Mains-powered project safety  - Page 1
Forum section: electronics / projects

author: Sivand12
author_rank: Contributor
timestamp: November 20, 2014, 11:15:10 am
content: I am building a general-purpose inverter (60Hz AC --> 40-140 kHz AC) for powering flyback transformers, induction heaters, etc. The project link I'm referencing can be found here:http://uzzors2k.4hv.org/index.php?page=multiinverterI am aware of the hazards present from the mains-level voltage; however, what I am more concerned about is the fact that schematic shows the a direct connection to unisolated mains.If it is possible to do so safely, I would like to use this circuit without an isolation transformer. However, that brings up several question:1. The author built his inverter within a salvaged ATX power supply metal enclosure. Can I ground the metal case, or would this cause problems due to the fact that the circuitry runs unisolated?2. If I did ground the metal case, I would certainly need to ensure that the MOSFET heatsinks did not make contact with the case---correct? Is there a practical way to isolate the heatsinks from the case?3. I realize it would be exceedingly dangerous to leave the case ungrounded, with the unioslated circuit. If this were necessary (due to 1 and 2), I suppose enclosing the ATX case with plastic would provide enough safety?Also keep in mind that I would be the only person using this inverter--it will not be sold/installed/used in an environment with anyone else, just to clear up potential liability concerns.Thanks!

author: rs20
author_rank: Super Contributor
timestamp: November 20, 2014, 11:42:44 am
content: 1. You mention ATX power supplies, which is interesting because half (loosely speaking) the PCB in an ATX power supply is live, unisolated. The flyback transformer is mounted on that PCB after all, so one half of it has to be unisolated. Thewhole pointof a grounded case is that if something goes horribly wrong and a live wire breaks off and contacts the case, the fault causes a short to ground and will blow a fuse/trip an RCD, etc, rather than just silently turning the case of the device into a deathtrap.2. Dunno, will yield to others who have more experience with this. But can't just choose a heatsink that's mounted on the PCB (so electrically isolated from ground) and has a generous mechanical clearance from the case? If your idea is to bolt a live heatsink to the case, I suspect that's difficult to achieve safely. I suspect a sil pad separating live from earth (e.g. a sil pad between a live transistor and a grounded heatsink) is not considered acceptable.3. Still exceedingly dangerous. Ground the case.

author: LukeW
author_rank: Frequent Contributor
timestamp: November 21, 2014, 01:14:09 am
content: Just "do what an ATX power supply does" and you're good, basically.- Use a metal case.- No need for an extra plastic case around the metal case.- YES securely connect the metal case to mains earth.- The heatsinks with the MOSFETs don't have to be touching the outside case, they can be on the PCB.- Although it doesn'thaveto be tied to earth, I think it's good practice to avoid having a heatsink connected to a live high voltage rail if you can avoid it, even with the heatsink enclosed inside the case. At least label the heatsink, to avoid problems for future repairers.- Use insulating washers and bushes (eg. mica) to mount the devices on the heatsink.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 21, 2014, 01:26:19 am
content: Quote from: Sivand12 on November 20, 2014, 11:15:10 amA standard PC power supply case is connected to the mains power green-wire safety ground. BY DEFINITION. DO NOT REMOVE THIS PRIMARY SAFETY FEATURE!!!!QuoteFor what reason would you NOT ground the case?There are many ways of isolating heatsinks from the chassis/enclosure/ground. You can see some of them at work in the existing power supply circuitry.QuoteUNDER NO CIRCUMSTANCES SHOULD YOU FAIL TO GROUND THE CHASSIS OF THE POWER SUPPLY!!!!If (2) and/or (3) seem like barriers, then you are doing something wrong!QuoteSo you are saying that it is "safe" as long as only YOU are using, and it is a potential death-trap for anyone else? That is unacceptable in my world. You NEVER KNOW what is going to happen!See the previous sentence about UNDER NO CIRCUMSTANCES..... etc.

author: rs20
author_rank: Super Contributor
timestamp: November 21, 2014, 02:11:37 am
content: Quote from: LukeW on November 21, 2014, 01:14:09 amAnd a Sil-Pad between the device and the heatsink itself, right? But I'm wondering -- is a Sil-Pad considered acceptable for isolating live from earth? You know, you need special fancy class-Y or class-X (I forget) caps for that job, seems like something that shouldn't be left to a random bit of Sil-Pad. Or am I being unfair to Sil-Pad?