Of blader dit eens door:Multivibrator
The circuit has two states:
State 1':
- Q1 switched on
- Collector of Q1 at 0V
- C1 charging through R2 (and Q1)
- Voltage at base of Q2 is the voltage across C1. This is initially low, but rising as C1 charges.
- Q2 switched off (assuming base voltage < 0.6V)
- C2 discharging through R3 and R4
- Output voltage high (although a little lower than the supply voltage because of the C2 discharge current through R4)
- This state is self-maintaining until the voltage at base of Q2 reaches 0.6V, at which point Q2 switches on, and the circuit goes into the following state.
State 2
- Q2 switched on
- Collector of Q2 (output voltage) goes from +V to 0V
- This step change on C2 causes a negative going pulse on the base of Q1, which rapidly switches it off.
- Q1 switched off, its collector rises to about +V.
- C1 discharging through R1 and R2
- C2 charging through R3 from -V through 0v to +0.6v (this might be considered a discharge rather than a charge)
- Voltage at base of Q1 is the voltage across C2. This is initially low, but rising as C2 charges.
- This state is self-maintaining until the voltage at base of Q1 reaches 0.6V, at which point Q1 switches on, and the circuit flips back into state 1.
Initial power-up
When the circuit is first powered up, neither transistor will be switched on. However, this means that at this stage they will both have high base voltages and therefore a tendency to switch on, and inevitable slight asymmetries will mean that one of the transistors is first to switch on. This will quickly put the circuit into one of the above states, and oscillation will ensue.
Period of oscillation
Very roughly, the duration of state 1 (high output) will be related to the time constant R2.C1 as it depends on the charging of C1, and the duration of state 2 (low output) will be related to the time constant R3.C2 as it depends on the charging of C2 — and these time constants need not be the same, so a custom duty cycle can be achieved.
However, the duration of each state also depends on the initial state of charge of the capacitor in question, and this in turn will depend on the amount of discharge during the previous state, which will also depend on the resistors used during discharge (R1 and R4) and also on the duration of the previous state,
etc. The result is that when first powered up, the period will be quite long as the capacitors are initially fully discharged, but the period will quickly shorten and stabilize.
The period will also depend on any current drawn from the output.
Because of all these inaccuracies, more sophisticated timer ICs are commonly used in practice, as described above. Gevonden op:
how-to.wikia.com/wiki/How_to_build_an_oscillator_circuit
