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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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5th Jun 2021, 11:25 am | #1 |
Octode
Join Date: Jun 2005
Location: Sheffield, South Yorkshire, UK.
Posts: 1,227
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Surprise today
I tried to use a power supply that had been unused for 3 years (2V, 90V, 120V) and it tripped the wall socket .My kitchen has been rewired recently to 'latest standards' , professionally. I found that the Neutral pin on the power supply is s/c to earth and case . Is this correct? It is a home made device which worked well last time. I can post photos and circuit later.
Last edited by Pete_kaye; 5th Jun 2021 at 11:27 am. Reason: Am I in wrong section? |
5th Jun 2021, 11:29 am | #2 |
Pentode
Join Date: Sep 2020
Location: Weymouth, Dorset, UK.
Posts: 135
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Re: Surprise today
Thats why it tripped neutral and earth must remain separate thruout your instalation the only place a joint is allowed is at the service head or elsewhere on the DNO side of the network
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5th Jun 2021, 11:33 am | #3 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Re: Surprise today
You normally wind up with a couple of volts on neutral with respect to earth due to ohmic drop in the neutral connections. Short neutral to earth and the earth will carry a bit of the current, plenty enough to trip an RCD.
So no-surprise. It doesn't always happen, but enough to be quite common. David
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5th Jun 2021, 11:35 am | #4 |
Dekatron
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,130
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Re: Surprise today
I assume that there was no RCD in the circuit when it "worked well", so a s/c neutral to earth would have gone undetected. Any such connection will trip an RCD every time as there will be an imbalance between the current through the phase and that through the neutral - exactly what an RCD is designed to detect.
If you wish to use this power supply, it will be necessary to ensure isolation between the neutral and anything earthed.
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5th Jun 2021, 6:20 pm | #5 |
Octode
Join Date: Jun 2005
Location: Sheffield, South Yorkshire, UK.
Posts: 1,227
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Re: Surprise today
Am I mistaken but was there a sticky thread on home built power supplies like mine? I can’t find it anywhere. I want to compare circuits.
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5th Jun 2021, 6:52 pm | #6 |
Octode
Join Date: Jun 2005
Location: Sheffield, South Yorkshire, UK.
Posts: 1,227
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Re: Surprise today
Found it. Via vintage-radio.com projects to build
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5th Jun 2021, 7:03 pm | #7 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,577
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Re: Surprise today
The question is 'Why'?
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6th Jun 2021, 11:40 am | #8 |
Hexode
Join Date: Mar 2014
Location: Fareham, Hampshire, UK.
Posts: 482
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Re: Surprise today
To eliminate a possible voltage between neutral and earth on old installations with insufficient earthing arrangements. Can’t think of any other reason.
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6th Jun 2021, 12:31 pm | #9 |
Hexode
Join Date: Mar 2020
Location: Rochdale, Greater Manchester, UK.
Posts: 453
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Re: Surprise today
Hi - I wondered if the quote "I found that the Neutral pin on the power supply is s/c to earth and case" means that a resistance check revealed that as a fault condition that has arisen, or inspecting the wiring showed an intentional direct wired connection? Two different scenarios that would require different remedial action.
Good luck with it Chris |
6th Jun 2021, 1:43 pm | #10 |
Octode
Join Date: Jul 2009
Location: Cheshire, UK.
Posts: 1,738
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Re: Surprise today
Was it built to the circuit shown here?
https://www.vintage-radio.com/projec...y-set-psu.html I can't see why it would have a short from the case ground to neutral, unless there is a fault such as a trapped wire to the transformer or another fault. Edit: maybe not that one as the first post mentions three outputs of 2V, 90V & 120V. David |
7th Jun 2021, 10:28 am | #11 |
Dekatron
Join Date: Dec 2007
Location: Haarlem, Netherlands
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Re: Surprise today
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7th Jun 2021, 10:30 am | #12 |
Octode
Join Date: Jun 2005
Location: Sheffield, South Yorkshire, UK.
Posts: 1,227
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Re: Surprise today
Sorted after an early breakfast. It turned out that a short was occurring due to worn insulation on the N pin to transformer which only happened when the back plate was screwed in. It gave good voltages except for the one labelled 1.4v reading 1.7 .
Last edited by Pete_kaye; 7th Jun 2021 at 10:58 am. |
7th Jun 2021, 12:20 pm | #13 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
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Re: Surprise today
That will need sorting as 1.4V valves don't like any sort of overload. Is it stabilised? (I think it should be if it isn't). Maybe it will read closer to 1.4V on load.
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7th Jun 2021, 1:07 pm | #14 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
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Re: Surprise today
The circuit you mentioned is for later miniature valve battery sets and includes a stabilised 1.4V. The example you have has been modified for earlier radios that use 2V filaments and a grid bias battery. Most likely the extra voltages are created using simple potential dividers.
I built a similar one but with a higher AC volts from the transformer and no voltage doubling. In mine, the grid bias voltages are taken from the HT supply by 'stealing' some HT voltage with a zener. |
8th Jun 2021, 9:01 am | #15 |
Octode
Join Date: Jun 2005
Location: Sheffield, South Yorkshire, UK.
Posts: 1,227
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Re: Surprise today
This is the circuit. It was built by the late Trevor Beamond(G3VLF) and given me by his sister recently.
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8th Jun 2021, 2:00 pm | #16 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
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Re: Surprise today
Looks good, the Zener in the HT centre tap is a bit odd but I guess the HT volts was too high. Providing the transistor that supplies the 1.4V is still OK, I would expect the voltage to fall under load but I suggest you test it with a dummy load.
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8th Jun 2021, 3:57 pm | #17 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
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Re: Surprise today
Doesn't the HT- to centre tap zener provide the -ve bias supply voltage scaled by the resistor chain for the different bias voltages?
A variation on a resistor in the same place to develop bias voltages for a set with a known constant HT current load (such as one with single ended class A output). There're a few neat "wrinkles" in the design to get the differing HT and LT voltages too!
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11th Jun 2021, 11:04 am | #18 |
Octode
Join Date: Jun 2005
Location: Sheffield, South Yorkshire, UK.
Posts: 1,227
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Re: Surprise today
I connected a 39 ohm across the 1.4 v and it went down to 1.44v . I think is ok
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11th Jun 2021, 12:08 pm | #19 |
Dekatron
Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
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Re: Surprise today
Hi Peter I think you got the numbers reversed, neat power supply could be tempted to build one , Mick.
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11th Jun 2021, 1:15 pm | #20 | |
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Re: Surprise today
Quote:
Cheers Mike T
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