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 Post subject: Re: Grounding Techniques
PostPosted: 26 Oct 2016, 00:54 
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Joined: 07 Dec 2015, 14:21
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Found the article.

It shows how to measure leakage current and pick the best polarity. Just one more trick to get the noise down.


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 Post subject: Re: Grounding Techniques
PostPosted: 24 Nov 2016, 00:20 
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Joined: 20 Nov 2016, 08:33
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I read the Grounding and Shielding doc where Bruce has suggested the use of a 120 Ohm 1/2 watt resistor in parallel with a 0.1-0.22uF capacitor for chassis grounding. I did the following calculations :
1. Impedance of 0.22uF capacitor at 60Hz (1/(2 * pi * f * C)) = 12K
2. Effective impedance of the cap and resistor in parallel = 119 Ohms
3. Current through resistor and cap = E/R = 110 / 119 = 0.92 Amps
4. Power dissipated in resistor I^2*R = 0.92^2 * 120 = 101 Watts

How will 1/2 watt resistor work? I'm sure Bruce is not wrong. What am I missing?


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 Post subject: Re: Grounding Techniques
PostPosted: 25 Nov 2016, 18:28 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Somewhere you got off track. There is virtually no current flow in the resistor. It only has to pass the small amount of leakage (often built up by the power transformer) and any EMI that hits the chassis. A 1/2 watt resistor is fine as the typical values I measure in my designs is about 10-30 millivolts. Often it is nearly zero.

Keep asking questions if it is not yet clear. Grounding is a very important thing with AC mains powered equipment and getting it wrong can have serious consequences.

Good listening
Bruce

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 Post subject: Re: Grounding Techniques
PostPosted: 25 Nov 2016, 22:02 
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Hi Bruce,

With regards to chassis ground (in the article) you say "The key thought is to maintain the shock protection while using it as an EMI shield."

In order to provide shock protection, if the chassis got live by accident it should ground it before causing harm. In that case won't the above calculations hold true?


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 Post subject: Re: Grounding Techniques
PostPosted: 26 Nov 2016, 21:35 
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Hi, Not really. If there is an internal short to the chassis it will go directly to the earth ground connection at the AC mains input. Effectively bypassing the resistor. If there is a significant short inside the equipment to its signal ground it should cause the fuse to blow. This also would not put anything through the resistor. The resistor and capacitor and sometimes a pair of solid state rectifiers (one going each way in parallel) isolate the chassis from the circuitry but allow it to be an EMI shield. You could do this without any earth connection to the chassis but it would be a major and dangerous violation of any electrical codes I know of. The rule there is that any exposed metal part of the unit (applies to anything really connected to the AC mains) needs to be at earth ground or double insulated from accidental contact. This is nearly impossible for diy gear particularly if it is connected to other gear by cables or wires. If the metal chassis was left floating and not connected to the internal circuitry it would accumulate EMI and have no way to discharge it. The presence of the EMI field would contaminate sensitive internal circuitry. Many of us recall that in long past days it was common practice to use the chassis as both the shield and signal ground usually with the negative side of the internal power supply tied to it as well. Also there were only two wire AC mains and no established polarity of them. It worked....but then signal to noise ratios were lower and interconnection of units could be problematic. When the polarized AC mains plugs (one prong larger than the other) came out it helped quite a bit with interconnections but still allowed for a fault inside the chassis to energize the case. With most quality gear now using the IEC or some other variation of three wire input power connections the issue of energizing the case is much less likely. It would now take a double fault. All that being said...if there was an unusual condition that caused a significant amount of power to exist between the grounds then the resistor being only a 1/2 watt would fail immediately and open the circuit. The capacitor would still be there, but the actual current through it would be very low and not harmful. The failure of the resistor would almost certainly cause hum or noise in the output and thus lead to a trouble shooting session.

I hope this helps as grounding is not a simple subject and gives many folks trouble. It can easily turn a great project into a poor performer.

Good listening
Bruce

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 Post subject: Re: Grounding Techniques
PostPosted: 27 Nov 2016, 03:54 
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Joined: 20 Nov 2016, 08:33
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Sorry Bruce, but I'm not following. It would really help if you could draw a simple diagram.

Thanks for taking the time to clarify this.


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 Post subject: Re: Grounding Techniques
PostPosted: 28 Nov 2016, 22:52 
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Joined: 20 Nov 2016, 08:33
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Specifically, I am not following where the chassis internally connected to AC ground? And if so what good will the parallel connection with the resistor and capacitor do?


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 Post subject: Re: Grounding Techniques
PostPosted: 29 Nov 2016, 16:22 
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Hi, The following is a really brief explanation. There are books written on this subject and it is not always intuitive how it works. If you are good with math I'll see if I can find a reference I saw a while back on the web that went into more depth that I will now.

The chassis is connected to the earth ground AC mains at the entry of the AC mains to the chassis. It is easiest IMO to use one of the many IEC power entry chassis mounted connectors to do this. You then can use a detachable power cord (very handy if you move stuff around). The IEC connector will have a ground lug on it that goes directly to the chassis. Some of the connectors are all metal on the part that goes into the chassis and will connect to the chassis that way, I prefer to use the ground tab too though. Now the chassis is connected to the earth ground and any fault inside it will go right to the earth ground. If you don't provide some path for internal circuitry leakage currents (usually in the millivolt range) to escape the circuitry they will cause problems as there will be a potential difference between the circuitry gound and the chassis. By capacitive and inductive coupling you will end up with hum and noise. The X2 capacitor provides a path for high frequency stuff and the resistor handles low frequency stuff. If you should directly connect the two grounds then you can set up alternate paths for both power (both inside and on the earth ground) and signal return paths. This will cause both to flow through the same path. Yes the chassis is a path. While the resistance of the path may be quite small it is finite. With any power flowing through it there will be a voltage developed. It too can be small but unfortunately it will be added to the one you want in the active circuit and thus be amplified. The result is hum and noise. This is normally called a ground loop. It can originate in the AC mains or signal connections between various pieces of your equipment.

I hope this helps

Good listening
Bruce

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 Post subject: Re: Grounding Techniques
PostPosted: 30 Nov 2016, 09:51 
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Grounding:

After seeing, and participating in, the discussion on 3-wire cords, and seeing other discussions on grounding in general, I think that relating a few experiences may be useful. First, a few things:

a) I have always, well, at least since I was 22, lived in houses with radiators on cast iron risers from a central boiler that was also directly connected to the (copper) water service. Meaning that I have had a good ground available in every room of the house/apartment.
b) In general, I have linked all my audio components, even those with 3-wire cords from the factory, together and thence to a good ground. For those components not equipped with an actual ground lug, I have used a spade lug on an exterior screw in some unobtrusive location.
c) And, just for common sense, kept the path as short as possible.
d) Using fairly robust fine-stranded copper wire, usually 12-gauge.

I have never had any hum problems due to this method. And on at least one occasion, when a very-vintage power-transformer chose to fail on a very vintage, very cheap solid-state amp, the fuse blew on the amp rather than giving me or anyone else a shock.

When I was living and working overseas, our villa had central AC, all ductwork, no suitable, similar local grounds available but the ground in the receptacle (NEMA style). Being the boss, I had our electrician drive a ground rod outside (even though we were in a desert country we were very near the Gulf, so the water table (brackish) was about two meters down). From there, I came up in insulated #8 stranded wire to my work bench, and to the living room for the grounds, one for my hobby, the other for the stereo.

Point being, after all this, I would not rely on the third leg of a receptacle being always a “good ground”, especially as compared to another receptacle or other local ground such as a water pipe. Nor, these days, would I rely on a water pipe much as with the advent of shark-fittings, PEX, dielectric unions and other devices and materials, continuity is often interrupted.
And, especially in older installations, BX (armor-flex/MC) cable relied on the exterior armor for the ground, and older RX (Romex) permitted the ground wire to be one gauge smaller than the primary wires. Such grounds are adequate for appliances, tools, and similar, but not so much for audio.

As things stand right now, I use the heating system standing radiators on black-iron or copper risers, verified all the way to the main municipal water service as grounds for the five operating stereos and my workbench, as described above. No hum issues.

I very strongly urge individuals who own/operate/maintain audio equipment, especially older/vintage/legacy equipment however well restored to install a separate, high-quality ground rather than rely on the mains ground, which can be problematic or worse. Keep in mind that the tools that verify correctly polarized receptacles and grounds are pretty broad-brush devices, such that resistance of a few ohms will not show as a failure – but could be damaging to equipment, or its users. I know that this might be difficult for renters or apartment dwellers. But in those cases, be absolutely sure to make sure, and be sure, again, that _all_ your equipment is connected to the same ground and not several. And if all your equipment shares the same preferably dedicated circuit, that is even better.


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 Post subject: Re: Grounding Techniques
PostPosted: 02 Dec 2016, 05:17 
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Joined: 20 Nov 2016, 08:33
Posts: 5
Can I summarize this as....

1. Chassis directly connected to mains power ground (Earth/3rd leg of the IEC connector) - protects from shock
2. Power ground - This is the -ve pin of the DC supply (possibly after transformer and rectifier). This is connected to #1 via the resistor and capacitor in parallel. It handles the leakage current in the circuit.
3. Signal ground - (outer connector of the RCA jack) - This is connected directly to #1.

Please confirm.


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