# forward/reverse voltage

Discussion in 'General Electronics Chat' started by tgregor, Sep 28, 2008.

1. ### tgregor Thread Starter New Member

Sep 28, 2008
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I have an optocoupler that has a max forward voltage of 1.65 and a max reverse voltage of 6.0 does this mean i can only apply a max of 1.65v to the collector? or can i apply 5v? I get confused about forward a reverse voltages.

2. ### hgmjr Moderator

Jan 28, 2005
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The input to the opto-coupler is an LED so you will need a current limiting resistor to avoid exceeding the maximum current applied to the input.

Applying a reverse voltage in excess of the rated 6V should be avoid also.

hgmjr

3. ### beenthere Retired Moderator

Apr 20, 2004
15,815
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The foreward voltage is the voltage necessary to place the internal LED into conduction. As long as there is that much voltage present, the LED will emit light. Then it's a matter of limiting current to less than the maximum rating with resistors.

The reverse voltage is the greatest voltage the LED can withstand in reverse bias. More than 6 volts will send the LED into avalanche conduction and destroy it.

4. ### tgregor Thread Starter New Member

Sep 28, 2008
2
0
Thank you that makes sense to me, the textbook answer i got online did not make that much sense. So if I were to put a 5v signal from an AVR for no more then 2 seconds the chip can withstand it especially if i put a pull down resistor on it.

5. ### beenthere Retired Moderator

Apr 20, 2004
15,815
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If the voltage is in the reverse direction, then a resistor will not help. Add a diode in series with the LED to block the reverse voltage.

6. ### Ron H AAC Fanatic!

Apr 14, 2005
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What is the part number of your optocoupler?