of the configuration and that the 555 output was
driving a transformer connected to ground.
This meant that the 555 output was
essentially shorted to ground for 98% of the
time. I first simply moved one side of the
transformer to +V which corrected the heating
issue, but I still didn’t get as high a voltage.
The second change I made was to increase
the value of R2 to 10K. With the 1K in the
circuit, I measured about 50V across C2 (with
Vcc = 5V). With 10K, I measured about 80V.
The last changes were to insert a PNP buffer
between the 555 and transformer and then
insert a 100K potentiometer in series with R2.
These two changes allow me to adjust the
voltage from about 10V to over
100V with a 5V supply.
Replacing the 555 in
Schematic 14 with the PIC 555
will yield similar results. Use
mode 5 and range 0 with a fixed
frequency of about 2 kHz.
With a 100K load, a duty
cycle of 90% yields about 23V,
while a duty cycle of 50% yields
70V.
Lower duty cycles don’t give
much higher voltages. Keep in
mind that the PIC duty cycle is
based on the ratio of output on
(high) to period. With the PIC
output on, the 2N3906 is off.
That’s a Wrap
I have enjoyed writing this series
and would like to thank Nuts & Volts
for asking me to write it. I’ve learned
quite a bit about the 555 and its
many uses from reading and
implementing the circuits described
in 555 Timer IC Circuits by Mr. Mims
and hope that you have as well.
If you have any questions about
the PIC 555 replacement or any of
the examples in this series, feel free
to email me at k3pto@arrl.net. NV
SCHEMATIC 14. Modified 555 DC-DC converter.
SCHEMATIC 13. 555
DC-DC converter.
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