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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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Hello,

I have a step-down transformer (240v to 110v), and I want to change it to produce 120v instead of 110v, is it okay if I just reduce the number of coils in its primary to produce the desired voltage? how can I test the electrical components in its board (capacitors, resistors), to check if they can handle the increase in voltage (+10v)? or should I look at some sort of datasheet for each component?

also if want to discharge a capacitor, can I connect it to a fan or light bulb to slowly discharge it instead of shorting the legs of the capacitors? I don't have resistors laying around.

Thanks.
 

Alec_t

Jul 7, 2015
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is it okay if I just reduce the number of coils in its primary to produce the desired voltage?
In general, no. There would be a risk of saturating the transformer core and the transformer could overheat.
If there is space to do so you could wind extra turns on the secondary.
should I look at some sort of datasheet for each component?
Yes, definitely. But never run components at their rated maximum values if you want them to have a long and happy life.
 

electrocutedPillow

Jul 21, 2022
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In general, no. There would be a risk of saturating the transformer core and the transformer could overheat.
If there is space to do so you could wind extra turns on the secondary.

Yes, definitely. But never run components at their rated maximum values if you want them to have a long and happy life.

Thank you for the quick, straight-to-the-point, and informative answer. I will have to read about transformer core saturation, as it is a new concept for me.
 

Minder

Apr 24, 2015
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Much easier to add turns to the secondary,
As per Alec_t , you should never if possible reduce the primary winding using the same input voltage.
.
 

CircutScoper

Mar 29, 2022
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Hello,

I have a step-down transformer (240v to 110v), and I want to change it to produce 120v instead of 110v, is it okay if I just reduce the number of coils in its primary to produce the desired voltage? how can I test the electrical components in its board (capacitors, resistors), to check if they can handle the increase in voltage (+10v)? or should I look at some sort of datasheet for each component?

also if want to discharge a capacitor, can I connect it to a fan or light bulb to slowly discharge it instead of shorting the legs of the capacitors? I don't have resistors laying around.

Thanks.

Not to contradict the sage advice you've already received, but reducing primary turns by only 1 - 110/120 = 8.3%, would decrease primary inductance (and therefore increase primary current) by a modest 1 - (110/120)^2 =16%.

Unless you intend to run the transformer right at its rated power limit, my guess is it would tolerate that.
 

Minder

Apr 24, 2015
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The down side for me if this is a E/I lamination TXFR is that the pri is wound on first, which would mean removing the secondaries.
I have 'modified' many over the years, both EI and Toroidal (easier), and never touched the PRI on any of them.
 

CircutScoper

Mar 29, 2022
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The down side for me if this is a E/I lamination TXFR is that the pri is wound on first, which would mean removing the secondaries.

In which case EP's question is probably only academic -- case closed.
 

electrocutedPillow

Jul 21, 2022
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Thank you all for your input, I will have to read more about the topic before attempting to modify the transformer. also, my question is not purely academic, as I do have a transformer that we used for a Japanese heater (that uses electricity for a spark and a fan to burn kerosine). since we no longer have the heater, and since we do have many devices from the USA (120v) I need to modify the transformer to make it compatible with the devices.

Thanks again for your answers.

PXL-20220721-173921152.jpg
 
Last edited:

crutschow

May 7, 2021
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reducing primary turns by only 1 - 110/120 = 8.3%, would decrease primary inductance (and therefore increase primary current) by a modest 1 - (110/120)^2 =16%.
You are assuming that the transformer won't saturate from that, but that's iffy, since most commercial transformers run close to saturation in normal operation (to save on core cost).
I have a step-down transformer (240v to 110v)
The label says 220V to 110V, so at 240V it will likely produce 120V out.
 

Alec_t

Jul 7, 2015
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Most appliances designed to run from mains will tolerate a considerable variation in the mains voltage.
 

Minder

Apr 24, 2015
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Thank you all for your input, I will have to read more about the topic before attempting to modify the transformer.
What I usually do is remove the outer insulation and determine if there is enough room to add windings, you require increasing the secondary by 10v, as there is usually ~5t per volt, that would equal around 50 turns, what I do is wind on a 10t test winding and measure to determine the T/V
The extreme method is to remove the EI laminations to enable the wind, but do not if at all necessary.
Toroid types are Much easier!
Crutschow has a point, have you measured it?
 

Minder

Apr 24, 2015
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...Also this type of device has at least a 10% voltage rating tolerance.
 

electrocutedPillow

Jul 21, 2022
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You are assuming that the transformer won't saturate from that, but that's iffy, since most commercial transformers run close to saturation in normal operation (to save on core cost).
The label says 220V to 110V, so at 240V it will likely produce 120V out.
I tested it with a multimeter, it outputs 106v
 

Minder

Apr 24, 2015
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Something wrong there? It is not even producing the label value? Are you in N.A.?
I know Japan is both 60hz & 50Hz but there should not be that discrepancy?
All the equipment I have installed of Japanese origin, has had a 10% voltage tolerance rating.
 

electrocutedPillow

Jul 21, 2022
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I opened it, but it does not have any board. just the coils and a black small component, which I guess is a small capacitor (second image). here are a few images of what is inside.
PXL-20220721-195900406.jpg

PXL-20220721-200106757.jpg

PXL-20220721-200229593.jpg

PXL-20220721-200408912.jpg
 

electrocutedPillow

Jul 21, 2022
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Something wrong there? It is not even producing the label value? Are you in N.A.?
I know Japan is both 60hz & 50Hz but there should not be that discrepancy?
All the equipment I have installed of Japanese origin, has had a 10% voltage tolerance rating.
It is old, so I guess maybe it needs some rewiring done, maybe, I am not sure. I am currently not in NA. does the frequency of the Alternating Current affect the voltage output of a transformer??
 

Minder

Apr 24, 2015
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Well if you are thinking of modifying it that is the best kind to do it with.
Toroidal. !
You just need to remove the outer insulation wrap and wind on what you need, find out the turns/volt like I said, they are much less on a toroidal, about 2 - 3 t/v Use the same gauge as the existing secondary.
The freq. does affect the output, what is your power V & F ?

.
 

electrocutedPillow

Jul 21, 2022
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Well if you are thinking of modifying it that is the best kind to do it with.
Toroidal. !
You just need to remove the outer insulation wrap and wind on what you need, find out the turns/volt like I said, they are much less on a toroidal, about 2 - 3 t/v Use the same gauge as the existing secondary.
The freq. does affect the output, what is your power V & F ?

.
Yeah I am glad it turned out like this. just one thing, one of the lines from the input cable is connected directly to the output? the blue one? is that neutral? shouldn't the primary and secondary circuits be disconnected in a transformer?
 

Minder

Apr 24, 2015
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How many conductors enter the transformer? If only three, then it could be a auto-transformer.
Or does it have four all together?
If four, measure the resistance between all and see if they are isolated from one another..
 

crutschow

May 7, 2021
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it could be a auto-transformer.
Yes, since isolation is not needed for this purpose, it could likely be an autotransformer since they are smaller and cheaper to build.

But you can still easily add windings to put in series with the output to increase the voltage.
The freq. does affect the output,
It doesn't appreciably affect the output voltage for a given output current.
 
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