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 Post subject: Another way to "buck" the input voltage
PostPosted: Oct Thu 25, 2018 2:20 pm 
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Rather than install a separate bucking transformer, simply add a winding to the existing power transformer. To see if there is room, take off the outer covers and see if there is enough space between the winding bundle and the laminations to slip in something like 20 or 22 gauge wire. In some cases, you might have to remove some paper insulation, and possibly redo some of the connections to the lead-outs.

What winding to add? First, decide how much drop you want. Example: set designed for 117, and your line is 125. The required drop is 8 volts.
Next find the winding constant for the transformer. Using whatever wire will fit, install a temporary winding---2 or 3 turns is OK, but a few more will give a more accurate answer. Apply the design voltage (117 for this example) to the primary and measure the voltage in the temporary winding. From this, calculate the turns to voltage ratio. Something like 2:1 or 3:1 is typical. (2.5:1 for one I am working on).

Using the ratio, and the desired drop, calculate the required number of turns for the buck winding. Using 8 volts and 2.5 turns per volt, we get 20 turns.

Installing the coil:

First, add some insulating tape to the winding bundle to replace whatever paper was removed. (I use the 1 mil 3M polyester tape (yellow---I think it's called #77). If there are any exposed wires, use 2 layers of tape.

Install the winding. For a typical set drawing 100 VA or less, #22 wire is about right, but use #20 if there is room. With this size wire, you don't need separate lead outs---just bring out the wire in some kind of sleeving. Be sure to use coated magnet wire......new is best, but used maybe OK if there is no sign of cracking or flaking in the coating.

Finally, wrap some tape around the outside of the new winding.

When hooking everything up, the new winding goes in series with the existing primary. To get the phasing right, measure the primary voltage to verify it is lower than the line. (You have a 50-50 chance of getting it right the first time)

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Another way to "buck" the input voltage
PostPosted: Oct Thu 25, 2018 4:07 pm 
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Hi Mark,

Another option for sets that use a 5-volt rectifier is to replace that with a solid-state rectifier, then wire the 5v winding in series with the primary, making sure phasing is correct.

The 5v winding always has insulation good for many hundreds of volts, so no issue.

- Leigh

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 Post subject: Re: Another way to "buck" the input voltage
PostPosted: Dec Thu 13, 2018 4:43 pm 
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Is there a schematic that shows the solid state rectifier in place and the 5 volt winding in series with the primary of the power transformer?

Thanks,
Carlos


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 Post subject: Re: Another way to "buck" the input voltage
PostPosted: Dec Thu 13, 2018 9:39 pm 
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Here you go:
Attachment:
buck.jpg
buck.jpg [ 29.28 KiB | Viewed 1245 times ]

This is drawn for the traditional rectifier with filament and cathode combined.
To get the "polarity" of the buck winding correct, measure the no-load voltage across one-half of the HV winding (ie center-tap to one end). Then reverse the connection on the buck winding and measure again. The correct wiring is the one that gives the lower voltage.

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 Post subject: Re: Another way to "buck" the input voltage
PostPosted: Dec Thu 13, 2018 9:48 pm 
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Why not simply add a finned power resistor onto the AC return side of the switch? A 28 ohm/50 watt unit will drop the incoming line voltage by about 12 volts.
https://www.mouser.com/ProductDetail/Vi ... esLacR8%3d

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 Post subject: Re: Another way to "buck" the input voltage
PostPosted: Dec Thu 13, 2018 10:01 pm 
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fifties wrote:
Why not simply add a finned power resistor onto the AC return side of the switch? A 28 ohm/50 watt unit will drop the incoming line voltage by about 12 volts.
https://www.mouser.com/ProductDetail/Vi ... esLacR8%3d

Of course!!---but this thread is about bucking.
The resistor approach is "brute force" and uses more energy.

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 Post subject: Re: Another way to "buck" the input voltage
PostPosted: Dec Thu 13, 2018 10:49 pm 
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Now Pix old boy, you know that members on this board are famous for hijacking, lol!

My suggestion was simply offered because it seemed like the least expensive (5 bucks for the resistor) and easiest method (connecting in series between the AC switch and lead going to the rectifier) route to take.

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 Post subject: Re: Another way to "buck" the input voltage
PostPosted: Dec Thu 13, 2018 11:42 pm 
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The additional winding is a good idea. In my experience, power transformers are not all that easy to open when: They have wire insulation falling off, they have been overheated so the inside is dry, brittle. Therefore, a risk and may create irrecoverable damage. They are potted...

The diode replacement has some merit, IMHO some sort of inrush limiting should be considered, transformer could take a substantial "whack" when turned on charging the first cap, the second cap if its value has been increased, could cause a surge in the choke or field coil, if any, an additionally aged component.
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Yes, it really happened, my kids liked the "thump" sound from the old Silvertone TV (CT-15) chassis. They would repeatedly pull-on/push-off the set, then the fuse popped. The transformer was shorted. I don't recall if a diode went out or not but Sears still had the power transformer!
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Never did the resistor option, just don't like throwing extra heat around in a chassis...
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I have placed a bucking transformer within the underside of a chassis works fine.
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I do use a bucking transformer for vintage Christmas lighting. The transformer is withing a blue 4" round electrical utility box with a fused primary. Electrical cable clamps and a 16ga. extension cord cut in half complete the device. Doesn't look all that nice but it works and runs cold. If I were to do more it would be to install a one-time temperature limit fuse. It is good for 3-amps, so a radio would work too...

So, pick your poison...

YMMV!

Chas

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