Using LEOs and Power s1,1pply configurations.

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Using LEOs

When using a light-emitting diode (LED) as an indicator, use the following formula to determine series resistance for various voltages: R = (V- 1·7) x 1000 +/,where R is resistance in ohms, Vis supply voltage (d.c.!), and I is LED current in milliamps.

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To operate a LED directly from the 240V mains, a better scheme is to use the second circuit shown. In this, a capacitor is used as a voltage dropping element. A 1N4148 diode or similar across the LED provides the rectification required. As the voltage drop across the LED is negligible compared with the supply, capacitor current is almost always exactly equal to mains voltage divided by capacitive reactance X c.

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At 50Hz, 0-47!!Fwill result in a LED current of about 16mA. Resistor R, is included to limit turn-on transients. A value of 270 ohms should be adequate.

Power s1,1pply configurations

No circuit losses are allowed for. At low voltages allow for 0·6 V diode drop.

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Voltage multiplier circuits

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Zener diodes

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Voltage regulators

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Variable voltage types eg 317,338 series

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Op-amp standard circuits

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Bridge circuits in measurement

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