Published:2009/7/9 21:16:00 Author:May | From:SeekIC
Three-terminal regulator device (LM78LXX) has Vout=Vin, until the input rises 1.5 to 2 V above the output when the regulated voltage Vreg = XX is obtained. A differential of 1.5 V between input and output is necessary to light the LED. Thus, the LED lights when Vin rises above Vreg + IR + 1.5 V, where I is typically 6 mA (a zener diode could be used in place of R). For input voltages much higher than necessary to light the LED, a current-limiting resistor in series might be necessary. A useful automotive application is shown in Fig. 14-9(b). The circuit indicates when battery voltage is above 13.5 V which indicates (in conjunction with gn ammeter) whether the alternator/regulator/battery system is operating correctly. With the engine off, the battery voltage drops to 12 V and the LED extinguishes. The circuit requires no calibration.
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