages which is good. Well, maybe I just do it in hardware then, firing up the old Weller. I'd have to buy a bag of varicaps but those are cheap.

Are you trying to sim a VCO? Simulating oscillators is always tedious.

You could use a diode model, which does include variable capacitance.

Try to get the Skyworks sample kit. It has a lot of varicaps, and many other cool things.

Q is charge and X is the cap voltage.

Q = (expression) * X

just means that (expression) is the capacitance. Seems silly to me.

I tried a capacitor that changes value abruptly at 0 volts, like the example. That works. If I try to change C abruptly at a non-zero voltage, the sim crashes. Some conservation thingie was violated.

No, it's a circuit where a capacitive sensor is employed and I want to mimic the sensor output. The capacitor itself will be inside a somewhat resonant circuit so it has to behave like a real and clean capacitor.

But that's a voltage-controlled voltage source. I'd need a real cap.

I'd need the old AM tuning diodes, tons of capacitance. What I need is a variable range of several hunded pF around a base of 1000pF or slightly above. Or I have to make Jim's method work but so far no luck. Could have to do with the durn Windows 7 restrictive file writing "privileges".

If we had all our hardware going I wouldn't need this. It is intended to give a SW engineer a fake signal in file format that he can plug into his algorithms so he doesn't have to wait for HW to be done.

I can somehow kludge it with a voltage-variable resistor because in contrast to capacitors that works in LTSpice. But it won't be pretty.

-Lasse

I take it you haven't tried my subcircuit? ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

Same way I did my varicap models and Joerg's voltage variable one. (The varicap model uses a Table to handle the non-linearity of a varicap.) ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

I have not given up yet but so far I got all kinds of error messages. The next step would be to try it on an XP machine where I'd have to get LTSpice going again. XP was IMHO the last known good OS and I've had error message in other software caused by Windows 7 (meaning they didn't happen on an XP machine).

What kind of error messages?

If you use the LTspice _symbol_ I made, you need to open the .ASY file with a text editor and change the path to wherever you've located the subcircuit definition library.

The latest version, on the website, shows how more clearly. ...Jim Thompson

Would something like this work?

The cap is modulated by voltage ZZ, which in this case makes the cap ramp from 1F to 20F. That in turn sweeps the LC ringing frequency down.

...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

It would be trivial to add tabular data to this model to characterize a non-linear transducer.

I just need to scratch my head and remember how to have a subcircuit call external tabular data so I can make it a generalized transducer model ;-) ...Jim Thompson

And there is the first problem. Windows 7 no longer allows writes to the program directories. Well, it does but secretly stashes them some place else but then access become an issue. I'll get to the ground of that, hav to for my CAD as well, just not right now. The usual, swamped in work plus honey-do stuff.

And, we had to get a Mexican Burger at one of our favorite watering holes. Along with an Old Republic red ale.