Astable Multivibrator Cold Startup Process

Thread Starter

jakj

Joined Oct 27, 2018
5
I am trying to understand this simulation: http://www.falstad.com/circuit/e-multivib-a.html

I am trying to figure out why C1 suddenly begins discharging exactly between 2.84ms and 2.85ms. (It's easier to see if you turn up the current speed.) The capacitor charging current suddenly starts going right when it was going left before.

I'm looking at the transistors' Vce and they go down (at slightly different rates, like in reality, which is why it starts oscillating). It's when Q1/Q2 Vce are both in the 60mv range and C1/C2 are in the 610mv range that the discharge starts. And then they go back and forth with each oscillation taking longer until they stabilize and the circuit is in normal operation.

I'm not looking for an explanation of the oscillation: There are dozens of those explanations everywhere. I'm trying to figure out how this thing gets started from cold and reaches its equilibrium. The only thing I've been able to find so far is "one of the transistors will conduct better than the other, driving one to saturation and the other to cutoff" but that's no explanation at all.

I feel like if I could just understand why the capacitor switches that first time, I might understand the whole thing. Hopefully.
 

Thread Starter

jakj

Joined Oct 27, 2018
5
I've already seen those. I'm looking for an explanation from circuit start, when power is first applied, not the main cycle after everything has gotten going. At the very start of "time", both transistors are on and both capacitors are charging, and then something<--(the thing I don't understand) causes one to start discharging which starts an increasing oscillation until the circuit stabilizes into its normal oscillation. I know it's "small differences in manufacture, thermal energy randomness, etc.", I know *why* but not *how*. I've watched that simulation in slow motion and I cannot see what causes the one to start discharging the very first time.
 

ericgibbs

Joined Jan 29, 2010
18,766
hi jaki,
Under certain conditions, when all the component voltages are in balance, the astable will not start running.
Its only the initial imbalance of the circuit that starts it running.
E
 

Jony130

Joined Feb 17, 2009
5,487
This circuit will not start oscillation only if you would have a perfectly symmetrical circuit (all components are perfectly matched).
In reality, it cannot happen because this circuit has a positive feedback loop. So, even the smallest difference together with the positive feedback ensures proper start-up.

And this type of situation will only happens in PSpice simulations software.

When I do a simulation and use the ideal components the circuit do not want to start.

10.png

But when I add tiny asymmetric into the circuit (Rc2 = 330.00001Ω) The circuit starts to work.

12a.png


We have a similar situation in many circuits. For example in this NAND gate RS flip-flop circuit:

11.png

Can you tell me what logic state we will have at the Q1 and Q2 output after the first power-up?
 

MrChips

Joined Oct 2, 2009
30,706
For example in this NAND gate RS flip-flop circuit:

View attachment 162837

Can you tell me what logic state we will have at the Q1 and Q2 output after the first power-up?
Theoretically, it is indeterminate what state the flip-flop will attain after power-up.
In practice, the two gates might have different switching speeds and the flip-flop will power-up in a preferred state. In a real design, don't depend on it.
 

Thread Starter

jakj

Joined Oct 27, 2018
5
I appreciate everybody replying, but I already clearly understand that small differences in reality cause the oscillation when an ideal situation would not. I am trying to understand the process the circuit goes through due to those small differences. I am trying to understand how the voltages and currents go through the circuit in the process of stabilizing into a regular oscillation. I am trying to actually understand this rather than simply accepting it as functional and moving on.
 

ericgibbs

Joined Jan 29, 2010
18,766
hi jaki,
Can you imagine all the possible combinations of what you are asking in starting the astable.?
Any one of the components could be responsible.
Not counting voltage and temperature variations,
E
 

crutschow

Joined Mar 14, 2008
34,280
Remember that is is an astable circuit. It's rather like balancing a pin on it's point.
In theory you could select a position where it would stay balanced but, in reality it will always fall over due to some slight imbalance.
The circuit is similar. The slightest imbalance of any sort will cause it to start oscillating.
 
Top