Input Protection for Optocuplers

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Hi,

I need to feed some digital inputs to the AVR from a external source.
This equipment will be used as a vehicle datalogger where the inputs will be isolated via a optocoupler (PC817)

The inputs of the optocoupler will be connected to the vehicle horn, lights etc.

The logger will log the events and also count the total duration of each inputs.

Is there any way to protect the optocouplers? I am fearing a load dump from the inductive and resistive loads might damage the optocouplers.

The vehicle is a 24V system.

_____________________ Love and Peace keeps PrOgraMmerS happy :)

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This Thread shows one approach.

There are certainly other ways to tackle this. I don't see it mentioned in this Thread, but elsewhere it was noted that the input reverse polarity protection diode might be redundant given the zener.

You will have to scale the resistive divider for normal operation at 24 V, but this may give you some ideas.

JC

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thats a very well explained post.
Thanks JC

_____________________ Love and Peace keeps PrOgraMmerS happy :)

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I dont want to connect any external signal directly to the avr. I got this http://electronics.stackexchange.com/questions/80597/optocoupler-input-protection for protecting the opto couplers.

_____________________ Love and Peace keeps PrOgraMmerS happy :)

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Whether you are protecting the input to the opto or to a uC doesn't matter, the input protection circuitry is to clamp the signal.

The two circuits are almost the same.

My circuit has the extra diode on the input, and the cap which was part of a LPF with the resistors.

A vehicle power supply can typically have both negative voltage spikes, and high frequency noise, in addition to the positive voltage spikes.

JC

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DocJC wrote:
A vehicle power supply can typically have both negative voltage spikes, and high frequency noise, in addition to the positive voltage spikes.
For a 12V automotive system 300V to 400V plus or minus.
How to select power line polarity protection diodes by Soo Man (Sweetman) Kim (Vishay) (EE Times, designlines Automotive, 6/30/2012)
May want to also consider the EMP due to a direct or close lightning stike.

"Dare to be naïve." - Buckminster Fuller

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saumya73 wrote:
This equipment will be used as a vehicle datalogger where the inputs will be isolated via a optocoupler (PC817)
My experience with automotive optocouplers for a camshaft sensor is the lifetime of the optocoupler is about 2 years or 50km.
Three failures until the final correction was made.
Alternatives to optics are by magnetics or by capacitance.
May want to consider making the optocoupler relatively easy to replace.

"Dare to be naïve." - Buckminster Fuller

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Thanks every one for the valuable info.

Is there any ideas for power supply?
I have been using LM2576 for a long time, though not a automotive grade, but want to switch to a good power supply

_____________________ Love and Peace keeps PrOgraMmerS happy :)