High voltage input protection - Page 1
Forum section: electronics / beginners

author: VDave
author_rank: Newbie
timestamp: November 13, 2017, 02:36:48 am
content: Hi,I'm doing my first project that would be directly connected to the 120v AC (north america) and I'm looking for information about protection circuits that should be present on the main AC line in a power supply.I know that a fuse is a "must", but what else would you put and what do I need to take in consideration?Thanks a lot!

author: PCChazter
author_rank: Contributor
timestamp: November 13, 2017, 07:04:00 am
content: You should know that in Canada, anything connected to mains voltage *legally* needs to be CSA approved. That being said, what are you connected to line voltage? If you are just trying to make your own power supply for powering a project, you may be better off finding an already built power supply suitable for your project. Or are you trying to make some measurements?Sent from my LG-M703 using Tapatalk

author: Rerouter
author_rank: Super Contributor
timestamp: November 13, 2017, 07:25:15 am
content: Cannot say for your particular countries regulation requirements.Off the mark, use a proper IEC input socket for your power input, you can get ones with a built in fuse holder. (no need for test and tag, no need for strain relief)next to the jack should be a mains power switch, if you want to play safe use a double pole to break active and neutral, this way if an outlet is wired funny you are safer with it switched off.If your system is noisy, or is highly susceptible to mains noise, you may need a mains filter, X and Y filter capacitors with an optional common mode choke. If your running it directly to a transformer, then you likely don't need a mains filter, as the transformer attenuates any frequencies higher than mains.Ground your casing if its metal, use a wedge lock washer if you want to make sure it never comes off.Do not rely on only solder for mains connections, you must mechanically support them, IEC sockets will generally accept crimp terminals, IF your running mains directly to your PCB, use a connector, not direct wire to board, leave 2x 3mm holes nearby to the connector to zip tie the loom to the board so the plug never tugs loose.To give an idea of why the respect of using say insulated crimp terminals instead of soldering. if the 2 mains conductors touch, there can be a fault current measured in hundreds of amps for a general home wiring circuit. solder joints if the wrong flux is used or the joint experiences vibration becomes brittle over the years, meaning a wire can break off and touch places it shouldnt, the crimp, not so much.

author: VDave
author_rank: Newbie
timestamp: November 14, 2017, 03:21:09 am
content: Thanks for the replies!PCChazter: I've looked at the option to but a power supply, but budget dosen't fit what I need and also make few things more complicated. For the CSA part, I don't plan to sell this project, so I don't really care about that. Also, anything I build is not connected when I'm not using it and I always have a fire extinguisher with meRerouter: I'm not familiar with the X and Y filter capacitors and common mode chokes. Do you know a good place to get info about that? (Good placement, part selection, etc.) I will google it, but if you know a good reference, it would be nice!Thanks for the tip about the terminals instead of soldered wire.

author: Rerouter
author_rank: Super Contributor
timestamp: November 14, 2017, 09:47:21 am
content: X and Y filter caps are special types of capacitors, built to fail safe in a certain way,The X capacitor is between live & neutral, it shunts differential mode noise, before it leaves or enters your device, this is stuff like step loads, where there may be a fast spike or dip across the wires as it gulps some power, or turns off something that was consuming very quickly,The Y capacitors is between the each wire and ground, this shunts common mode noise, e.g. noise common to both conductors in respect to ground, say your ground gets a spike because another device has a ground fault, e.g. a Metal-Oxide-Varistor handling a mains voltage spike, this keeps that noise from coupling in to your product, or is your device uses something like a MOV to suppress spikes to keep that noise from other devices.https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/Common mode chokes are like a current transformer, with Live on 1 side, and Neutral on the other, it uses the magnetic coupling of the 2 currents to smooth almost all but common mode noise. now the name seems wrong, but it comes from the fact that its wired across the common mode effected wires,As far as this part goes, they again build these things for you, as they are so commonly used to make up for noise sensitivity issues,https://www.google.com.au/search?q=iec+socket+filter