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 Post subject: Stupid bucking transformer/fuse question
PostPosted: Sep Wed 22, 2021 3:55 pm 
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Joined: Jan Thu 24, 2013 11:50 pm
Posts: 3545
Location: Bristol, Rhode Island
Hi all,

I'm going to install a bucking transformer under the hood of a Zenith 8S154 I just purchased and recapped, to keep it cool. I would also like to add a 1A fuse.
My question: Should I wire the fuse before or after the bucking transformer, or does it not matter?

Thanks.


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 Post subject: Re: Stupid bucking transformer/fuse question
PostPosted: Sep Wed 22, 2021 4:10 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: S. Dartmouth MA 02748-1225 USA
Before...

May have to use a a slo-blo fuse...

Chas

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 Post subject: Re: Stupid bucking transformer/fuse question
PostPosted: Sep Wed 22, 2021 4:14 pm 
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Joined: Jan Thu 24, 2013 11:50 pm
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Location: Bristol, Rhode Island
Thanks Chas. Yep, 1a slowblow is what I have. The radio's currently pulling .6a or so.


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 Post subject: Re: Stupid bucking transformer/fuse question
PostPosted: Sep Wed 22, 2021 6:41 pm 
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Joined: Jan Thu 24, 2013 11:50 pm
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Location: Bristol, Rhode Island
Follow-up: Would I need to install a three prong cord with this, as shown in the schematic, or could I just go with a two-prong?
Haven't wired one directly into a set yet, so want to make sure I do it right.

Thanks,

Ted


Attachments:
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Last edited by Diverted on Sep Wed 22, 2021 7:18 pm, edited 1 time in total.
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 Post subject: Re: Stupid bucking transformer/fuse question
PostPosted: Sep Wed 22, 2021 6:53 pm 
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Location: Long Island NY
The fuse should be the first thing the power hits when it gets inside the radio. That way if anything goes wrong with the bucking transformer the fuse will prevent anything untoward from happening.

What schematic? The radio sure didn't have a three wire cord when it came from the factory. Is it some internet DIY bucking transformer diagram? If so, they may have shown a ground connection for completeness, since it would need to be carried through on a bucking transformer with an outlet intended for any appliance to be plugged in. But it wouldn't really make a whole lot of difference if you are plugging in a radio that has a two-wire cord and plug.

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 Post subject: Re: Stupid bucking transformer/fuse question
PostPosted: Sep Wed 22, 2021 7:19 pm 
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Location: Bristol, Rhode Island
Sorry Chris, spaced it out and forgot to attach the pic. It's been added above.
I was assuming I would just eliminate the earth ground as shown in the above pic.

Thanks!


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 Post subject: Re: Stupid bucking transformer/fuse question
PostPosted: Sep Wed 22, 2021 9:14 pm 
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Since it's AC voltage you are fusing, I think it makes only a few micro seconds of difference which leg of the power cord has the fuse, AC pulsing back and forth at 60 cycles/sec.
Before we adopted the polarized standard for AC, it was common to see a switch on one side and the fuse holder on the other.
Put it on the hot side ahead of the switch to be the most correct, but again, right after the switch might be more practical and the difference is a nanosecond.

Only thing stupid here is wastied title space with a stupid word. :D

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 Post subject: Re: Stupid bucking transformer/fuse question
PostPosted: Sep Wed 22, 2021 9:42 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Minneapolis, MN USA
Why is slo blo recommended for this application ?
Fast fuses are already slow enough, slow blow even worse.
For a 1 amp nominal draw and exposed to a 200 percent over load...
A fast blow will take 5 seconds max to open.
A slow blow will take between 5 and 30 seconds to open.


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 Post subject: Re: Stupid bucking transformer/fuse question
PostPosted: Sep Wed 22, 2021 10:33 pm 
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Personally, I would choose a 1A fast-acting fuse. Usually slow-blow fuses are used for equipment like motors with very large starting currents. Motors can easily draw double their normal current for a few seconds while bringing the load up to speed. Tube electronics effectively has a soft-start characteristic, so you don't need such a long time lag. So long as the fuse isn't loaded to more than 75% of its rating, you shouldn't have trouble with nuisance blowing.

As Chris108 said, the fuse should be the first component after the power cord, and in the hot lead. If the radio uses a power transformer, the metal chassis should be earthed thru the third prong for safety. (Transformerless AC/DC radios have the chassis connected to a listed Y-class capacitor instead.)

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 Post subject: Re: Stupid bucking transformer/fuse question
PostPosted: Sep Wed 22, 2021 11:12 pm 
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Diverted wrote:
Thanks Chas. Yep, 1a slowblow is what I have. The radio's currently pulling .6a or so.
Your fine with your choice, if the radio did not use a ground do not install one. Turn-on very shortly after turn off the inrush can exceed a standard fast blow leading to nuisance fuse blows. Most transformer power TV's had slow blow as well as other motor/transformer power equipment in the home. The fuse is to protect the premises not the device.

Recently retro-fitted a China Dehumidifier, custom, a soldered in slow blow of 3.8 amps went when the was a surge from a power line failure (Little Compton address). Replaced with a renewable 3AG type in a inline holder. Welcome satisfaction as well as adequate protection... Dehumidifier was just out of its 3 month warranty, an online purchase too...

Chas

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 Post subject: Re: Stupid bucking transformer/fuse question
PostPosted: Sep Thu 23, 2021 12:32 am 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island NY
I think a 1-amp slow blow fuse would be fine in this application. Don't forget, a bucking transformer is being added to the system so its inductance will be added to that of the radio power transformer. The instantaneous inrush current could be higher when the switch is closed than it would be for the radio alone.

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 Post subject: Re: Stupid bucking transformer/fuse question
PostPosted: Sep Thu 23, 2021 5:33 am 
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Joined: Dec Sat 28, 2019 4:18 pm
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Location: Corinth, TX
I sort of agree with westcoastjohn and chris: Put a 1 amp fast blow fuse in the hot lead ahead of everything and because the set is transformer powered, install a three conductor power plug - and make sure that the wall outlet is properly wired.

TexasJohn :)


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 Post subject: Re: Stupid bucking transformer/fuse question
PostPosted: Sep Thu 23, 2021 1:56 pm 
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Joined: Jan Thu 16, 2020 12:29 am
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westcoastjohn wrote:
Since it's AC voltage you are fusing, I think it makes only a few micro seconds of difference which leg of the power cord has the fuse, AC pulsing back and forth at 60 cycles/sec.
Before we adopted the polarized standard for AC, it was common to see a switch on one side and the fuse holder on the other.
Put it on the hot side ahead of the switch to be the most correct, but again, right after the switch might be more practical and the difference is a nanosecond.

Only thing stupid here is wasted title space with a stupid word. :D



Well, the notion of which line has the fuse, the phase or the neutral is very important.

In some countries, the apparatus had both phase(active or live) fuses and a neutral fuse. This is not legal in others and in those countries it was required that the neutral fuse got disabled.

The reason is that if the fuse is in the neutral line and it blows, the fault on the active line could lift something to the line potential in the equipment, or elsewhere. So the blown neutral fuse itself, would not prevent the transmission of power from that equipment, say to a user's body, or possibly to other equipment it was plugged on to elsewhere.

So the fuse, should always be immediately on the active wire as it enters the apparatus , regardless if that apparatus has an earthed chassis or not and never on the neutral.


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 Post subject: Re: Stupid bucking transformer/fuse question
PostPosted: Sep Sat 25, 2021 5:21 am 
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Location: Vancouver Island, Canada
Thanks for the clarification and you are right, a blown fuse on a neutral leaves the device energized and that would present a shock hazard.

_________________
Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.
[:l>)


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 Post subject: Re: Stupid bucking transformer/fuse question
PostPosted: Sep Sat 25, 2021 2:42 pm 
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Location: Victoria, Australia
A moot point that wiring codes vary internationally and who can do domestic wiring is another factor. It seems your codes were fairly hap hazard and for a considerable time phases could be inverted and plugs reversed. A reason for a three wire plug would be to stop it being reversed and the fuse not being in the active line.

Regulations here are strict; A two pin plug top cannot by design, be reversed and DIY house wiring is banned. Reversal within a socket is rare. This must be done by the appropriate electrician and Insurance ramifications and other penalties apply. Regulations here even in early radio times always stated "That if any metal on a mains powered device (240V here) could become live and could be touched" it had to be grounded. Whilst the insulation in our transformers was as good as "double insulation", We still tend to ground transformer sets, especially if the transformer is a shielded type, or there are line caps to a metal chassis.

It is therefore imperative that steps are taken to ensure the fuse in always on active, and the bucking arrangement is an "Auto transformer", like Variacs (slide regulators) and provide no isolation. It is good policy with mains / utility powered sets to unplug it before attempting to change bits.

One has to be careful. I did work in OH&S & have seen a wide range of stupidity.

Marc


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