This is only a preview of the December 2020 issue of Silicon Chip. You can view 0 of the 112 pages in the full issue. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Power Supply for Battery-Powered Vintage Radios":
Items relevant to "Dual Battery Lifesaver":
Items relevant to "A Closer Look at the RCWL-0516 3GHz Motion Module":
Items relevant to "Balanced Input Attenuator for the USB SuperCodec, Part 2":
Articles in this series:
Items relevant to "Automated tyre inflator/deflator":
Items relevant to "Infinite impedance AC source":
Items relevant to "Controlling model railway points with a servo":
Items relevant to "Flexible Digital Lighting Controller, part 3":
Purchase a printed copy of this issue for $10.00. |
“Infinite impedance” AC source
The resonant circuit shown in the
upper part of the adjacent diagram,
when driven with a sinusoidal voltage, will deliver a constant amplitude
alternating current into a wide range
of resistive loads. The mathematical
proof that this is the case is given in
the PDF at www.siliconchip.com.au/
Shop/6/5350
The concept of a constant alternating current may seem like an oxymoron, but it is not. After all, a constant
direct current source or sink is just
a load-independent current. So why
not a load-independent alternating
current? The theory is not new, but I
wanted to test it in reality.
If you’re wondering what “infinite
impedance” means, consider that a
zero impedance has an unchanging
voltage amplitude regardless of the
load current. A circuit with infinite
impedance is the opposite; it delivers an unchanging current amplitude,
regardless of the voltages it needs to
generate to do so.
As the equations are a bit complicated, I thought it would be handy to visualise the multi variables in a graphical
form, bypassing the heavy-duty mathematics. I used the GNUplot software
to generate the graphics (available free
from www.gnuplot.info/).
These plots are also in the PDF linked
above, in Appendix A. These plots can
88
Silicon Chip
be used as a starting point for circuit
design. I had 60nF capacitors and a
1.25mH inductor at hand. From the L/X
graphic shown below, the point corresponding to these components gives an
operating frequency of around 18kHz
and an impedance of around 145W.
Substituting these component values into the equations from the proof
gives a frequency of 18.4kHz, and the
impedance as 144W.
To drive this resonant circuit, I used
the Touchscreen DDS Signal Generator (April 2017; siliconchip.com.au/
Article/10616) and an LMC6482AIN
op amp, as shown in the lower circuit.
For measuring and monitoring the
output, I used a True RMS auto-rang-
Australia’s electronics magazine
ing DMM and a two-channel digital
storage oscilloscope.
To prove that the load current of
the resonant circuit is independent of
the load resistance, I carried out three
tests, with 50W, 100W and 200W load
resistors. The PDF mentioned above
has screengrabs showing these three
test conditions. Each time the load resistor value was doubled, the output
voltage also doubled, thus maintaining a constant output current.
The amplitude of the output current
can easily be controlled by adjusting
the ratio of the op amp feedback resistors, R2:R1. A power op amp or audio amp could be used instead of an
op amp, to allow for higher currents.
siliconchip.com.au
A simple control loop and added
synchronous rectification could make
this circuit useful for driving LEDs.
Other applications await.
Note that another way to achieve a
similar result would be to use an op
amp monitoring the voltage across
a shunt in series with the load, and
using negative feedback to provide
the required drive voltage to match
the shunt voltage to a reference sinewave.
However, such a circuit may suffer
from stability problems, necessitating added compensation components
which would reduce its precision.
Mauri Lampi,
Glenroy, Vic. ($90)
Controlling model railway points with a servo
This controller was created to operate a set of points (or turnout) on a OO/
HO model train layout using a small
9g model servo (eg, Jaycar YM2758).
For simplicity, each set of points has
its own small microcontroller with
just three inputs. Potentiometers VR1
and VR2 set the two positions, while
switch S1 selects between them.
It runs from a DC supply of at least
9V and 1A. This is reduced to 5V by
7805 regulator REG1, which is adequate to operate a 9g servo. Power for
the microcontroller is decoupled by
schottky diode D2 and a 220µF filter
capacitor, to prevent motor current
surges affecting its operation.
The controller should be close to the
servo and the points, due to the weak
drive and to minimise power losses in
the wires. So switch S1 may be several metres away, and thus its wiring
is susceptible to interference.
Circuit
Ideas
Wanted
siliconchip.com.au
The 1kW/100nF RC low-pass filter
between S1 and the GP3 input of IC1
(pin 4) reduces the effects of EMI, and
it is advisable to use twisted pair wires
and/or grounded shielding to further
reduce the chance of interference.
Potentiometers VR1 and VR2 are
wired across the micro's supply, and
the wiper voltages are stabilised by
100nF capacitors which perform two
functions. They reduce the effects of
stray electric fields and also provide
the low source impedance required by
the micro's internal analog-to-digital
converter (ADC). The servo signal has a
330W series resistor to protect IC1 from
accidental shorts at CON1.
A heartbeat LED, LED1, flashes to indicate when the circuit is operational.
Setup is simple. With the power off,
set VR1 & VR2 to their mid positions
and the points also in their mid positions. Turn the power on and adjust
one potentiometer to set the points to
"ahead" or "turn". Then change the position of switch S1 and adjust the other potentiometer. The points are then
operational, controlled by S1.
Note that if the points are hard
against either end position and the
servo is trying to move the points
more, the servo will be destroyed in
little time. To prevent this, the mechanical link between the servo and
the points should not be rigid. You can
use an open-coil spring or provide a
U-shaped loop so that there is some
compression or extension of the link.
The software was written in PICBASIC Pro. The Servo_Dual_Posn_
SC.BAS and Servo_Dual_Posn_
SC.HEX files are available from
siliconchip.com.au/Shop/6/5638,
along with a PDF of the PCB pattern.
George Ramsay,
Holland Park, Qld. ($80)
Got an interesting original circuit that you have cleverly devised? We will pay good money to
feature it in Circuit Notebook. We can pay you by electronic funds transfer, cheque or direct to
your PayPal account. Or you can use the funds to purchase anything from the SILICON CHIP Online
Store, including PCBs and components, back issues, subscriptions or whatever. Email your circuit
and descriptive text to editor<at>siliconchip.com.au
Australia’s electronics magazine
December 2020 89
|