I'm trying to understand the basic flip/flop circuit as explained here : http://www.electronics-tutorials.ws/waveforms/astable.html
I'm having some trouble understanding how the capacitors work in the circuit, especially the negative voltage shown on my meter. I've highlighted the pieces of text that confuse me and the questions I'm trying to answer.
I'm a bit puzzled by the following piece of text:
The instant that transistor, TR1 switches "ON", plate "A" of the capacitor immediately falls to 0.6 volts.
The way I understood it
- If a transistor is turned OFF, it's base is < 0,6V and its collector will be high (reaching Vcc).
- If a transistor is turned ON, it's base is at around 0,6V, and its collector will be close to 0V. (on my demo circuit it is reading 0,08V).
Now, coming back to the quoted text. If TR1 switches on, why would plate "A" of the capacitor C1 fall to 0,6V ? Why doesn't it fall to 0V ?
I would imagine that the C2 plate connected to the TR1 base would fall to 0,6V, but not the plate A of C1.
Another thing I found puzzling was :
This fall of voltage on plate "A" causes an equal and instantaneous fall in voltage on plate "B" therefore plate "B" of the capacitor C1 is pulled down to -5.4v (a reverse charge) and this negative voltage turns transistor TR2 hard "OFF". One unstable state.
OK, so before, when TR1 was off, I can imagine plate A being close to Vcc (as it is connected to the collector of the transistor that is turned off), and plate B being 0,6V (as it is connected to the base of the transistor that is turned on).
So when TR1 turns on, the collector voltage of TR1 will drop, as will the voltage on plate A.
So how am I suppose to read this negative voltage on the capacitor ?
- plate A drops from 6V -> 0,6V (a 5,4V drop)
- plate B also needs to drop 5,4V ? from 0V to -5,4V ?
I understand that this negative voltage will cause TR2 off.
Capacitor C1 now begins to charge in the opposite direction via resistor R3 which is also connected to the +6 volts supply rail, Vcc, thus the case of transistor TR2 is moving upwards in a positive direction towards Vcc
So here I guess we're talking about C1 charging from -5,4V to 0,6V (the point that turns TR2 back on). If somebody could point me to an article / explanation that explains how I should interpret this negative voltage that would be great.
I guess I'm having trouble with
- the negative voltage.
- the charging (in different directions ?) / discharging of the caps.
- the fact that plate A of C1 drops to 0,6V.