Buck Boost for Step Up from 12VAC to 120 VAC

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SAAPI

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I have a weird application where my client only has 12 VAC - I need 120 VAC for my project at fairly minimal current - any reason I can't use a buck-boost transformer in reverse?

Everything I know about transformers tells me this is fine - but you never know?
 
SAAPI said:
I have a weird application where my client only has 12 VAC - I need 120 VAC for my project at fairly minimal current - any reason I can't use a buck-boost transformer in reverse?

Everything I know about transformers tells me this is fine - but you never know?
Click to expand...
Some transformers yes, some no. Ask the manufacturer or check the literature.

It would probably not be a buck/boost transformer, just a regular 120:12 V transformer in reverse.
 
220225-1345 EST

Why are so many electricians hung up on how to apply a transformer?

Fundamentally a tightly coupled power transformer is a bidirectional device. But you have to understand how a real world power transformer is designed and operates.

You have to understand transformer core materials, their magnetizing curves, and what criteria were used in the design of the transformer.

If a transformer has been designed as a step down transformer ( forget the buck boost criteria because this only provides a major benefit for small voltage changes or convenience of assembly ), then it is usually rated for its use at full load. This means a transformer designed for 120 V input and 12 V full load output which is a ratio of 10 to 1 in voltage will have a turns ratio of less than 10 to 1. This is to compensate for internal voltage drop at full load. Internal impedance is from resistive losses in the wire primarily, and leakage inductance.

If you apply 12 V input and fully power load the secondary ( original primary ) ( this can not be as great as the design direction of power flow ), then you will get less than 120 V output. Core saturation will not be as great as it would be if driven in its normal orientation.

If the original ratio was 9.5 to 1 to compensate for internal impedance, then when run in reverse the approximate output will be about 109 V from a12 V input.

.
 
gar said:
220225-1345 EST

Why are so many electricians hung up on how to apply a transformer?

Fundamentally a tightly coupled power transformer is a bidirectional device. But you have to understand how a real world power transformer is designed and operates.

You have to understand transformer core materials, their magnetizing curves, and what criteria were used in the design of the transformer.

If a transformer has been designed as a step down transformer ( forget the buck boost criteria because this only provides a major benefit for small voltage changes or convenience of assembly ), then it is usually rated for its use at full load. This means a transformer designed for 120 V input and 12 V full load output which is a ratio of 10 to 1 in voltage will have a turns ratio of less than 10 to 1. This is to compensate for internal voltage drop at full load. Internal impedance is from resistive losses in the wire primarily, and leakage inductance.

If you apply 12 V input and fully power load the secondary ( original primary ) ( this can not be as great as the design direction of power flow ), then you will get less than 120 V output. Core saturation will not be as great as it would be if driven in its normal orientation.

If the original ratio was 9.5 to 1 to compensate for internal impedance, then when run in reverse the approximate output will be about 109 V from a12 V input.

.
Click to expand...
"a real world power transformer is designed and operates"

that was my worry, my EE 101/102 only covered transformers in a pure form. For this application I'm lightly loading the output - I'll load it and test it. Just picked the Buck-Boost Transformer, which also is used as a step-up transformer and comes in a package that works for my application.

BTW - I had never heard of Buck-Boost transformers until we move our operation into a new facility - with the building wired at 208V at my electrician recommended them - it was a cheap date to fix our power problems.

thanks for the insight.
 
A buck-boost is basically a standard transformer, but with secondary insulation rated to withstand the primary voltage, in addition to the dual-voltage primaries and secondaries.

They can be used for standard transformer purposes, too.
 
petersonra said:
Some transformers yes, some no. Ask the manufacturer or check the literature.

It would probably not be a buck/boost transformer, just a regular 120:12 V transformer in reverse.
Click to expand...
Yes roughly but you would need the 120V a bit higher to allow for the voltage regulation.
 
It hasn’t been mentioned yet, but there’s no practical advantage in using buck-boost vs a conventional step-up transformer for this big of a voltage ratio.

I guess in this case, the extra 12V would help compensate for the ratio compensation effect of using a transformer in reverse.
 
gar said:
220225-1345 EST

Why are so many electricians hung up on how to apply a transformer?

Fundamentally a tightly coupled power transformer is a bidirectional device. But you have to understand how a real world power transformer is designed and operates.

You have to understand transformer core materials, their magnetizing curves, and what criteria were used in the design of the transformer.

If a transformer has been designed as a step down transformer ( forget the buck boost criteria because this only provides a major benefit for small voltage changes or convenience of assembly ), then it is usually rated for its use at full load. This means a transformer designed for 120 V input and 12 V full load output which is a ratio of 10 to 1 in voltage will have a turns ratio of less than 10 to 1. This is to compensate for internal voltage drop at full load. Internal impedance is from resistive losses in the wire primarily, and leakage inductance.

If you apply 12 V input and fully power load the secondary ( original primary ) ( this can not be as great as the design direction of power flow ), then you will get less than 120 V output. Core saturation will not be as great as it would be if driven in its normal orientation.

If the original ratio was 9.5 to 1 to compensate for internal impedance, then when run in reverse the approximate output will be about 109 V from a12 V input.

.
Click to expand...
A bit of a diversion, but this has been discussed and IIRC pretty much all "power transformers" do not have compensated windings.
 
Heck, a small inverter would do the trick without all the considerations of a reversed transformer.
Or, as mentioned in #8, what does the device need? Might be a lot better to derive that from the 12v than all the way up to 120 and back down.
 
zbang said:
Heck, a small inverter would do the trick without all the considerations of a reversed transformer.
Or, as mentioned in #8, what does the device need? Might be a lot better to derive that from the 12v than all the way up to 120 and back down.
Click to expand...
But control transformers, which often supply power to something that might use a small power supply, generally are compensated.
 
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