Bucking? Transformer

[linearone]

I can think of nobody specifically to pose this question to.
I have seen mention on various electronic forums and I think this one as well, a technique of using the secondary of a second transformer, just the secondary I think, in series (I think) with the primary of a high voltage transformer to reduce the output voltage of the main transformer. One guy said to use a high current, low v secondary like you would find in a 12v power supply. The goal was to lower the voltage of the main unit if it was say a flea market find and was slightly to high.

I've still never seen this in a diagram or even a picture of it but I've seen it mentioned a few times. How Would something like this work?or am I missing something? Can someone draw it out?

 

[DoctorAudio]

This diagram is wired/phased to "buck" (the 12.6 volt secondary lowers the primary voltage by 12.6VAC) To wire it for 12.6V of "boost", you invert the polarity of the secondary. The transformer needs to be the same VA as the plate transformer it's feeding. The amount it boost/bucks is determined by the secondary voltage of the transformer. A 24V secondary would give 24VAC of boost/buck.

 

[SuperLid]

It is also referred to as an "autotransformer". It will boost or buck.

 

[linearone]

This diagram is wired/phased to "buck" (the 12.6 volt secondary lowers the primary voltage by 12.6VAC) To wire it for 12.6V of "boost", you invert the polarity of the secondary. The transformer needs to be the same VA as the plate transformer it's feeding. The amount it boost/bucks is determined by the secondary voltage of the transformer. A 24V secondary would give 24VAC of boost/buck.

View attachment 63570

In this setup , yes , I can understand that you would need it to be the same va as the main transformer but isn't there a way to accomplish this with a smaller unit? I saw a website that was showing something about it only needing to be the total power of the bucking.

 

[DoctorAudio]

Yes, most of the line current flows through the secondary of the boost/buck transformer so in theory, this winding is what needs to be of sufficient gauge to handle the amps the line is drawing. So lets say the plate transformer is 120 in and 1200 out at 1 amp, it will be pulling roughly 10 amps from the 120V line at full load. The Boost/Buck transformer needs only the secondary to be beefy enough to withstand 10 amps. One way to get a 24V Boost/buck on the cheap is to use a transformer out of a junk Pyramid/Astron/Tripplite power supply. A transformer from a 35 amp supply can be used to Boost/Buck up to 35 amps of line current. You can use 2 of them in series/parallel to Boost/Buck a 240V line.

here is a handy PDF from Hammond:
Hammond Boost/Buck catalog

 

[linearone]

Yes, most of the line current flows through the secondary of the boost/buck transformer so in theory, this winding is what needs to be of sufficient gauge to handle the amps the line is drawing. So lets say the plate transformer is 120 in and 1200 out at 1 amp, it will be pulling roughly 10 amps from the 120V line at full load. The Boost/Buck transformer needs only the secondary to be beefy enough to withstand 10 amps. One way to get a 24V Boost/buck on the cheap is to use a transformer out of a junk Pyramid/Astron/Tripplite power supply. A transformer from a 35 amp supply can be used to Boost/Buck up to 35 amps of line current. You can use 2 of them in series/parallel to Boost/Buck a 240V line.

here is a handy PDF from Hammond:
Hammond Boost/Buck catalog

Could you draw out how this would be connected using a second transformer as you described?

 

[DoctorAudio]

I don't have a scanner or printer right now and I got a lot going on for the next couple days but when I get a minute I can hand draw it and take a pic with my phone and post it up.
(Handy Andy might be able to CAD a diagram up for you....)

Do you know how to phase transformer windings? (start and finish of each winding)

 

[linearone]

I don't have a scanner or printer right now and I got a lot going on for the next couple days but when I get a minute I can hand draw it and take a pic with my phone and post it up.
(Handy Andy might be able to CAD a diagram up for you....)

Do you know how to phase transformer windings? (start and finish of each winding)

Unless they are marked I've just been verifying with a variac. I've successfully series and parallelled transformers before.

 

[DoctorAudio]

This example is using a pair of 120VAC primary 24VAC secondary Pyramid/Astron/Tripplite transformers with the primaries in series for use on a 240V line. This is configured for "Buck" mode to reduce 240 to 216. For "Boost" mode, everything is the same except swapping both secondary windings end to end to get 24 volts of boost. (264VAC) The "dot" on each winding is the "start" lead which is the end of the winding closest to the core/bobbin. Phasing parallel transformers is important so they don't fight each other. Remember when bridging two transformers like this they must be identical voltage and current wise. You can test it first with a variac set for 24VAC before you hit it with full voltage.