Optocoupler question....

Apr 23, 2010 4 Replies
T

Hi all,



I have a need to detect when an ac voltage has dropped.



I thought I'd use an opto coupler (4n25 or 4n37 etc) as the isolation part. The problem I have is that I need an output that will supply power when the led part of the isolator is dark.



Currently I have breadboarded a simple cicuit using one of these devices, but I can't figure out how to get the output inverted. Every configuration I come up with turns on when the input voltage is present (led on).



This should be fairly simple but my mind will not wrap itself around it.



The system currently uses a 50 VAC signal that is present at all times, except during a fault condition, when it drops out. I want to get rid of that buzzing beastie and go electronic. I'd rather not have a relay or transformer that is active all the time, as with that solenoid device just when the fault condition is present.



Any useful input is always appreciated....



Thanks,


- Tim -


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J

Can't you just hang an inverter or transistor behind it?

If you find a solution that makes real power out of an AC line that sits at zero volts let me know, I'll buy the patent :-))

Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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T

Well the circuit will be using 12 volts DC on the alarm side. Maybe I could have described it a bit better. If it was straight TTL and I had more space, a logic solution would have worked.

But alas it is a moot point now as I have just discovered that the 50 VAC line was activated through a simple microswitch with no control relays to be concerned with. The rest of this machine has several relay logic circuits to protect it. Long story short - I just moved the wire to the other contact on the microswitch and presto! the 50VAC is present during fault only.

I guess I should have checked on that before posting a request for help. I will still use the optocoupler circuit though to activate the rest of the disable system.

Thanks for the reply,

- Tim -

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J

Would have been easy as well, using a transistor and a couple of resistors. Or a FET and one resistor, with the FET driving the big alarm siren.

Those are my favorite solutions. Time to head for the pub then for a cold one :-)

[...]
Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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M

You should check that the 50vac normally present was not a part of some other failsafe circuit(s). I have known well meaning mechanical engineers make modifications to kit that effectively defeated panic buttons by removing the drive voltage from the emergency stop in a very similar fashion to bodge something else into the chain.

Swapping a normally closed contact for a normally open one means that you have no way of telling whether the wiring harness is correct. That is until you hit the panic button and nothing happens. If a connector is left disconnected after servicing or becomes open circuit then your safety protection is entirely illusory.

Do make sure you have not inadvertantly disabled some other failsafe. It is extremely annoying to hit the big red button on dangerous kit in an emergency and just get a "click" sound.

Regards, Martin Brown

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