Voltage Buck for ST120

Here's the deal. In my travel trailer I have a couple TVs that operate on 12V. But when the battery charger is working to charge the AGM batteries, the voltage will sometimes go as high as 14.5 volts. This is normal for the "bulk" charge of AGM batteries. But the TVs hate it and they will shut down. For under $20 I bought a 12V buck regulator. Put one behind each TV and no mater what the input voltage is, the output is always EXACTLY 12V.

I have an ST120 that wants to see no AC voltage above 117v +/-. I'm using a variac as the voltage in my house is usually around 126v. Every time I turn the variac on I need to reset it. Guess I could just leave it on all the time, but I chose to shut it off when not using it.

I've searched but I'm not finding buck regulators for these AC voltages and 5amp current capability.

Anyone know of such a device?

Rbertalotto wrote:Here's the deal. In my travel trailer I have a couple TVs that operate on 12V. But when the battery charger is working to charge the AGM batteries, the voltage will sometimes go as high as 14.5 volts. This is normal for the "bulk" charge of AGM batteries. But the TVs hate it and they will shut down. For under $20 I bought a 12V buck regulator. Put one behind each TV and no mater what the input voltage is, the output is always EXACTLY 12V.

I have an ST120 that wants to see no AC voltage above 117v +/-. I'm using a variac as the voltage in my house is usually around 126v. Every time I turn the variac on I need to reset it. Guess I could just leave it on all the time, but I chose to shut it off when not using it.

I've searched but I'm not finding buck regulators for these AC voltages and 5amp current capability.

Anyone know of such a device?

This may get you close or below 117V AC

Dynaco Heathkit Fisher Scott Vintage Radio Bucking Transformer Voltage Reducer

https://www.ebay.com/itm/Dynaco-Heathkit-Fisher-Scott-Vintage-Radio-Bucking-Transformer-Voltage-Reducer/193936058041?hash=item2d277d66b9:g:648AAOSwWY5ajym3

I believe this unit will give you a sine wave and keep the voltage setting better than a variac.

https://www.amazon.com/dp/B000FBF08Q/?coliid=IU9VDYAMCXA9A&colid=292PN48OYLYA8&psc=1&ref_=lv_ov_lig_dp_it

Looks like I'll just stay with the $55 variac. Most of these "Voltage Regulators" are "120V +/-5v".....Not good for what we want. And they start in the hundreds of dollars for not programable. A programable one is $2500!!!

Many of them simply limit the voltage to 140V and then shut down......Yikes!

https://www.circuitspecialists.com/variac-variable-ac-ps05kva_with_digital_display.html

Circuit Specialists 5 Amp Variable Output Autotransformer with Digital Display...$54

Rbertalotto wrote:Looks like I'll just stay with the $55 variac. Most of these "Voltage Regulators" are "120V +/-5v".....Not good for what we want. And they start in the hundreds of dollars for not programable. A programable one is $2500!!!

Many of them simply limit the voltage to 140V and then shut down......Yikes!

https://www.circuitspecialists.com/variac-variable-ac-ps05kva_with_digital_display.html

Circuit Specialists 5 Amp Variable Output Autotransformer with Digital Display...$54

Thats a very nice little variac Rbertalotto.... I just might snag that too for my other projects.

Maybe this will help:

https://blueglowelectronics.files.wordpress.com/2018/11/bucking-transformer-slides.pdf

I'm using the recommended by Bob 5A circuit specialists variac but am considering the bucking transformer. Blueglow has lots of info and youtube content.

That "Brown Box" is awesome.....but at $350....a bit rich.

That "Buckminister" (simple version) is the one I built a couple of times. It cost about $35 to build one.
Knocks my 123 or 124 VAC down to about 115/116.
I run my VTA ST 70, SP 14 and PH15 off the bucker and have been using it for about 4 years.
By the way, that bucker info is from Rob Robinette's 5E3 site.

https://robrobinette.com/5e3_Modifications.htm#Bucking_Transformer

i'd go with a bucking transformer for sure. A year or two ago, I research every power conditioner I could find, from $200 to $5000.
NONE of them will keep a voltage at a set level. All they do is shut down if voltage is more than about 130vac or lower than 110vac

tubes4hifi wrote:i'd go with a bucking transformer for sure.  A year or two ago, I research every power conditioner I could find, from $200 to $5000.
NONE of them will keep a voltage at a set level.   All they do is shut down if voltage is more than about 130vac or lower than 110vac

Yes... To me a bucking transformer is the best way to go and it's permanent.

The bucking transformer post link I made the other day is good for bringing the voltage down from 124 to 116 but sadly is only good for a max of 4 amps so that in my opinion is not enough amps for the VTA 120.

Im in the same situation. I measured 122v AC on my line yesterday and I still think thats to much and will be hard on the tubes for the 120 after the voltage has been stepped up.

I run a Variac that Bob suggested. I have a kill a Watt ( that Bob suggested ) plugged into the Variac. I have a small portable Ground Fault Interrupter ( that someone on this forum suggested ) plugged into the Variac. And I installed a parts express grounded power cord into my ST-120 ( groiund wire to one of the transformer mounting bolts ). I look at the voltage reading on the Kill a Watt periodically -- if it is above 118 I turn it down. If it is too low I leave it along, because the ST-120 is overkill for my small room anyway. Sounds good, and I am well protected against shorts etc and power interuptioons.