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Old 20th April 2009, 05:37 PM   #321
AndrewT is offline AndrewT  Scotland
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Hi,
fit a mains failure mute.
This can be either a mute on the input of the power amp, or a mute on the output of the power amp or a mute inside the circuit to reduce the gain, or a mute on the output of the source.

All or any of these options can be triggered by loss of mains voltage (equivalent to switching off).
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Old 9th May 2009, 04:27 PM   #322
Artie is offline Artie  United States
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Is it just me, or does this article have a serious potential safety hazard in it? Forgive me if I'm wrong, but here's what I see:

In Rob Elliot's article, (which is what this appears to be based on), he shows how to create the "loop breaker" circuit. This is illustrated with a diagram that shows the "Power Supply Circuit" on the inside of the "zero volt" area. The implication, (in the diagram), is that the complete power supply circuit, including the transformer, is connected to the zero-volt line, rather than true ground. A couple of paragraphs down, Rob points out, (in italics, no less), . . . Do not use any loop breaker circuit to isolate the transformer core, as it is unnecessary and dangerous to do so.

Unfortunately, Nuuk doesn't include this line in his article, but he still uses the diagram. To make matters worse, he suggests isolating the transformer from the chassis with rubber washers, in order to minimize mechanical vibration.

Now we have a situation where some newbie might be taking measurements inside his "live" amp, with a transformer that has in internal short to its frame. The largest chunk of metal in his circuit may have live AC on it, without blowing any fuse or tripping any breaker.

On a sidenote: IMHO, the "loop breaker" circuit shouldn't really be included in a "beginners" guide to power supply design. Its a "cute" way to resolve a ground loop problem, but its not something that I've ever seen utilized in any piece of electronic gear, consumer or pro, in forty-something years of electronics tech. I don't believe that all of this equipment is out there as a safety hazard. Just my 2-cents worth.

Maybe the diagram could be editted to something like this:

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Old 9th May 2009, 05:16 PM   #323
AndrewT is offline AndrewT  Scotland
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Hi Artie,
somewhat facetiously, I'm going to ask you to describe how you would connect a toroid transformer as you have shown?
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Old 9th May 2009, 05:30 PM   #324
Nuuk is offline Nuuk  United Kingdom
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I love it when Andrew comes out with the big words!
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Old 10th May 2009, 12:39 AM   #325
Artie is offline Artie  United States
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A lot of beginners won't be using toroids. Rob Elliot, himself, stresses the quote I have in bold above. I just think, for the sake of safety, that it be mentioned.
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Old 10th May 2009, 08:13 AM   #326
Nuuk is offline Nuuk  United Kingdom
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Artie, I'm still trying to understand what you are saying (but that's a wake-up to me that something in the PSU article may need clarifying further).

When I have used a clamp-mounting type of transformer (ie with a metal frame), I have always connected the frame to mains earth. So even if I use rubber washers under it, it is still connected to mains earth, albeit using a wire rather than one of the bolts through the frame to the equipment chassis.

Come to think of it, now you bring up this subject, I would suggest that this is another reason for beginners to use a toroid transformer, although we all know that DIYers will often use what they have in the parts box if it saves them money!

Quote:
Do not use any loop breaker circuit to isolate the transformer core, as it is unnecessary and dangerous to do so.
This is what I don't understand! Isolate the transformer core from what? From the mains, or the circuit that it is powering?

But thanks for bringing this up. If there is something that needs correcting, or clarifying in the PSU article, then it will be once we understand what that is.
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Old 10th May 2009, 11:21 AM   #327
Artie is offline Artie  United States
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Good morning Nuuk.

Quote:
Originally posted by Nuuk
When I have used a clamp-mounting type of transformer (ie with a metal frame), I have always connected the frame to mains earth. So even if I use rubber washers under it, it is still connected to mains earth, albeit using a wire rather than one of the bolts through the frame to the equipment chassis.
Thats great, but you don't actually say that on your webpage article. Its important to note that the quote, you have above, is from Rob Elliot's webpage. Not me.

There's two different issues here:

1. Apparently, according to Rob, its not unheard of, for a transformer to short internally to the frame. (Although, I've never seen it happen.) I'm just afraid that a beginner might not take the time to follow the links to Rob's page, and read the whole article, and then might miss the part about not isolating the tranny frame from ground. The problem is compounded by the fact that the diagram doesn't show it.

2. The second issue is the inclusion of Rob's "loop breaker" circuit in a "beginners guide". Thats far from a universally accepted practice. (I've never seen it done, anywhere.) That doesn't make it a bad idea, but it opens a whole new can-'o-worms for the beginner. More opportunity for mistakes, if you will.

I hope I haven't blown this up into more than it is, but you stress safety in your first post. Its just something that caught my eye. Perhaps, a short sentence on grounding the tranny would suffice.

Anyway, its a great article.

Artie
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Old 10th May 2009, 11:40 AM   #328
Nuuk is offline Nuuk  United Kingdom
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Artie, I agree about grounding the metal-cased trannies and have added that warning to the article. In my own case, I have only used toroids for some years now so thanks for bringing this issue to my attention.

But I am still unsure of your other point. The diagram that you query on a previous post is Rod Elliot's. If there is a mistake with it, or you think that there is, surely he is the one to ask about it.
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Old 11th May 2009, 09:25 AM   #329
Artie is offline Artie  United States
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Again, I don't mean to belabor the point, but the reason I mentioned it to you, rather than Rob, is because he uses that diagram in an article on "grounding", in a "Tips and Tricks" section. You use it in a beginners guide to power supply design. Consequently, you don't include all of the text that Rob does leading up to that diagram. (And after.)

The two pertinent texts from Rob's site are:

Quote:
The only exception is if a double insulated mains transformer is used, but these are rare. Should the transformer be of "conventional" construction (not a toroidal), then the transformer body - the steel core - must be connected to chassis directly. Do not use any loop breaker circuit to isolate the transformer core, as it is unnecessary and dangerous to do so.
and . . .

Quote:
In some cases, a transformer may be fitted with an electrostatic shield, which are lamentably uncommon in hi-fi transformers. Where provided, these too must be connected directly to the main earth point, and not via the loop breaker. Again, this is unnecessary and potentially dangerous.
Its probably not a huge deal. I just wanted to point it out since you use Rob's (somewhat) misleading diagram.
Feel free to use the one I've modified, if it helps. (I'm not sure it does.)

Artie
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Old 11th May 2009, 09:49 AM   #330
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Artie

The two pertinent texts from Rob's site are:

Do not use any loop breaker circuit to isolate the transformer core, as it is unnecessary and dangerous to do so.

and . . .

In some cases, a transformer may be fitted with an electrostatic shield, which are lamentably uncommon in hi-fi transformers. Where provided, these too must be connected directly to the main earth point, and not via the loop breaker. Again, this is unnecessary and potentially dangerous.
the reference to dangerous in both quotes is wrong.
The Disconnecting Network shown in ESP and Nuuk's sites can pass domestic rates of fault current to the protective earth (PE).
The one I tested showed no signs of damage to any of the components not even the low power resistor (600mW) and low voltage capacitor (50Vdc).
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