Hi again,Although I have had some neons fail on household sockets, I've got many that have worked for years, so unless you know something I don't, I'd say neons are reliable. At 50 for £2 I'm not too worried about having to replace one now and then. Also this is an optocoupler it's just using a neon instead of an IR LED, it may not be one piece, but an optocoupler none the less. Some of the datasheets for visible light optotransistors suggest neon as a possible light source. That's what gave me the idea of using one. I know that transformer less 220ac to 5vdc are technically sound but I'm not over keen on them, too many components. I find it interesting that people are happy to use a transformer but then say that a transformer less circuit isn't safe. Why do they trust 2 copper coils wrapped around each other? If the insulation failed or it was overloaded, the resultant meltdown wouldn't be very isolated. If a capacitor/resistor/diode in a transformer less circuit failed it is highly unlikely to fail as a short, most likely open circuit, just like a fuse!
I'm trying to avoid using capacitors and I don't see why flicker is an issue, the optotransistor turns on and off very nicely using the neon as is.
I just noticed the OP wants to measure overvoltage so I have to plead guilty to going off topic.
If it's just over voltage protection that's needed what about a varistor?
Well reliability is also affected by quality control, and apparently there is a variation because i have had neons blow out after only a few months and some that last for years. I have had in the past five years two blow out already. Sometimes they just get dimmer, and dimmer as time goes on. Sometimes they also start to flicker noticeably.
But sometimes reliability is not measured by the performance of just one device, it is measured by two or more devices such as in this case a neon vs an LED. The LED will last a long time if driven properly and quality control is pretty good.
One thing i do remember is that if the series resistor for a neon is made larger the neon lasts longer but yes with less brightness.
You say you are using an opto coupler but you know what i meant, and that is a commercially made opto coupler with internal LED not neon.
You may not mind changing a neon bulb, but the real question is can your application tolerate a down time while it waits for that replacement.
The 'flicker' problem may or may not affect your application depending on how bad the LED or Neon flickers and how the transistor turning on and off 100 times per second may affect your application. It may matter, it may not matter. If it does not matter, you dont need a cap, if it does matter, you do need a cap. See how simple that was?
Transformer isolation for commercial use is not the same as a general purpose transformer. They are wound differently. The typical wall wart is much safer than a 'regular' transformer because for the wall wart the windings are DOUBLE isolated. First, they are not wound one on top of the other, they are wound side by side, which puts a physical distance between ANY two turns of both windings. Second, they have a spacer that separates the two windings made of some insulating material so even certain crush directions wont even cause them to touch. Of course they also have the usual wire insulation, but that's not considered as part of the protection really that's of course just so the individual turns dont touch.
This is coming from someone who worked in the power industry for many years who did design and some actual hands on testing of power equipment ranging in power from less than a watt to maybe 30k Watts, physical size about one square inch to cabinet designs that were 5 feet tall, 6 feet wide, and 2 feet deep.
It's always your choice what you want to do with your own design, i only make suggestions based on experience and education in electrical engineering science.