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 Post subject: Doubts on power supply
PostPosted: 03 May 2017, 10:40 
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Joined: 29 Dec 2016, 03:06
Posts: 18
I've found this project that uses a 24VAC wall wart supply to get high voltage through a backwards installed transformer. What makes me suspicious is that the 24VAC supply input is floating, not connected to chassis nor earthed. But then the final high voltage has no earth connection neither, as the chassis has no connection to earth. Isn't it dangerous?

http://diyaudioprojects.com/Tubes/Unive ... Preamp.htm


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PostPosted: 03 May 2017, 13:08 
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Joined: 08 Aug 2009, 03:11
Posts: 2133
Location: Chilliwack, BC
Not dangerous, because it's isolated from the line.

Cheers!

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PostPosted: 03 May 2017, 14:00 
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Joined: 29 Dec 2016, 03:06
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But the B+ is 250V, after passing through the step-up transformer, and it's quite high! What could happen if a cable with that voltage contacts with the chassis, and you touch it?


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PostPosted: 03 May 2017, 15:47 
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Joined: 06 Apr 2009, 10:08
Posts: 1375
Location: US Pacific Northwest
JordiMB wrote:
Isn't it dangerous?
The answer is, "It depends". There are numerous appliances which have non-grounded chassis. In general, the approach to making this "safe" is called "double insulation". In the US, the standard used by manufacturers is UL1097. If this standard is followed, then the appliance without a grounded chassis can still get the UL seal of approval. This is why you'll find lots of stereo equipment with metal chassis and two pronged plugs.

WARNING: YOU ARE RESPONSIBLE FOR YOUR OWN BUILDS. READ THE FOLLOWING AT YOUR OWN RISK!

In practical terms, this is a matter of educated usage. If you are building something for yourself, and you are reasonably confident in your construction technique, earth grounding a chassis is not strictly required. Also, If your chassis is non-conducting, and your controls have isolation, then it's not particularly dangerous. Remember that current always flows in a circuit (read "looped path"). To be shocked, there has to be a path through your body for the current to flow. In Mark's design, the wall wart transformer provides one layer of isolation from earth ground, the wooden chassis provides a second. This is a "reasonable" approach. It will not however, pass the tests of UL1097 because there is exposed metal.

The real problem with this design approach is if the preamp output is connected to an amplifier with an earth ground referenced signal ground. In this case, the initial isolation layer is defeated and the risk increases.

At the heart of the issue is that there is really no definition of "Dangerous" (except maybe for a pissed off wife). What we are really addressing is "risk" not "danger". And you can never completely eliminate risk. You can provide some degree of mitigation, but that's the best you can do. And in the end, how much risk you are willing to accept for yourself, is your own business.

Just remember, if you are selling something, then there is the issue of liability law with which you need to contend. This is probably an area best addressed by an attorney. Or someone that runs a business and has consulted their own legal counsel.

I hope this helps.

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PostPosted: 04 May 2017, 00:59 
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Joined: 29 Dec 2016, 03:06
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So the correct way to build a project like this would be using a non-conductive housingwith no metal parts exposed?

(Not selling, just a personal project)


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