1. Semiconductors

25. Reverse biased pn junction When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown .

26. Diodes Have Polarity ( They must be installed correctly .) The PN Junction Diode Anode (+) Cathode (-)

29. Diode Ratings Rating Abbreviation Designated As Significance Breakdown Voltage V BR PIV, PRV, V BR , or V RRM Voltage at which avalanche occurs; diode is destroyed if this rating is exceeded. Average Forward-Current I O I O Maximum allowable average current. Maximum Forward-Surge Current I FSM I FSM Maximum instantaneous current. Maximum Reverse Current I R I R Maximum reverse current.

31. Signal Diodes (small current) Signal diodes are used to process information (electrical signals) in circuits, so they are only required to pass small currents of up to 100mA. General purpose signal diodes such as the 1N4148 are made from silicon and have a forward voltage drop of 0.7V. Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V and this makes them suitable to use in radio circuits as detectors which extract the audio signal from the weak radio signal.

32. Rectifier Diodes (large current) Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC), a process called rectification. They are also used elsewhere in circuits where a large current must pass through the diode. All rectifier diodes are made from silicon and therefore have a forward voltage drop of 0.7V. The 1N4001 is suitable for most low voltage circuits with a current of less than 1A.

33. Bridge Rectifier They have four leads or terminals: the two DC outputs are labelled + and -, the two AC inputs are labelled . converts AC to DC

36. LED The cathode is the short lead and there may be a slight flat on the body of round LEDs Never connect an LED directly to a battery or power supply! It will be destroyed almost instantly because too much current will pass through and burn it out. LEDs must have a resistor in series to limit the current to a safe value, for quick testing purposes a 1k resistor is suitable for most LEDs if your supply voltage is 12V or less. Remember to connect the LED the correct way round!

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