Power Supply & Signal Grounding

[peebee]

Just curious:

In lots of home audio-video equipment, there's no ground conductor in the power cord, no 3-prong plug.

Is there any signal reference? Are the signals ungrounded/isolated?

What happens when a signal connection is made to a different piece of equipment? Say it's a cable TV box -- does the cable TV system become part of a fault current path? Say it's a TV with a grounded plug -- does that become the ground reference for all equipment?

I'm used to older equipment with linear supplies and a solid ground bond. I have no idea how signals are referenced in newer equipment.

 

[IRstuff]

Absolutely.

I decided during freshman yr to go digital, because I tried to debug a light organ that had an 2-prong plug and it turned out that one prong was intended to be tied to neutral and become the reference. Unfortunately, I used a 3-prong oscilloscope and when I attached the ground lead on the scope probe, a 741 op amp exploded in my face. Way too dangerous;-)

I've had other situations where I routinely fried RS-422 receivers, until I realized that the source and sink were being powered up from separate and isolated AC supplies, this allowing very high ground potential differences to be equalized through the input emitter on the differential receiver. TTFN

 

[peebee]

Yup, UL double insulated doesn't help much when you start poking around inside with long thin metal objects.

Wow, I'm surprised they reference to neutral in the organ. That doesn't seem possible with anything with a signal output intended for connection to other equipment, such as A/V equipment, or rather, it seems like it would lead to large currents flowing through the patch cords.

 

[Lewish]

Well, let's see if the old analog engineer can shed any light on the question. First off, you didn't identify what country you are in, but I am guessing from the way you worded the question that you are in the U.S.
If you look at your new equipment with the 2 prong plug, you will notice that it is polarized. One blade is wider than the other. The manufacturer has assumed that you are in a newer home and that your wall outlets will allow the plug to be plugged in only one way. Thus, the neutral is the wide blade on the left as you look at the wall outlet. That statement, of course, assumes that someone hasn't installed the outlet upside down.
Anyway, this insures that all you equipment has the same neutral connection, and thus are all at the same potential. If you follow the wiring of the house back to the breaker panel, you will find that the ground lead and the neutral lead are attached to the same rail in the breaker box, and that rail is connected to a ground wire, which goes to earth ground.
So, even if some of your equipment has 3 prongs, and some has 2 prongs, it is all referenced to the same ground eventually. Now the last part of that statement is where problems arise. Because everything is ground referenced back at the breaker panel, it is possible for there to exist a diffence in ground potential if some equipment is plugged into wall plug A and other is plugged into wall plug B. You will then get ground current flowing between the devices and will get 60 Hz plus harmonics noise on you interconnecting cables.
So, what to do.... Well, if you have ground noise, then connect a grounding wire between the chassises of the various pieces of equipment. That is what I had to do for my audio equipment. Fortunately, the manufacturer put a convenient lug for that purpose on each chassis. Problem solved.

I hope this helps.

 

[peebee]

Wow, that's scary.

In a past life, I was a member of a power trio band. Amps are typically provided with a 3-prong plug, but often that still was not enough to prevent one of us from getting a shock while touching the guitar & mic at the same time. Using a single power strip to power both amps helps that somewhat, but does not always eliminate it.

And I've personally witnessed many recent installations where the neutral & hot were reversed at the receptacle. And at my 100-year old house, with 60-year old wiring, where both hot & neutral have indistinguishable white cloth insulation, the chances of any existing receptacle being wired correctly are about 50/50. Any time I work on the electric, I'll get the receptacle tester out. But there's still lots of receptacles both new and old around the country that are wired backwards.

Even with proper wiring, it's not uncommon to have relatively large potentials on both the neutral & ground conductors, as well as a voltage between them.

Seems like all this could be imposing some rather large currents on the A/V patch cord & CATV shields.

It also seems like this could be causing lots of neutral-ground bonds downstream of the panel in violation of NEC.

 

[peebee]

Here's another related question: what kind of power supplies do most A/V sources use now (eg, VCR's, DVD, etc.). Are they all switching power supplies? The equipment doesn't seem heavy enough to have any large transformers in it, but maybe the power requirements are just very low. It seems like most stereo receivers still have linear supplies in them.

I'm too lazy to open my own stuff up.

 

[jbartos]

Suggestion to the previous posting: It is good not to be lazy to have a good peace of mind.
DC-DC power conversion is convenient over switching since it eliminates transformers in comparison to the DC-AC-DC conversion.
AC-DC is also convenient over switching.
AC-AC conversion; it just depends on an application and criteria, e.g. harmonic content.