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AC opto isolators

HI all,

I am wanting to check a AC line (15VAC) using a opto isolator, basically just having it trigger a logic output so I can detect if the AC line is there or not. I think a AC opto would do it, though I am not sure if the AC ripple would cause the opto to give false outputs unless it was designed to work in that way.

My second option is to use a normal DC opto, convert the AC to DC, though the same problem is there with ripple, I could smooth that out with a small value capacitor, my only concern is that when the AC line fails, that the smoothing capacitor will keep the opto on for a fraction of a second after the AC fails.

The problem with that, I could probably time it correctly with a small value capacitor around 10UF, which should supply enough smoothing to keep the optos running stable and the value is small enough that when the AC fails it discharges pretty fast. The only thing which worries me about doing that is when the capacitor starts to "get old" and drift in value or temperature causes it to drift, that it could cause the circuit to malfunction. I could use a much larger value so its back tot he problem that the opto may not turn off fast enough after the AC line fails.

At the moment that's the only way I can think of monitoring the AC line, will probably work fine though thought I would ask and see if anyone else has any ideas on this type of thing ?

Cheers, Chris

Reply to
exxos
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AC to DC and smoothing cap is easy and cheap. The value you'd be dealing with would probably be low enough that it would be practical to get a high quality polyester or polypropylene cap that would last for a long time without changing value.

If you need to detect the loss of AC power fast, a 555 timer wired as a "missing pulse" detector would be one way. It is in the application notes for the 555.

There are ICs that detect the loss of DC and provide a reset pulse when the DC level falls to 4 point X volts . . . Usually called "power supervisor circuits" (or something like that)

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simple to apply and comes in a TO92 package

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Reply to
default

You are correct that there will be a short interval around the zero crossing of the AC wave where the opto will turn off and this iwll result in a glitch. Your approach to putting a small capacitor across the input is a standard approach to this problem. The value of the capacitor needed will depend on the on - off paramaters of the opto and the value of current limiting resistor you use with the 15 volt input. If you are so inclined, you can either do a first order aproximation calculation based on the time constant or even go ahead and solve the diff-eq to determine what value of capacitance is needed to keep the voltage above the turn off point.

As far as the capacitor aging, as long as your timing tollerances aren't so tight that you can't afford any extra delay, you can increase the capacitance by a safety factor to account for this. On the other hand, if your tollerances are that tight, you are probably going to have issues anyway and may need to re-evaluate your design.

Reply to
Noway2

Yes that would work well. You could probably detect the loss within about one and a half cycles or two (or perhaps less than one cycle if you ran two

555).
Reply to
CWatters

Yeah actually was thinking of electrolytic caps though makes a lot more sence to use polyester! I will check out the 555 sheet thanks!

Chris

Reply to
exxos

ahh, forgot to add that I am using one of those at the moment, its a DS1812 (I think) though it only works when the 5V rail is failing, which isn't actually helping at all :-\

Chris

Reply to
exxos

An AC coupler will work very well, In practice the opto will go off for a very short time at the crossing point if the collector load is quite high, you will only need a small cap to get rid of it.

Reply to
cbarn24050

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