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“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