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# Multisim 8 question - negative voltages?

M

#### Marcin Szczepaniak

Jan 1, 1970
0
Hi!

I am trying to simulate some kind of regulated power supply using Multisim 8.

Look at the circuit (It amplifies voltage at 200 miliohm serial resistor)

It is minimal circuit which shows my problem.

As you can see at picture - LM324 produces negative voltage relating to ground!
But it is impossible - opamp is powered with +12 and 0V voltages.

When i change LM324 with 'virtual opamp' it works as expected - there is no
negative voltage at output.

I know that LM324 is far from ideal but i think it doeesn't have ability to
produce negative voltages, especially when there is no capacitor or inductor in
this circuit.

My question is - what am i doing wrong? What should i set in Multisim to have
correct simulation ?

greetings
Marcin

F

#### Fred Bloggs

Jan 1, 1970
0
I am trying to simulate some kind of regulated power supply using Multisim 8.

Look at the circuit (It amplifies voltage at 200 miliohm serial resistor)

It is minimal circuit which shows my problem.

As you can see at picture - LM324 produces negative voltage relating to ground!
But it is impossible - opamp is powered with +12 and 0V voltages.

When i change LM324 with 'virtual opamp' it works as expected - there is no
negative voltage at output.

I know that LM324 is far from ideal but i think it doeesn't have ability to
produce negative voltages, especially when there is no capacitor or inductor in
this circuit.

My question is - what am i doing wrong? What should i set in Multisim to have
correct simulation ?

Who knows- are you doing DC analysis with an unrealistic limit on the
maximum number or iterations? IIRC, the LM324 is problematic for SPICE
in general, and the real part has a tough time getting Vo below 0.7V
while sinking currents in the 3mA range anyway, and your circuit
requires it to go to 0.24V- not going to happen.

W

#### Winfield Hill

Jan 1, 1970
0
Hi!

I am trying to simulate some kind of regulated power supply using Multisim 8.

Look at the circuit (It amplifies voltage at 200 miliohm serial resistor)http://szczepan.sieradz.info/tmp/multisim.jpg

It is minimal circuit which shows my problem.

As you can see at picture - LM324 produces negative voltage relating to ground!
But it is impossible - opamp is powered with +12 and 0V voltages.

When i change LM324 with 'virtual opamp' it works as expected - there is no
negative voltage at output.

I know that LM324 is far from ideal but i think it doeesn't have ability to
produce negative voltages, especially when there is no capacitor or inductor in
this circuit.

My question is - what am i doing wrong? What should i set in Multisim to have
correct simulation ?

How much negative voltage? Spice models like to include input offset
voltage, and often they use the part's worst-case spec'd value. As
for the output range, most opamp models fail to accurately limit near
the part's voltage extremes, which is ground in the case of the
LM324.
If you study the model, you'll see why it goes negative. But if the
model has any integrity at all, it should be setup so it can't go far.

F

#### Fred Bloggs

Jan 1, 1970
0
Winfield said:
How much negative voltage?

Almost -4V at a gain of 1....

Spice models like to include input offset
voltage, and often they use the part's worst-case spec'd value. As
for the output range, most opamp models fail to accurately limit near
the part's voltage extremes, which is ground in the case of the
LM324.

Input right at Vcc/2...
If you study the model, you'll see why it goes negative. But if the
model has any integrity at all, it should be setup so it can't go far.

Not a viable suggestion....

J

#### John Larkin

Jan 1, 1970
0
Hi!

I am trying to simulate some kind of regulated power supply using Multisim 8.

Look at the circuit (It amplifies voltage at 200 miliohm serial resistor)

It is minimal circuit which shows my problem.

As you can see at picture - LM324 produces negative voltage relating to ground!
But it is impossible - opamp is powered with +12 and 0V voltages.

When i change LM324 with 'virtual opamp' it works as expected - there is no
negative voltage at output.

I know that LM324 is far from ideal but i think it doeesn't have ability to
produce negative voltages, especially when there is no capacitor or inductor in
this circuit.

My question is - what am i doing wrong? What should i set in Multisim to have
correct simulation ?

greetings
Marcin

You have a bad simulator *and* a bad opamp model. A real-life circuit
can't do this.

John

M

#### Marcin Szczepaniak

Jan 1, 1970
0
Dnia 10.02.2007 John Larkin said:
You have a bad simulator *and* a bad opamp model. A real-life circuit
can't do this.

So which simulator should i use to have proper results?
Could you recomend some software to me ?

W

#### Winfield Hill

Jan 1, 1970
0
Marcin said:
So which simulator should i use to have proper results?
Could you recommend some software to me ?

I don't agree with John's remark that your results
are due in part to a poor spice simulator. It's the
opamp model you're using that's at fault. The best
simulator package (most use the same Berkeley spice
engine, with minor enhancements) can have a few poor
models in its library, and even the worst simulator
surely has a few excellent models in its library.

Most simulators let you add models to their library,
and most serious spice users spend plenty of time
seeking better models or making our own. Here, you
need a better LM324 macromodel.

One suggestion: make a schematic of your '324 model
by translating its netlist to a drawing. This will
let you see for example how its output stage works.

One other suggestion: test your models first with
some simple test circuits. Then you'll know in
advance if you can place much faith in the results.

J

#### John Larkin

Jan 1, 1970
0
I don't agree with John's remark that your results
are due in part to a poor spice simulator.

What's bad about Multisim (aside from the price) is bait-and-switch
business practices, rotten support, and buggy device models.

John

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