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Thread: Fan out current of a 555?

  1. #1
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    Default Fan out current of a 555?

    Does anyone know what this is offhand? A quick Google doesn't show it.

    I'm using a 555 to give the lasers a test 50-50 duty cycle and just wondering how many I can drive from one chip without adding a tranisitor to up the current.

    TIA
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  2. #2
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    mixedgas is offline Creaky Old Award Winning Bastard Technologist
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    Quote Originally Posted by 450nm View Post
    Does anyone know what this is offhand? A quick Google doesn't show it.

    I'm using a 555 to give the lasers a test 50-50 duty cycle and just wondering how many I can drive from one chip without adding a tranisitor to up the current.

    TIA
    Fairchild NE55 data sheet says this

    Low Output Voltage VOL
    VCC = 15V
    ISINK = 50mA
    VCC = 5V
    ISINK = 5mA - 0.05 0.35 V

    High Output Voltage VOH
    VCC = 15V
    ISOURCE = 200mA
    VCC = 5V
    ISOURCE = 100mA

    depending on which 555 you have its between 2 and 8 LS TTL for the fanout

    I often end up adding a 1K pullup to 5V on pin 3 when driving old TTL.

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