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Old 9th August 2007, 04:00 PM   #11
ilimzn is offline ilimzn  Croatia
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Originally posted by StoneT
If you start with the available rectifiers you either have half wave Voltage doubler or full wave bridge.
With a half wave rectifier, assuming everything is connected the right way and no DC potential between neutral and earth (a big and dangerous assumption, I know) you end up with your earth, a common at earth potential and rectified 240V line, (340V if feeding a cap). Fine. However If the previous assumptions were wrong you could end up with: Earth, a rectified line at near earth potential with whatever DC offset there may be and a common at 240V (340V) below this.
So why is this a problem? Well assuming your chassis is earthed and the circuit common isn't tied to ground anywhere within, the greatest danger is from the connection terminals (speaker / RCA etc). The common for these would then be several hundred volts away from earth. Even assuming no-one touches these it would nicely fry any other piece of equipment you connect.
And of course fry anyone who touches two chasis with a 340V DC potential difference.

I thought maybe you could have a small circuit to test which wire is neutral and then route the mains the opposite way if connected wrong
Wouldn't help anyway as there is always some difference between ground and neutral, which, if nothing else, will trip the differential safety switch for your mains.

That leaves the bridge. With this you get 2 rails symmetrically again at +/- 120V(170V)
Check again, you don't. You get one at 340DC and one close to neutral, by a diode drop. Again, your 0V is neither neutral nor ground and if connected accidentally will blow equipment.

In other words, you must use some form of galvani isolation, period. Either for the mains, or for all other signals (in the latter case everything on the enclosure must be isolated from the mains in any form, or isolated, period. Just think old tube TV sets with no mains transformer and series connected heaters. There is a reason they were wood or later plastic.
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Old 9th August 2007, 04:02 PM   #12
ilimzn is offline ilimzn  Croatia
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Originally posted by kevinkr
I'd do some research into the consequences of this design choice and I think you'll be able to persuade yourself quite effectively of the ill-advisability of this move.
Or, as someone recently very eloquently put it: a transformer costs less than a coffin and a funeral.
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Old 9th August 2007, 04:04 PM   #13
kevinkr is offline kevinkr  United States
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Originally posted by ilimzn

Or, as someone recently very eloquently put it: a transformer costs less than a coffin and a funeral.
At the very least..
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 10th August 2007, 10:08 AM   #14
StoneT is offline StoneT  United Kingdom
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OK, I really should have done a bit more thinking about this before making that post.
Thanks everyone for the replies.

Originally posted by kevinkr

I thought Miles spelled it out pretty well in his response to the original poster.
That's just the thing, he said there could be things wrong with the supply (which I was already aware of) but didn't say why these things matter.
To my mind if you have a bridge rectifier, both input terminals are treated the same and so it doesn't matter if they are reversed. and in that respect I was right.

Originally posted by ilimzn

Check again, you don't. You get one at 340DC and one close to neutral, by a diode drop.
OK, I did check again. I did a very quick spice sim to see what the output is.
Result: We are both wrong!
The attached image shows the outputs for resistive load and cap input respectively. My '0V point' is at ~120V ac either way.
Not very useful for connection to anything else!

It is therefore only possible to use the half wave rectifier which I fully understand is highly inadvisable.

One thing this has shown me is I had assumed like ilimzn that if you use a transformer and ground the lower output of the bridge rectifier, your 0v line is only a diode drop away from the lower rectifier input. So grounding one terminal of the transformer instead would only raise the common by the diode drop. This seems to be true for a resistive load but for cap input, grounding the transformer terminal would produce rectifier output as if the transformer wasn't there. You are basically making one terminal into a 'neutral'

I guess I'll be buying a HUGE isolating transformer for my OTL then.
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