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Topic: Method for safely interfacing arduino with variable voltage input signals? (Read 1 time) previous topic - next topic

Docedison

There's a guy here who has a tag line about theory and practice...
This is one of those cases where there is a disconnect between theory and practice.
Yes an Opto-Isolator would work and work well... in a nice safe transient free environment.
However the transients present In the OP's situation might well be fatal to the Led's.
I built an Opto-isolated sensor board for a Center Pivot Irrigation Controller some years back
and I used P2500 series opto's (Panasonic) for the interface to the controller.
It lasted for 2 minutes before spikes from the Motors (440VAC 3 Phase @ 1 KW) destroyed all the connected
isolators. The switching transients) destroyed the LED's in all the devices connected.
The cure was to remove the opto's and replace them with ldr's and NE2 lamps.
This was the main reason for my suggestion about relay's and Yes the opto's were well protected
with Zeners and diodes for over voltage and negative going spikes as well.
Unfortunately this was one of the times I chose to ignore my employers thoughts about transients...
I made the mistake of just current and spike limiting the inputs and I had no real good idea of how hostile that environments can be.
I found out, thus my relay suggestion.

Bob
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

scottdrums89

Hello all,

Sorry for not getting back to this sooner - had limited internet access while traveling for Thanksgiving. 

I was staying away from relays due to the fact that I will not have access to the electronics (being used off-site) as it is being moved from machine to machine in order to change out the relay to match each machine's control voltage.  I'm trying to go for a simple, fairly idiot-proof design since I'm not going to be the one making the final connections, so I would like to avoid having multiple inputs for various voltage ranges (with each connected to a corresponding relay) and leaving it in the hand of the installer to decide which is the correct one.

-Scott

dc42

Whilst I have no reason to disbelieve Docedison's tale, optoisolators should be suitable for the task in nearly all conditions, unless you have really huge transients. If you do have large transients, then it should be possible to protect the opto isolator from them, although zener diodes may be too slow for this purpose and TVS devices may be more suitable. The solution I proposed in reply #2 will work for all those voltages and is also insensitive to polarity.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

scottdrums89

Hi dc42,

I agree with you that a bridge rectifier and optoisolater are likely the best components for my situation, or, at the very least, a good place to start.  The one thing I'm hanging up on a bit is how to regulate the current through and/or voltage across the LED.  Would you suggest using both a zener (for regulation) and a TVS diode (for protection) in parallel with the LED?  Something else?

-Scott

dc42

Opto isolators work over a wide range of input currents, so the 1K series resistor I suggested will work for the whole voltage range you are interested in. Most opto isolators have max forward current of 60mA, so 1K is good up to 60V absolute max peak voltage. If there is a danger of transients greater than this, then I suggest splitting the series resistor in two and using a 20V or 24V TVS diode at the mid point.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

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