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AN871 APPLICATION NOTE

Benefits

Due to these specific characteristics and properties SCR’s and triacs offer to the designer substantial ben-

efits such as:

– Easy to drive

– Low consumption drive circuit

– Ideal switch for pulse operation (ex : discharge of capacitor)

– Automatic switching off

– Reverse polarity protection (SCR)

– Bidirectional switch - current and voltage - (triac)

– High ruggedness

EXAMPLES OF APPLICATION

Pulse operation circuits

SCR’s and triacs are really the most suitable switch for high current pulse operations. They provide very

powerful solutions especially in ignition circuits.

Ignition circuits. Used for a very long time (30 years) in small engine ignition, it is today an alternative

solution to meet the new requirements in car ignition. In fact, capacitive discharge ignition circuit provides

a higher efficiency:

– energy save

– low pollution.

The various existing configurations operate similarly: the energy stored in a capacitor is transferred to a

spark gap through a transformer and a SCR.

Figure 3 and 4 show the typical ignition circuits using SCR’s.

Figure 3. Ignition circuit with SCR: storage capacitor connected in series with input voltage source

and pulse transformer

+V

(A)

C

+V

(B)

C