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Help with optocoupler optocoupler connect to schmitt trigger 74HC14

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Zero999:
How many channels do you need? The 74HC14 has six and is cheap.

I'm not sure what you're asking. The output impedance of the 74HC14 depends on the supply voltage. At low supply voltages the impedance increases, as there isn't so much voltage drive on the output transistors' gates. At higher supply voltage, the transistors' gates are driven more hard on, so the output impedance is much lower. This means if an output is shorted to 0V and is set high, the current will increase exponentially with the supply voltage.

I couldn't find a specification for 10V, just 2V, 4.5V and 6V. Refer to figure 33 on page 34 of the 74HC family data sheet. At a die temperature of 25oC, when the output is set low and the output is shorted to the positive rail, i.e. it has the same voltage as the supply, the output current will typically be 6.2mA, 32mA and 58mA for supply voltages of 2V, 4.5V and 6V respectively. At 3.3V the current will still be under the specified maximum current of 25mA, with the output short circuited, so it's safe to skip the current limiting resistor. To help matters the current reduces, as the die temperature increases, which to some extent will help to protect the IC to some degree, but it will probably still burn out if the output is shorted and the supply voltage is 4.5V or 6V.
https://www.dectel.ru/analogues/ms/HCT_USER_GUIDE.pdf

Danesh_SA:
Dear Zero999,
Thank you for your detailed answer, that's exactly my answer :-+.
I need 4 channel of 74HC14.
Now i got it.With 3.3V ,i connect 74HC14 directly to MCU ,there is no fear of burning :) !

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