What is the purpose of this diode ?

Thread Starter

yehdev_cc

Joined Jul 25, 2010
23

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Thread Starter

yehdev_cc

Joined Jul 25, 2010
23
Looks like it's a reverse polarity protection diode.
Thank you very much for the reply,

I see now, if the polarity is reversed, the circuit will function the same, I guess.

I have one more question, does it provide any "protection" ? if the input polarity is reversed (input = GND) and the diode is not present there, what might go wrong in this circuit ?
 

AnalogKid

Joined Aug 1, 2013
9,054
No, if the input polarity is reversed the circuit will not function, because there will be no current through the opto and discrete LEDs. That is, unless the reversed input voltage is high enough for the two LEDs to go into reverse conduction, acting like zeners. LEDs do not like this, and the diode you are asking about provides a lower voltage bypass path to GND to protect them.

ak
 

Thread Starter

yehdev_cc

Joined Jul 25, 2010
23
No, if the input polarity is reversed the circuit will not function, because there will be no current through the opto and discrete LEDs. That is, unless the reversed input voltage is high enough for the two LEDs to go into reverse conduction, acting like zeners. LEDs do not like this, and the diode you are asking about provides a lower voltage bypass path to GND to protect them.

ak
Why won't the diode conduct ?
If I understand the term "reversed polarity" right, instead of GND at the diode anode, there will be positive voltage, much higher - typically - than the forward ON voltage needed. And we'll get this voltage (nearly) at the same node on the zener cathode, just like the case of the normal polarity.

did I get something wrong there ?
 

BobTPH

Joined Jun 5, 2013
3,003
The diode will conduct if the input goes negative. It will limit the voltage across the optocoupler to a diode drop, which will protect it from reverse breakdown.

Bob
 

ian field

Joined Oct 27, 2012
6,539
The diode WILL conduct, but as noted, the circuit will not function in its normal way.
You can get AC input opto-couplers that have an inverse-parallel pair of IRLEDs, with one of those the reverse protection would be redundant - but you'd have to supplement the existing volt-drop zener by making that an inverse-series pair.

The AC input optos are/were fairly common on dial up modems. Although some manufacturers decided it was cheaper to use a regular opto and add a bridge rectifier.
 
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