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Basic & Safe Case/Chassis for chip amp
Basic & Safe Case/Chassis for chip amp
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Old 1st April 2011, 03:11 PM   #1
Baztien is offline Baztien  France
diyAudio Member
 
Join Date: Feb 2011
Location: Bath, UK
Question Basic & Safe Case/Chassis for chip amp

Hi all
I'm willing to build a basic looking case to house my chip amp and its power supply.

I first considered putting everything in a wood enclosure because wood is cheap, easy to cut, easy to drill through, good looking and non conductive.

I also considered buying an ABS case but didn't like the look of it.

But then I got concerned by the safety side of things when I was looking for the connections to make to earth my project.
I was planning to ground my pcb, speakers, transformer and its metal casing directly to the earth wire of the mains lead.

All the guides I've read mention a metal case to house amp and transfo (Rod Elliott, Decibel dungeon). So I understand you connect all the ground wires to your case and you then connect the case to your mains earth wire.

My options are:
-It is important to have a full metal case if you want to safely house both amp and transfo
-Is it good enough to have a bottom metal panel and the others as non-conductive material (it's the case of my mini Hi-Fi)
-My first idea of full non-conductive box is isolated enough as long as you ground all the necessary to the earth mains wire

Which one is recommended for a first project (cheap preferably)?

Many thanks
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Old 1st April 2011, 05:53 PM   #2
simon7000 is offline simon7000  United States
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Join Date: Nov 2008
Location: Oakmont PA
Quote:
Originally Posted by Baztien View Post
Hi all
I'm willing to build a basic looking case to house my chip amp and its power supply.

I first considered putting everything in a wood enclosure because wood is cheap, easy to cut, easy to drill through, good looking and non conductive.

I also considered buying an ABS case but didn't like the look of it.

But then I got concerned by the safety side of things when I was looking for the connections to make to earth my project.
I was planning to ground my pcb, speakers, transformer and its metal casing directly to the earth wire of the mains lead.

All the guides I've read mention a metal case to house amp and transfo (Rod Elliott, Decibel dungeon). So I understand you connect all the ground wires to your case and you then connect the case to your mains earth wire.

My options are:
-It is important to have a full metal case if you want to safely house both amp and transfo
-Is it good enough to have a bottom metal panel and the others as non-conductive material (it's the case of my mini Hi-Fi)
-My first idea of full non-conductive box is isolated enough as long as you ground all the necessary to the earth mains wire

Which one is recommended for a first project (cheap preferably)?

Many thanks
A metal case provides shielding from outside interference (EMI). For safety all you really need to do is be sure the safety ground is connected to the power transformer frame. I don't know which codes apply to where you live. But the current trend by ISO and UL are to require high levels of insulation to make sure AC line voltages do not make it to any surface consumers can come in contact with.

So if your mains transformer has side by side windings safety grounding may not be required.

Using the safety ground as a drain wire for outside interference is not a good idea. It has too much inductance to be useful and may even bring in some noise such as light dimmers and food processors.

If it works fine in a wood box then EMI is not a problem for you. Next step is to line the box with aluminum foil, finally a solid aluminum box unless you are crazy then a silver plated copper box should be quite the thing.

Of course proper fuse size is extremely important.

ES
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Old 4th April 2011, 09:08 AM   #3
Baztien is offline Baztien  France
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Join Date: Feb 2011
Location: Bath, UK
Many thanks for the reply

Quote:
Originally Posted by simon7000 View Post
Using the safety ground as a drain wire for outside interference is not a good idea. It has too much inductance to be useful and may even bring in some noise such as light dimmers and food processors.
I'm not sure I understand that correctly. When you say ground "outside interference" do you mean "the metal case providing the shielding"?

My transformer is rated 72VA so I calculate my fuse size by 72VA/250V=0.29A
I understand it should be slow fuse, is this correct? Also I don't think I will find a 300mA fuse easily so I'm thinking of going higher to 500mA

thanks for the input
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Old 4th April 2011, 02:08 PM   #4
simon7000 is offline simon7000  United States
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Join Date: Nov 2008
Location: Oakmont PA
Quote:
Originally Posted by Baztien View Post
Many thanks for the reply



I'm not sure I understand that correctly. When you say ground "outside interference" do you mean "the metal case providing the shielding"?

My transformer is rated 72VA so I calculate my fuse size by 72VA/250V=0.29A
I understand it should be slow fuse, is this correct? Also I don't think I will find a 300mA fuse easily so I'm thinking of going higher to 500mA

thanks for the input
The safety ground is just able to route fault currents to a safe place. If you ground the frame of your transformer and then isolate if from the chassis you have met that need. Some folks think that interference such as radio pickup or dimmer noise can be reduced by grounding a metal case through the safety ground. Not only does that not work, but it often makes things worse.

250 & 300 ma are standard fuse values. I would use a 250ma moderate time delay fuse. If the fuse blows at turn on you can size it up. Picking a fuse value that does not blow early but still protects is tricky, because you don't actually have all the information such as what is the maximum inrush current during initial charging.
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Old 4th April 2011, 03:53 PM   #5
Baztien is offline Baztien  France
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Join Date: Feb 2011
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Quote:
Originally Posted by simon7000 View Post
If you ground the frame of your transformer and then isolate if from the chassis you have met that need.
Is it not unsafe to keep some metal work like the chassis not grounded?
According to Elliott and decibel dungeon the chassis should be star earthed for safety.
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Old 4th April 2011, 04:25 PM   #6
simon7000 is offline simon7000  United States
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Join Date: Nov 2008
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Quote:
Originally Posted by Baztien View Post
Is it not unsafe to keep some metal work like the chassis not grounded?
According to Elliott and decibel dungeon the chassis should be star earthed for safety.
The new standard is to have sufficient insulation from any input power source. So if your power cord has insulated conductors inside an insulated jacket and you use an insulated bushing where it enters you have a good start. If your power switch has is a push on-off type with two separate insulators between you and the AC that works. So if you ground the transformer frame of a dual bobbin type and keep it insulated from the rest of the case you should meet all of the current codes. Good insulation is often tested with several thousand volts so a single layer of tape doesn't qualify.
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Old 11th April 2011, 11:19 AM   #7
Baztien is offline Baztien  France
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Join Date: Feb 2011
Location: Bath, UK
Thanks for your reply Simon

I think I'm gonna go for an aluminium case which I found recently. I've only got 4 aluminium panels on it. Is it enough to protect against EMI?
Also my local store hasn't got medium delay fuse. I could order some on-line but if it's not the right size fuse I'll have to keep ordering and paying the pp.
Can I used a slow fuse instead or is it definitely a medium?

regards
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Old 11th April 2011, 02:32 PM   #8
simon7000 is offline simon7000  United States
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Join Date: Nov 2008
Location: Oakmont PA
Quote:
Originally Posted by Baztien View Post
Thanks for your reply Simon

I think I'm gonna go for an aluminium case which I found recently. I've only got 4 aluminium panels on it. Is it enough to protect against EMI?
Also my local store hasn't got medium delay fuse. I could order some on-line but if it's not the right size fuse I'll have to keep ordering and paying the pp.
Can I used a slow fuse instead or is it definitely a medium?

regards
If EMI is a REAL problem you need to totally encase the unit. Four sides should be fine. If troubles turn up you might be able to use aluminum foil to screen things, but it really is only effective at RF frequencies.

Slow blow fuses should be fine.
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Old 12th April 2011, 04:25 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by simon7000 View Post
The new standard is to have sufficient insulation from any input power source. So if your power cord has insulated conductors inside an insulated jacket and you use an insulated bushing where it enters you have a good start. If your power switch has is a push on-off type with two separate insulators between you and the AC that works. So if you ground the transformer frame of a dual bobbin type and keep it insulated from the rest of the case you should meet all of the current codes. Good insulation is often tested with several thousand volts so a single layer of tape doesn't qualify.
This description does not address the requirement to have:

"all exposed conductive parts must be connected to the Safety Earth".

nor

"the PE must be permanently and mechanically fixed to the chassis".
__________________
regards Andrew T.
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Old 12th April 2011, 04:54 PM   #10
simon7000 is offline simon7000  United States
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Join Date: Nov 2008
Location: Oakmont PA
Quote:
Originally Posted by AndrewT View Post
This description does not address the requirement to have:

"all exposed conductive parts must be connected to the Safety Earth".

nor

"the PE must be permanently and mechanically fixed to the chassis".
"" from?
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