protection diode orientation question

[earckens]

What is the best way to use a protection diode (1N4148) on the input of an opto-isolator input:
1. in series with a protection resistor at the positive supply line, cathode towards opto-isolator
2. in parallel over the opto-isolator input, cathode at the plus, anode at the minus of the opt-isolator (resistor in series on the positive supply line before the diode

 

[dknguyen]

1 if you have the voltage overhead

 

[earckens]

1 if you have the voltage overhead

ok, and why?

 

[dknguyen]

ok, and why?

No current flow in the reverse direction and uses the diode to dissipate heat in normal operation along with the resistor. Might as well use it since it's doing nothing 99.9% of the time.

 

[JimB]

What is the best way to use a protection diode (1N4148) on the input of an opto-isolator input

What are you trying to protect against?
Overvoltage?
Reverse polarity?
Something else?

JimB

 

[earckens]

What are you trying to protect against?
Overvoltage?
Reverse polarity?
Something else?

JimB

Mostly reverse polarity. Overvoltage is no issue and not solved with diode

 

[JimB]

What are you trying to protect against?

Mostly reverse polarity.

In that case a diode in series with the opto-isolator should be OK. (Your option 1)

But, if the reverse voltage is high compared with the reverse break down voltage of the opto-isolator, then ALSO add an anti-parallel diode across the diode of the opto-isolator. (Your option 2).

JimB

 

[dknguyen]

In that case a diode in series with the opto-isolator should be OK. (Your option 1)

But, if the reverse voltage is high compared with the reverse break down voltage of the opto-isolator, then ALSO add an anti-parallel diode across the diode of the opto-isolator. (Your option 2).

JimB

A diode with the appropriate breakdown voltage you want to clamp it to, of course. Zener or TVS is best.

 

[JimB]

A diode with the appropriate breakdown voltage you want to clamp it to, of course.

I was thinking that a simple junction diode in an anti-parallel connection would limit the reverse voltage across the opto-isolator to about 0.6 to 0.7 volts. No need for a zener.

JimB

 

[dknguyen]

I was thinking that a simple junction diode in an anti-parallel connection would limit the reverse voltage across the opto-isolator to about 0.6 to 0.7 volts. No need for a zener.

JimB

Oh you're talking about an reverse-over volted condition? (I was referring to a forward over-volted condition) Is that necessary if the series diode is already limiting reverse current flow to leakage currents?

 

[JimB]

Is that necessary if the series diode is already limiting reverse current flow to leakage currents?

I don't know.

What I was thinking was that using just a series diode for overvolt protection, there will be some unknown potential of reverse polarity across the opto.
The value of that potential will depend on the reverse resistance of the protection diode and the opto.
So, using an anti parallel diode across the opto will limit the reverse potential to o.7v (ish).
It just depends how thorough you want or need to be applying protection.

JimB

 

[earckens]

I don't know.

What I was thinking was that using just a series diode for overvolt protection, there will be some unknown potential of reverse polarity across the opto.
The value of that potential will depend on the reverse resistance of the protection diode and the opto.
So, using an anti parallel diode across the opto will limit the reverse potential to o.7v (ish).
It just depends how thorough you want or need to be applying protection.

JimB

Interesting view; and since overvoltage is not an issue in my case but reverse voltage is then I may conclude that an antiparallel diode is the better protection?
The maximum allowed reverse voltage is 6V for this particular optocoupler, who knows if, when using a series diode, this will not be exceeded?

 

[dknguyen]

Interesting view; and since overvoltage is not an issue in my case but reverse voltage is then I may conclude that an antiparallel diode is the better protection?
The maximum allowed reverse voltage is 6V for this particular optocoupler, who knows if, when using a series diode, this will not be exceeded?

Diodes are put Anti-parallel to whatever they are protecting and work by breaking down above a specified voltage and effectively act as a limited short so that the voltage across there terminals is not exceeded. Of course, parallel diodes or (stacked parallel diodes to get multiples of ~0.7V) would also work as long as your operating voltage is below whenever the diodes become forward biased.

 

[AnalogKid]

With the series diode and a large reverse voltage, there are three elements in series, two (essentially zener) diodes and a resistor. The current through the string is the maximum leakage current of the two diodes. Voltages are distributed based on the reverse breakdown voltages of the two diodes (usually not specified). Because both diodes are minimally conducting in the reverse direction, something neither is specifically built for, I prefer the reverse connected diode in parallel with the input LED and dealing with the extra resistor heat.

ak