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 Post subject: Safety Power Supply
PostPosted: Apr Wed 11, 2018 12:31 pm 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 759
Location: Tokyo
I found this circuit in May 1947 Radio Craft. Couldn't 1N4007 diodes be used instead of a tube rectifier?

Rob


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 Post subject: Re: Safety Power Supply
PostPosted: Apr Wed 11, 2018 12:46 pm 
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Posts: 24070
Location: Annapolis, MD
Yes.....but: regardless of what rectifier is used, the circuit is NOT safe. Connecting one side of the line to chassis creates a shock hazard---even if the connection only passes one-half of the cycle.

We need to get in our time machine, and get that author re-educated....

EDIT: both cycles of AC go to chassis.....one through a capacitor.

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 Post subject: Re: Safety Power Supply
PostPosted: Apr Wed 11, 2018 1:44 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1327
Location: Jackson, TN
Yes, agree with Mark's assessment.

The circuit would certainly limit fault current since each leg flows through a tube diode.

There would still be plenty of current available let to fry the grounded victim! :shock:

Tim


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 Post subject: Re: Safety Power Supply
PostPosted: Apr Wed 11, 2018 10:12 pm 
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Joined: Nov Sat 26, 2011 4:09 am
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Location: Texas. USA
pixellany wrote:
Yes.....but: regardless of what rectifier is used, the circuit is NOT safe. Connecting one side of the line to chassis creates a shock hazard---even if the connection only passes one-half of the cycle.

We need to get in our time machine, and get that author re-educated....

EDIT: both cycles of AC go to chassis.....one through a capacitor.
"John" apparently, and erroneously, thought that a 'diode' isolates so that connection though a diode is not "directly to the chassis." All the second diode does is introduce another diode drop, not make the supply 'safe'.

It's a line powered half wave rectifier with an extra, useless, diode to chassis.


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 Post subject: Re: Safety Power Supply
PostPosted: Apr Wed 11, 2018 10:20 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2223
Location: Saskatoon
And to answer the original question about using 1N4007 diodes instead of the 6H6, it would make the circuit even more dangerous because the solid state diodes would not provide any significant current limiting.


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 Post subject: Re: Safety Power Supply
PostPosted: Apr Wed 11, 2018 11:28 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 17936
Location: Albuquerque, NM 87123
Fixed bias for amplifiers of 120 volts DC? Maybe for some transmitters. I doubt the 6H6 could deliver much current though; one little mistake and the tube is history.


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 Post subject: Re: Safety Power Supply
PostPosted: Apr Thu 12, 2018 2:11 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 759
Location: Tokyo
So, the article was not peer reviewed before publication. I'm shocked.

Perhaps the author meant the circuit was safe as long as it was turned off: with the heaters cold, there's no current flow, so the chassis would be isolated from the AC line. A DPST on/off switch would be cheaper and easier, though.

Rob


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 Post subject: Re: Safety Power Supply
PostPosted: Apr Thu 12, 2018 2:31 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 26968
Location: Detroit, MI USA
Regardless of what the author was thinking, it's still not safe and can't be made safe. You have to wonder whether people wrote in after reading the article, and a later statement was published regarding how unsafe it really was.

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Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: Safety Power Supply
PostPosted: Apr Thu 12, 2018 3:46 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2223
Location: Saskatoon
shinkuukan wrote:
So, the article was not peer reviewed before publication. I'm shocked.

There's a good chance you will be, if you build that thing. :P


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 Post subject: Re: Safety Power Supply
PostPosted: Apr Sat 14, 2018 5:29 am 
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Joined: Nov Tue 14, 2017 5:09 am
Posts: 949
Location: Austin, Texas
Speaking of using the wrong tube for a rectifier, I was recently surprised to find radios that used the type 37 audio triode as the power supply rectifier. They also used resistor power cords dropping 90 volts to run four 6 volt tubes. They must have been the definition of "curtain burner".

Jay


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 Post subject: Re: Safety Power Supply
PostPosted: Apr Mon 16, 2018 5:14 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 26968
Location: Detroit, MI USA
JnTX wrote:
Speaking of using the wrong tube for a rectifier, I was recently surprised to find radios that used the type 37 audio triode as the power supply rectifier. They also used resistor power cords dropping 90 volts to run four 6 volt tubes. They must have been the definition of "curtain burner".

Jay


You can design something cheaply, and it might marginally work well enough to sell. The worst case I ever saw was a set using two top cap tubes like a 78 as rectifiers and they had one side of the AC line connected directly to the metal tube caps and their clips. You wouldn't think some of that stuff could have legally been sold, but this was apparently before the UL cracked down on the smaller independents over safety considerations.

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Experience is what you gain when the results aren't what you were expecting.


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