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designideas are suggested. For a different LED operation voltage, some parameters should be recomputed in the same way as in the foregoing analysis. The lower the LED operation voltage is, the lower the ac input voltage can be. This Design Idea can also apply to ac at 60 Hz. 1. Use high-voltage through-hole resistors or series surface-mount resistors to achieve at least 400V withstand. A fuse is suggested for safety against shorts. 2. Safety warning for novice experimenters: Lethal voltages are present in this circuit; use caution when testing and operating it. If scoping, use an isolation transformer to float the circuit’s ac input from earth ground; do not float the oscilloscope chassis. The scope ground cannot be connected to the circuit without isolation. 3. Do not push the button with ac voltage applied. For safe maintenance, keep pressing the button to discharge C1 through R10 until D8 goes out.EDN References 1 Sheard, Steve, “Driver circuit lights architectural and interior LEDs,” EDN, Aug 11, 2011, pg 41, www.edn.com/4368306. 2 Babu, TA, “Offline supply drives LEDs,” EDN, April 21 2011, pg 58, www.edn. com/4369648. Battery-operated equipment often will benefit from a power-on indicator. The indicator, however, can waste significant Figure 4 The yellow and blue traces, respectively, present the voltage across C1 and R6 in the circuit at 220VRMS (at 50 Hz ac). The two traces remain at the same position when the ac input changes from 96VRMS to 260VRMS. power. In situations where a lowduty cycle blinking indicator provides an adequate indication of the power being turned on, the simple circuit described here should prove useful. A tiny, single-gate Schmitt-trigger logic inverter, the SN74AHC1G14, together with two resistors, a Schottky diode, and a capacitor form the timing generator of the blinker, shown in Figure 1. The output waveform has a period of about 0.5 sec and a very low duty-cycle value, of around 1%. The interval of lowoutput duration, TL, of the generator is expressed as TL=RTC×ln 1+ 2 −1 VCC VHYST RT 240k VCC TANTALUM SOLID RCH 2.2k 4.4V w ww.edn-europe.com MARCH 2013 | EDN Europe 39 EDNEQUATION DIANE , where VHYST is the hysteresis voltage at the input of IC1 and VCC is the supply voltage of IC1. For VCC=4.5V, the typical value for VHYST is 0.75V. For the required value of TL=0.5 sec, a value for RT of 200k was selected. The value of the timing capacitor, C, can be calculated from the equation, with a small amount of algebraic rearranging, as 7.45 μF. The nearest standard value is 6.8 μF; a tantalum solid-electrolytic capacitor is used for this value. To achieve the low duty cycle of the generator, the high-output duration, TH, is shortened by speeding up the time to charge capacitor C. This is done through the additional resistor, RCH, and the series-connected Schottky diode, D1S. The forward voltage drop at D1S is no more than 200 mV and can be neglected. The LED is on for approximately (1/100)×TL≈5 msec. Low-duty-cycle LED flasher keeps power draw at 4 mW Marián Štofka, Slovak University of Technology, Bratislava, Slovakia EDN DI5328 Fig 1.eps DIANE GND IC1 C + 6.8 μF 100 nF D1S TMM BAT42 D2S TMM BAT42 D1 1N914 D2 1N914 RS 68 RB 2k RE 2k Q1 LED 2N4403 Q2 2N3904 NOTE: LED IS A HIGH-RADIANCE TYPE. Figure 1 Q1 and Q2 function as a current source and push a constant current through the LED regardless of its forward voltage drop (within the compliant voltage limitations). The Schmitt inverter forms a classic square-wave generator, modified with RCH and D1S to produce an asymmetrical output.


EDNE MARCH 2013
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