Newsgroups: sci.electronics.design
Subject: Re: 220VAC input signal to 5VDC digital circuit
References: <[email protected]>
	 <[email protected]>
From: Tomi Holger Engdahl 
Date: 18 Jan 1999 01:34:11 +0200
Message-ID: 
Organization: Helsinki University of Technology, Finland
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Jonathan Bromley  writes: 
> John Woodgate wrote:

> said, regular opto-isolators can be pressed
> into service.  Consider using a *capacitor* to limit the 
> current to the isolator's LED, but don't forget to use
> a claas X capacitor that is OK for permanent connection to
> mains voltage (no need to use class Y, they're only for
> situations where cap failure could result in exposed parts
> becoming live).

And it is a good idea to put a series resisotr in series with the
capacitor. If there is no series resistor, then line surges
and turn on surges can easily damage the optosiolator after
the capacitor because of high surge current. And lots of
harmonics on the power line will cause the current through
the otoisolator to encrease some amoun. So a series resistor
in series with the capacitor is a good safety feature.

Using class X capacitors in this kind of mains connected circuit
is a very good idea for safety.

> If putting such an arrangement on a PCB, be sure to observe
> adequate creepage and clearance distances between traces:
> IIRC you should leave 3mm between conductors on the "hot" side,
> and preserve at least 8mm clearance between any "hot-side" 
> conductor and any "logic-side" conductor.

Those are very good safety tips. Enough clearance is essential
for safety.


-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web pages at http://www.hut.fi/Misc/Electronics/