Introduction: How to Get More Voltage From a Fixed Output Transformer

Picture of How to Get More Voltage From a Fixed Output Transformer

It can be annoying to not have the right transformer to power a project. For me it seems that my transformers available are always too under-powered. However, I recently found out that you can increase the voltage by essentially turning it into an autotransformer. All you have to do is hook the primary up to the mains supply plug like normal, then run another wire from the hot side of the primary and attach this to neutral secondary terminal. The load (your project) is wired to the neutral side of the primary and to the hot secondary terminal on your transformer. 
        The voltage that you get from this configuration will be the sum of the mains voltage, and the normal secondary voltage. Also please keep in mind that because this turns your transformer into an autotransformer, that there will be no mains isolation. Additionally, I recommend that you place something resistive in series with the mains leads to limit current

Hope this helps you, play safe.  :)

Troubleshooting: If you wire everything up and get the exact opposite of what you should (the secondary voltage is subtracted from the primary voltage) then just reverse the secondary wires.

Safety: Configuring a transformer in this manner can produce dangerous voltages, with little to limit the current. This
combination can be very dangerous. Always use proper safety precautions, and above all: common sense. 


tony54 (author)2013-06-11

You can also use this to REDUCE the voltage by running the secondary out of phase with the primary. That would be good for people with too high power line voltage. I hear this is becoming more common for the power company to do this in Autralia at least to increase the electric bill.
This way if you have a 10V secondary then you can have 130V or 110V AC from a 120V input. This would be good to make motors run slower without wasting power or making a 120V lightbulb last for 10 years instead of 1 year , 5 - 10x the lifetime for incandescent bulbs. Lookup the derating for a more accurate lifetime.
I don't recommend boosting the voltage unless you have something designed for the higher voltage.
This circuit could also help in "Brownout" low voltage periods if the power gets down to 100v instead of 120. Just remember the voltage is a RATIO not a strict voltage so if the power input goes down by 10% so does the output of the transformer.

Anoop M N (author)tony542015-02-21

yes you can, but by connecting suitable res is trance in series

iceng (author)2012-10-08

Good ible....

The dot on the windings always tells me ;
When I apply a positive voltage on a dot coil then there will be
a positive voltage on the other dot winding...

Great for understanding pulse transformers as you know
That positive voltage could be the momentary upper half of a sine wave.

I really feared organic chem in my schooling..


iTixle (author)iceng2012-10-09

What always confuses me is how AC can have a polarity. But somehow it does cause If I mix up the wires the transformer does the opposite and subtracts the voltages.

iceng (author)iTixle2012-10-09

Imagine a sinewave first positive above the zero reference line then the wave goes
negative ie alternating voltage...

Now the added coil winding will either be in phase THIS ADDS
or out of phase negative when the main is positive THIS SUBTRACTS

That is called polarity ( in Phase ) or reverse polarity ( out of Phase )..

Tough to text a subject without a chalk board to draw upon or an oscilloscope
to show the waveforms as described..


iTixle (author)iceng2012-10-11

Ohh I see now. So the polarity is used to describe phase relations. If two AC are in phase, then the two sine waves rise and fall at the same time. Am I right?

iceng (author)iTixle2012-10-11

Congratulations, You got it before any formal training.
At 50 or 60 cps ( hertz ) you can be in phase or 180 out of phase
 in transformer voltages.

Next will be the Lag or Lead relationship of AC Voltage & Current
known as power-factor on inductive loads like Motors.
when they are in phase the load is considered to be resistive.


Lectric Wizard (author)2012-08-08

Also remember that the current capacity of the transformer is now THE LESSER of the two ratings, ie if the secondary is only rated for 500mA then that is the max you can draw...

Lesser of which 2 ratings? Only the secondary matters, assuming that the 120 v line has a much greater rating ( which is almost always the case as breakers are in the 15000 ma and up range ). In any case you need not consider the primary as the load is in parallel with it.

iTixle (author)Lectric Wizard2012-08-08

Yeah that's true. This is why I like to add a resistor to limit current. :)

rimar2000 (author)2012-08-08

This is called auto-transformer.

Very clever, using a simple transformer.

iTixle (author)rimar20002012-08-08

I mentioned that. Thanks for your comment. :)

Lectric Wizard (author)2012-08-08

Don't forget you have to match the "polarity" of the primary & secondary. If when you hook it up , you get LESS than the input voltage (bucking),simple reverse the secondary leads. (This can be very useful if you want to test how well something will tolerate a "brown out" ) . Cheers!

I should have read the troubleshooting section ,sorry !

iTixle (author)Lectric Wizard2012-08-08

It's all good :)

I should have read the troubleshooting section ,sorry !

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Bio: I'm a sophomore in high school. I am proud to be a super-nerd. Things that bore my peers in school, fascinate me. One thing ... More »
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