bob800
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Mains-powered project considerations
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=multiinverter I 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!
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smaerd
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I do not know much about working with mains voltage aside from a few things.
For #3, The type of plastic is surprisingly important. Project enclosures typically use varrying grades depending on what they contain, don't
underestimate electricity to creep and crawl on static layers. A properly shielded and grounded(which you are still working on wish I could help more
there) project shouldn't pose much of an issue in a plastic case of nearly any variety though.
Also good tid-bit of advice I've heard from someone working in the field: when working with mains, it's usually a good idea to keep one hand in your
pocket as a habit so that if your bench becomes charged you don't make contact through your arms and to your chest/heart and is isolated to say one
hand.
Best wishes hope someone can actually help you out.
edit - some basic information I am sure you already know but may be beneficial for a searching passer-by: http://www.penguintutor.com/electronics/electrical-safety
[Edited on 15-6-2014 by smaerd]
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Metacelsus
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Quote: Originally posted by smaerd  |
Also good tid-bit of advice I've heard from someone working in the field: when working with mains, it's usually a good idea to keep one hand in your
pocket as a habit so that if your bench becomes charged you don't make contact through your arms and to your chest/heart and is isolated to say one
hand.
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For low frequency, low-to-medium voltage AC and DC, this is sound advice. However, when working with flybacks that use ~30 KHz at many kV, you will
still get badly burned even if you put one hand *near* a live wire.
Keep in mind that flyback circuits have the potential to generate a ton of interference, which, in my experience, can do strange things with GFIs and
sensitive CMOS electronics.
When in doubt, wear a Faraday suit! 
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NOV:5
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The way I read it, it Looks like its on a nema plug. That should have a breaker protecting it. Yes ground the case, mount it in a project box.
The TO-247 case (The MOSFET Case) should have a mica plate and thermal grease under it, the mounting hole is isolated as well.
Yes, normally when working with power you try to keep 1 hand out of the cabinet, (and don't lean against it either  ... The point is to not have a current path across your chest, so pay attention to
your feet also.
Really it looks like it's intended a a discrete entity that plugs in to power. Mains doesn't really come into play..
Remember, Remember...
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franklyn
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If you're concerned , use a ground fault circuit interrupter on the line cord. You can salvage
one with the cord from a discarded hair blower. http://www.youtube.com/watch?v=gFFvaLzhYew
It is really designed to be used with a circuit board having a common ground with the
a.c. neutral , instead of the floating chasis ground depicted , but it's better than nothing.
If your electronic devices do not suffer now from line noise , it won't affect your project.
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