17 Jun
2015
17 Jun
'15
2:14 p.m.
On 2015-06-17 13:28, Dave G4UGM wrote:
I found it easier to think of it in DC terms. So the
Cap charges through R5
+ R3 and R9 + R8.
As the Cap charges the voltage on the base of Q1 rises until it turns on,
which then turns on Q2.
While the cap charges, it steals the base current which
would otherwise
have gone to Q1, thus keeping Q1 turned off. When the cap nears the end
of the charge, more current goes to the base of Q1 which turns on,
turning on Q2, which raises the voltage over R8 and R9. Since the
voltage on a capacitor cannot change instantaneously, the voltage on the
base of Q1 rises while the cap discharges through the base of Q1,
keeping it hard on. As the cap discharges and charges in the reverse
direction, the base current of Q1 decreases and ultimately Q1 turns off,
turning off Q2 and lowering the voltage over R8 and R9, and the cycle
starts over.
For the circuit to work, I think (I may be wrong) the base current
supplied to Q1 by R5 and the pot has to be not quite sufficient to turn
it on. Also the cap is reverse charged for one half cycle.
I believe this is a classical astable multivibrator circuit, but not the
more common one with two cross-coupled transistors with capacitors from
the collector of one to the base of the other. The DEC circuit I think
can be seen a lot in old Siemens application books from the 1960s, such
as may be found here (note German books):
http://rainers-elektronikpage.de/SIEMENS-Fach---u_-Datenbucher/siemens-fach…
/Jonas
At this point the cap is then charged (or discharged) in the reverse
direction via Q2, D5 and R4 until Q1 turns off.....
At first glance I thought R9 might be there to
provide some hysteresis in
the
switching thresholds for the RC charge/discharge
but it looks like it acts
in the
opposite direction to that.
The base circuit of Q3 (the first stage of buffering) will draw current
from the
high-impedance side (R8,R9) of the oscillator
output, pulling up the C5,R9
junction when Q2 is off, so it will probably affect the oscillator and be
necessary to get the 'proper' functioning of the oscillator portion of the
circuit.
I included that in my LTSpice model....
... but it doesn't actually have that much effect...
Dave
G4UGM