G
gearhead
Guest
I built this voltage regulator a while back:
http://jhau.maliwi.de/mot/voltreg.htm
with the difference that I flipped the
entire circuit's polarity to
put the field switch in the ground,
because my bike generator's field switched
on the low side.
Notice especially the author writes:
"R4 is a feedback resistor that enhances
the switching characteristics. When you decrease
its resistance, switching occurs steeper, but
the hysteresis window (i.e., the voltage
that you need to reduce until the circuit
switches back on) is larger.
"Capacitor C1 stabilises the circuit at higher load.
I have observed that the switching behaviour
of the circuit without this capacitor is pretty
"sharp" and stable if the load draws less than
ca. 0.5 A (typically); however at higher current
I observed a tendency to oscillate. This can be
observed best with an oscilloscope, or a 12 V lamp
as load: it will start to glow, instead of being
switched completely on or completely off.
C1 eliminates this instability; a value of 100 nF
to 220 nF should do the job."
I built the circuit with a mosfet and observed the
same effect -- at higher loads, instead of switching
visibly on and off, the bulb in the test circuit
dims.
I no longer have that bike and the voltage regulator
I built, but it must have occupied a mental
back burner all this time because I recently had an
idea how to improve the circuit.
Although C1 alleviates the circuit's tendency to
oscillate, it surely slows every on-off transition.
It occured to me to reduce the value of R4 by an
order of magnitude. Then move C1 from its location
in the original circuit and put it in series
with R4 instead.
The small value for R4 ought to sharpen switching
enough to avoid the parasitic oscillation that
characterized the original circuit.
Now, the presence of a capacitor in the feedback
path will make the positive feedback transient.
Using R4=10k and C4=0.1uF for example would result
in a switching frequency on the order of 1 kHz (with
sharper switching events than the original circuit).
During calibration one could leave out
C1 and R4 and set the voltage in linear operation.
Then install C1 and R4; and
the circuit will oscillate around the setpoint.
http://jhau.maliwi.de/mot/voltreg.htm
with the difference that I flipped the
entire circuit's polarity to
put the field switch in the ground,
because my bike generator's field switched
on the low side.
Notice especially the author writes:
"R4 is a feedback resistor that enhances
the switching characteristics. When you decrease
its resistance, switching occurs steeper, but
the hysteresis window (i.e., the voltage
that you need to reduce until the circuit
switches back on) is larger.
"Capacitor C1 stabilises the circuit at higher load.
I have observed that the switching behaviour
of the circuit without this capacitor is pretty
"sharp" and stable if the load draws less than
ca. 0.5 A (typically); however at higher current
I observed a tendency to oscillate. This can be
observed best with an oscilloscope, or a 12 V lamp
as load: it will start to glow, instead of being
switched completely on or completely off.
C1 eliminates this instability; a value of 100 nF
to 220 nF should do the job."
I built the circuit with a mosfet and observed the
same effect -- at higher loads, instead of switching
visibly on and off, the bulb in the test circuit
dims.
I no longer have that bike and the voltage regulator
I built, but it must have occupied a mental
back burner all this time because I recently had an
idea how to improve the circuit.
Although C1 alleviates the circuit's tendency to
oscillate, it surely slows every on-off transition.
It occured to me to reduce the value of R4 by an
order of magnitude. Then move C1 from its location
in the original circuit and put it in series
with R4 instead.
The small value for R4 ought to sharpen switching
enough to avoid the parasitic oscillation that
characterized the original circuit.
Now, the presence of a capacitor in the feedback
path will make the positive feedback transient.
Using R4=10k and C4=0.1uF for example would result
in a switching frequency on the order of 1 kHz (with
sharper switching events than the original circuit).
During calibration one could leave out
C1 and R4 and set the voltage in linear operation.
Then install C1 and R4; and
the circuit will oscillate around the setpoint.