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# Problem simulating astable 555 circiut

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#### joe31

##### New Member
I´ve got the following problem in simulating an astable 555 circuit:

The frequency of the output signal varies.
It starts normal with a longer time for the first charging of
the 10µF capacitor. The next period seems to be normal but
then the time of the periods increases steadily until it suddenly
goes back to start again...

It should work at 1 Hz.
The pause time I need is 1/10 sec.
So the R1 has to be 14,43 kOhms and R2 has to be
115,42 kOhms.
Vcc = 9V and I put a capacitor between pin 5 and ground.

I tried other times and values even with different programms (multisim2001, circuit maker) but the output signal was always
weird.

Has anyone experienced the same problems or has an answer?

Joe

I have never had a problem simulating 555s. I use Linear Technology's SwitcherCAD III, which is free and works really well.
When you start a 555 multivibrator, the first part of the first cycle will be longer than all the rest, because the timing capacitor starts charging from zero volts, while on all successive cycles, it starts from VCC/3 (3 volts in your case). It sounds like you are experiencing more than this, which I can't explain. If this is your problem, and it really bugs you, you can put an initial condition on the capacitor. Different versions of Spice handle this in different ways. In SwitcherCAD III, you name the node (let's call it "cap"), and declare a Spice directive: .IC v(cap)=3.

joe31 said:
I´ve got the following problem in simulating an astable 555 circuit:

The frequency of the output signal varies.
It starts normal with a longer time for the first charging of
the 10µF capacitor. The next period seems to be normal but
then the time of the periods increases steadily until it suddenly
goes back to start again...

It should work at 1 Hz.
The pause time I need is 1/10 sec.
So the R1 has to be 14,43 kOhms and R2 has to be
115,42 kOhms.
Vcc = 9V and I put a capacitor between pin 5 and ground.

I tried other times and values even with different programms (multisim2001, circuit maker) but the output signal was always
weird.

Has anyone experienced the same problems or has an answer?

Joe

Can you post the circuit you are feeding your simulator? Sinc eyou mentioned that other simulators show the same wierd behavior, maybe there is a fundamental issue with the way it is being modelled.

I would suspect an issue with the 555 model. If needed, I have one that works properly.

Thank you all for your replies!

I made an update of multisim and now it works fine!

Joe

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