Sciencemadness Discussion Board
Not logged in [Login ]
Go To Bottom

Printable Version  
Author: Subject: Mains-powered project considerations
bob800
Hazard to Others
***




Posts: 239
Registered: 28-7-2010
Member Is Offline

Mood: No Mood

[*] posted on 15-6-2014 at 11:30
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!
View user's profile View All Posts By User
smaerd
International Hazard
*****




Posts: 1262
Registered: 23-1-2010
Member Is Offline

Mood: hmm...

[*] posted on 15-6-2014 at 14:05


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]




View user's profile View All Posts By User
Metacelsus
International Hazard
*****




Posts: 2502
Registered: 26-12-2012
Location: Boston, MA
Member Is Offline

Mood: Double, double, toil and trouble

[*] posted on 15-6-2014 at 19:41


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.


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! :P




As below, so above.
View user's profile View All Posts By User
NOV:5
Harmless
*




Posts: 24
Registered: 5-9-2014
Member Is Offline

Mood: Moodless

[*] posted on 12-10-2014 at 11:58


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...
View user's profile View All Posts By User
franklyn
International Hazard
*****




Posts: 3026
Registered: 30-5-2006
Location: Da Big Apple
Member Is Offline

Mood: No Mood

[*] posted on 13-10-2014 at 02:10


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.

.
View user's profile View All Posts By User

  Go To Top