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SN74HC132: accepts 12V input from fan tach signal?

Art Mecina
Genius 16585 points
Part Number: SN74HC132
Other Parts Discussed in Thread: SN74HCS00

Hello Guys,

Good day.

Our customer is using the SN74HC123 in the fan tach monitoring application. His fan tach is using 12V and the SN74HC123 is a 5V device.
Is the input clamped to Vcc, i.e. 5V ? Can the input accept a 12 volt fan tach signal?

Thanks and regards,

Art

  • 0
    TI__Guru* 96681 points

    Hi Art,

    Although HC inputs do include a positive clamp diode, I would recommend using a voltage-divider circuit to reduce the input signal to ~5V prior to applying it to the device. This will prevent any transients from exceeding the absolute maximum rating of the device, and it will reduce or eliminate any current that would be flowing from the 12V supply to the 5V supply, which sometimes can be problematic in systems.

    Also, I would recommend switching to the newer SN74HCS00, which is also a quad 2-input NAND gate with Schmitt-trigger inputs.