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If used as a LED sink, what happens if a 74HC595 pin goes high? Might the LED blow?

I expect that when a pin is low, current will sink and the LED will light.

What about the opposite condition? When a the pin goes high, might there be risk of blowing LED's due to reverse flow? Why or why not?

schematic

simulate this circuit – Schematic created using CircuitLab

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3 Answers 3

up vote 6 down vote accepted

The answer lies in the reverse voltage rating of the LED chosen. Common through-hole LEDs typically are rated at 5 Volts reverse, some LEDs higher. This means that for up to 5 Volts applied in reverse bias, the LED will behave as a regular diode, blocking current flow. Beyond this voltage, reverse breakdown might occur, potentially destroying the LED if the voltage is high enough.

In the schematic shown, the PWM signal is 0 to 5 Volts. Thus, even when the PWM signal is low, and the latch output is high, the maximum reverse voltage the LED is exposed to is less than 5 Volts. So the LEDs will be fine.

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Being a diode, your LEDs will not suffer any damage, provided that the voltage being applied is below is breakdown voltage. +5V is generally safe for diodes commonly used.

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Gah, beat me to it! –  fuzzyhair2 Jul 30 at 15:06
1  
+5V will make most LEDs DED (Dark Emitting Diodes). –  Spehro Pefhany Jul 30 at 15:31

Almost all LEDs are rated for at least 5V reverse voltage.

In actual practice, I've never seen one that broke down even at 12V.

(Pulsed) reverse bias is a normal operational condition for multiplexed LED displays.

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