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Old 26th September 2003, 03:12 AM   #1
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Default Proper Grounding Scheme

I am going make a new power supply for my preamp and amplifiers. Well... It is acutually a re-design and shrink of an old one.

As far as grounding goes, Do I simply connect the main earth from the AC inlet to my power ground on the circuit?

-Thanks
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Old 26th September 2003, 04:41 AM   #2
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Paul you can ground your ac to the power supply ground but if you notice nothing you buy comes with a three prong plug...you just judge whats good for you
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Old 26th September 2003, 04:52 PM   #3
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That is what I thought.
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Old 26th September 2003, 05:01 PM   #4
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Paul, I got in real trouble last time i mentioned to use a 2 prong instead of 3 prong plug. One of the moderators didn't like it....even though most commercial equipment today has a 2 prong plug, that equipment (hopefully) has been thoroughly tested to pass UI? specs. If something goes wrong, you definitely want the case grounded to main earth. Whether or not your power supply ground is connected to case...I don't know whats best. I'm still learning. BE CAREFUL. Later!

-Matthew K. Olson
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Old 30th September 2003, 01:47 AM   #5
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Originally posted by Mattyo5
Paul, I got in real trouble last time i mentioned to use a 2 prong instead of 3 prong plug. One of the moderators didn't like it....even though most commercial equipment today has a 2 prong plug, that equipment (hopefully) has been thoroughly tested to pass UI? specs. If something goes wrong, you definitely want the case grounded to main earth. Whether or not your power supply ground is connected to case...I don't know whats best. I'm still learning. BE CAREFUL. Later!

-Matthew K. Olson
Hi Matthew,

The moderators are not being ninnies about this.

The point of the 3rd prong is to provide a return path to force the mains circuit breaker to trip in the event of a wiring fault or malfunction that puts mains voltage on the enclosure. Without that 3rd prong, a metal enclosure with mains AC in it is an electrocution waiting to happen. Vintage tube guitar amps often lack this 3rd ground prong and they end up zapping a musician every so often. http://www.google.com/search?hl=en&l...+electrocution

Modern, commercially made equipment with 2 prong line cords either uses an external power supply (that is double insulated), have a plastic case to prevent finger contact with energized metal or are internally double insulated (like a hand power tool).

I work for a large ($1B US annual sales) corporation designing electronic scales and we use wall warts power supplies for just about all of our low end consumer products just to avoid these sorts of issues. When we bring mains power into the product we use a 3 wire power cord. We could deal with double insulating the product, but the 3rd wire is cheaper and ultimately safer.

For a DIY project, if you need to bring AC mains power inside a metal enclosure, then the 3rd power prong is the only safe way to go. There are other, saner ways to deal with ground loop problems.

Phil
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Old 30th September 2003, 02:35 AM   #6
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I understand completely Phil, thanks for the explaination though. I'm just warning Paul that if he does start suggesting to do a 2 prong approach that ...well...its not a good idea and that i have had previous experience on this board w/ regards to that suggestion. I understand that the moderators are right on and taking this stand for a reason, and a good reason at that. No arguments here. Paul, be careful in all you do, Phil, thanks for the thorough explaination

-Matthew K. Olson

actually, phil, question for you...suppose the case is grounded by the 3rd prong, but the center tap (from toroid 2ndary) is not grounded to the case...ie...floating ground...is there anything wrong w/ this? As I see it, if there somehow was a fault that charges the chassis, the voltage would just go right through to the 3rd prong and not hurt anyone....is this correct? thanks!
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Old 1st October 2003, 04:00 AM   #7
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Originally posted by Mattyo5
actually, phil, question for you...suppose the case is grounded by the 3rd prong, but the center tap (from toroid 2ndary) is not grounded to the case...ie...floating ground...is there anything wrong w/ this? As I see it, if there somehow was a fault that charges the chassis, the voltage would just go right through to the 3rd prong and not hurt anyone....is this correct? thanks!

Floating your power supply ground from earth ground is not a safety issue in any way. You will probably want some kind of a bleeder resistor connecting the two grounds to prevent static charge from building up in the circuitry.

I've designed electronics used with vehicle scales which have to survive direct lightning strikes. One of the things we do to help accomplish this is to float the electronics from power ground (actually we tie logic ground to earth ground with a 10K resistor in parallel with a small cap). The issue here is that when a lightning strike occurs, the radiating e-fields around the strike will generate different voltage potentials at each end of the interconnect cable. This is especially likely with vehicle scales since the part that the trucks sits on while being weighed is normally outdoors and the operator interface (the part that is plugged into mains power) can be located upwards of 1000 feet away from the scale.

Floating the power supply ground from earth ground permits any induced potential on the electronics inside the vehicle scale and operator display to equalize to an average value (half way between the potentials at each end) without damaging anything. This potential is then bled off to earth ground via the 10K resistor. If the power supply ground was connected to earth ground then destructive currents would flow through the interconnect cable during a lightning strike and fry everthing.

At the lighting test lab we dumped 6,000 A surges into the equipment to simulate the effects of a lightning strike. Very impressive to watch.

FYI, do you know why cow and horses are more vunerable to lighting than other animals? Their front feet and back feet are too far apart. If lighting strikes sufficiently close, the induced potential at one end of a cow is enough different from that at the other end to cause current flows lengthwise through the cow's body which stops it's heart.

Phil - a endless wealth of useless details at my fingertips -
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Old 1st October 2003, 06:06 AM   #8
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Default That AC ground

That AC ground can cause ground loops and hum in audio systems. Best thing to do is purchase an isolator plug, for your AC plug. You then have the option of using the AC gound or not, depending if you have hum.
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Jim W.
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Old 1st October 2003, 10:22 AM   #9
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Default CowPow!

"FYI, do you know why cow and horses are more vunerable to lighting than other animals? Their front feet and back feet are too far apart. If lighting strikes sufficiently close, the induced potential at one end of a cow is enough different from that at the other end to cause current flows lengthwise through the cow's body which stops it's heart."

My father in law raises cows. Should I visit his farm and install ground straps around his cows in the middle, to be on the save side?
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Old 1st October 2003, 11:40 AM   #10
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Jewilson, you need to read the rest of this thread. I have gotten in trouble w/ moderators saying that you should use an adapter to ...3 prong -->2 prong adapter...to isolate from AC ground. Its not a good idea. If the chassis becomes charged ...ac ground needs to be attached to the chassis so you don't get rail voltage when you touch the chassis. So, as has been said to me, be careful w/ what you suggest. Yes, many components have only 2 prongs, but these are carefully inspected etc. commercial components...VCR's, etc. ...and for us DIYer's....its best to ground chassis to AC ground. Just FYI. Later!

-Matthew K. Olson
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