Memories...
ae4jm said:
Can anyone help answer my question of how this circuit flip flops with the capacitors charging and discharging? What is this doing to the transistors? Schematic is attached. Any insight is much appreciated.
The schematic is a "textbook" astable multivibrator. The "secret" to its operation lies in the fact that voltage cannot instantly change across a capacitor. It can only change as fast as the resistors in series with it will allow.
The thing that makes analyzing this circuit tricky is that you have to assume conditions to exist before explaining how they exist. In this case, the key condition is that there is a strong reverse bias on the base of one of the transistors that holds it OFF while the other transistor conducts. Let's say that Q1 is OFF and Q2 is conducting.
Because Q1 is essentially out of the circuit (neither the base or collector are conducting), C1 charges through R1 and the forward biased BE junction of Q2. That provides base current for Q2, holding it ON. R3 is much higher resistanct than R1 and has little impact on the charging rate of C1. But, when C1 is fully charged, there's no further base current available for Q2 through that path and R3 then provides the base current needed to hold Q2 ON.
Meanwhile, the reverse bias on Q1 base is changing as C2 discharges through R2 and the CE path through Q2 (the "not yet explained part"). Even though R4 is a fairly low resistance, the CE resistance of Q2 is much lower and R4 has little effect. So, the 100k resisance of R2 dominates the discharging of C2. Eventually, C2's voltage gets to the point where it can forward bias Q1's base (about +5.5V) and Q1 turns ON.
Now, here's the trick. When Q1 turns ON, the left end (in your schematic) of C1 is suddenly pulled from essentially 0V (ground) to +6V. Since C1 had just charged to about 6V and since that voltage cannot instantly change, the right end of C1 pops up to almost +12V. That strongly reverse biases Q2. In fact, this action can be a problem since most transistors have a maximum reverse voltage rating between B and E of about 5V, so it's easy to damage a transistor. With the voltages in this circuit, it's not a problem, however.
As soon as Q2 cuts OFF, it's essentially out of the circuit (neither the BE or CE paths through the transistor are conducting) and the circuit has flipped (or flopped) as now Q1 is conducting and Q2 is OFF.
Where C1 charged rather quickly through R1 and the BE junction of Q2, now C2 charges through R4 and the BE junction of Q1. While that's happening, C1 is now discharging through the CE junction of Q1 and R3. Eventually, the voltage on the base of Q2 gets down from about +12V to about +5.5V and Q2 turns ON again. When it does, the circuit flips again and now the voltage across C2 now applies a high reverse bias to the base of Q1 and the action starts again.
Since the circuit will keep changing states as long as power is applied to the circuit, it will oscillate. If the feedback from Q1 and Q2 are resistors, the multivibrator will simply stay in the state it's set for. That's the basis for computer memory(static RAM anyway...dynamic RAM is different). If the feedback for one side of the multivibrator is a resistor and the other side like the astable, you have a monostable multivibrator (also called a one shot) which flips and then flops only once each time it's triggered.
Hmmm...been awhile since I've thought of this. Good mental practice for me.
And, yes...be careful of electrolytic polarities!