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Increasing the output voltage of a step-down transformer by reducing the number of coils

electrocutedPillow

Jul 21, 2022
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I am really glad I have joined this forum, I learned a lot of stuff just from one topic. never knew such a thing existed until now (autotransformer). certainly, I have a lot of readings to do.
btw, @Minder you said you modified a lot of transformers, do you cover the coils with a new wrap after you are done? since I don't see any grounding for the case, I imagine it would be a problem if the coils touch the metal case.

Edit: also, since from what it seems, it is an autotransformer, do I still need to add turns? or can I just change where the connection of the output with the coil happens, and I don't need to worry about saturation?
I can't thank you enough guys. this really turned out to be the best project for me to begin with so far.
 
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Minder

Apr 24, 2015
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Yes, after modifying, wrap with some kind of similar material, detect the 110v 'secondary' winding and add series turns accordingly.
The case should be earth grounded in most jurisdictions.
Especially now you do not have isolation with this kind of device.
 

electrocutedPillow

Jul 21, 2022
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Yes, after modifying, wrap with some kind of similar material, detect the 110v 'secondary' winding and add series turns accordingly.
The case should be earth grounded in most jurisdictions.
Especially now you do not have isolation with this kind of device.
Thanks, I found this link. this guy re-used the same wrappings he removed and it worked for him. hopefully, it will work for me too.
it is made of something called "Amide-modified Mylar heat-shrinking tape" according to the OP in the link.
No more questions for now. thanks for all the help.
 

crutschow

May 7, 2021
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can I just change where the connection of the output with the coil happens, and I don't need to worry about saturation?
Yes, if you can get at where the tap is made.

An autotransformer has just one winding, and a tap is made on that winding to get the desired voltage.
Thus for 120V from a 240V input the tap is about 1/2 from the bottom.
 

Bluejets

Oct 5, 2014
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Looks to me to be a standard old run of the mill torroid transformer but who knows unless it's ripped apart or tested by someone who knows what they are looking at.

If the former then it's knackered anyhow.
Chances of an amateur getting it up and running again are fairly slim to say the least.

And the primary is usually the first wind on the core.....
 

Minder

Apr 24, 2015
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Thanks, I found this link. this guy re-used the same wrappings he removed and it worked for him. hopefully, it will work for me too.
it is made of something called "Amide-modified Mylar heat-shrinking tape" according to the OP in the link.
.
I have done the same, if you are careful, it can be re-used.
Wind on 10t of any gauge wire to find out the turns/volt.
No need to connect to any other winding.
But when you wind on the final overwind and connect to the existing winding, make sure it is in the same wind direction , otherwise you will subtract instead of add.
 

roughshawd

Jul 13, 2020
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My guy at the shop told me....
More power bigger wire, more current more wraps.
 

Minder

Apr 24, 2015
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My guy at the shop told me....
More power bigger wire, more current more wraps.
He gave you a bum steer, more turns = higher voltage, i.e. your TFMR has a certain number of turns per volt designed in.
The Va (power) rating not only decides the gauge of wire, but it is also depends on the core mass.
 

Minder

Apr 24, 2015
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A typical auto-transformer could have windings in the 'buck' or 'boost' mode.
e.g. using the top primary connection and one of the upper taps, it can also server as low voltage secondary's.

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