ISIS - strange behaviour

[Boncuk]

When simulating the oscillator circuit as attached the output pin1 has a 510Hz square wave and output7 provides a triangualar wave.

As soon as I add U1 part D (not connected anywhere) the simulation doesn't show any results. Also, the CPU load drops from normally 91 to 92% to 4 or 5% - not working at all.

Anybody who can shed some light on this phenomenon?

Boncuk

 

[Sceadwian]

Never leave a node in a simulator floating, ever, and don't mistake an ouput line as floating, it has a path to ground on at least one side. If you're dropping a component that has no path to ground the entire simulation software will generally puke unceremoniously. It simply has nothing to base the computations it starts for that part. Most simulators 'grounds' are the analog version of a 'bit bucket' anything you send there will end there, it's not like a real world ground where you'll get feedback. The GND line in a simulator basically means '0'

Want to experiment? Take a simulated circuit that works in a known way (preferably high current), redefine all 'ground' nodes to pass to a node (realground) or something and link that to the simulators 'ground' node via a .001 ohm resistor. Then you'll see something more real.

 

[mneary]

I hope you wouldn't insert a floating op amp section into a real circuit. To eliminate the error message tie the 'unused' inputs to something. The vendor's app notes might have a suggestion.

 

[Boncuk]

Thanks both of you.

Of course I know not to leave input pins unconnected even if a part of an IC is unused.

However, it doesn't make any difference if I leave part D of the OpAmp unconnected or tie the inputs the way the unused output is forced high or low.

Again: As soon as I use the fourth part of the OpAmp the oscillator circuit won't run anymore which is kind mysterious.

Using the triangular output for PWM the PWM is of course disabled since the output will will stay at +6V constantly.

There is absolutely no error message.

 

[mneary]

I thought you said U1 part D was not connected anywhere.

Can you post a schematic of the example which doesn't work? Maybe try resistors on the inputs.

 

[Boncuk]

Hi mneary,

when I posted part D of the IC isn't connected anywhere I of course meant it was not connected to the circuit, more or less connected as a separate circuit.

I added U1D again and forced the output low with a voltage divider.

As you can see by the simulation result its output is about 490mV. The output pin7 (supposed to have a triangular wave form) stays constantly on VRef (+6V) supplied from U1C.

Also the CPU-load indicates only 4 to 5% whereas with the working circuit without U1D the load varies between 91 and 94% and the oscilloscope operating.

I left the simulation alone for about one hour but there wasn't any change.

I wonder for the reason because I also simulate more complex designs without any malfunction normally.

It looks like U1D is turning off the rest of the circuit.

I would have liked to try any other OpAmp, but ISIS is very limited concerning working ICs.

Regards

Hans

BTW, try to write U1 double dot D. This is what you get: U1

 

[mneary]

Thanks for the clarification. I wonder if U1D pin 13 being hooked to +12 confuses the simulator. The model might not account for that condition since the LM324 isn't within its common mode range. How about pin 12 to ground and pin 13 to the junction of R6 and R7? I know it doesn't make sense, but....

 

[Boncuk]

The only difference is the output of part D goes high if I reverse the input pins. Nothing else changes.

CPU-load remains at 4%.