# Transformer without separate primary and secondary windings...

Gold Member

## Summary:

The person in this video makes a transformer that does not have separate primary and secondary windings. How is this supposed to work?

## Main Question or Discussion Point

I have always longed to make a system where I can get AC over a wide range of voltages. This system would help me test how my other experiments behave when they are powered from the mains. In order to get different voltages, I need to design my own transformer where the secondary has been tapped at different places so as to give different voltages.

On YouTube, I found this video. The person winds transformers at his home. The video is not in English, so you can jump to the section where he shows his circuit:

The input is on the left (where his finger is), and the output is on the right. As you can see, he starts the primary winding, but instead of ending it, he bends it into an output terminal, and thereafter continues winding the secondary coil.

My question: I have seen transformers that have two wires for input (live and neutral). Here there is only one input wire. So, will the neutral be common from the input to the output? One end of my load will connect to the A, B, C, D or E terminals on the right, and the other end will be directly attached to the second input wire (that has not been attached to the transformer) - is this correct?

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tech99
Gold Member
The primary can be isolated from the secondary or it can have a connection. Usually it is safer to have it isolated from the secondary. A transformer using just one winding, without a separate primary and secondary and with taps for taking off various voltages is called an auto transformer, but does not provide isolation from the mains.
The transformer you show will produce dangerous voltages.

The circuit in question is called autotransformer.

5kVA and 418V with a mains reference? Don’t build that unless you intend to run it from a proper isolation transformer. Even then, get some qualified supervision.

berkeman
Gold Member
The transformer you show will produce dangerous voltages.
5kVA and 418V with a mains reference? Don’t build that unless you intend to run it from a proper isolation transformer. Even then, get some qualified supervision.
No, I do not intent to exactly build the transformer shown in the video. I am using the video for learning purposes. I will be happy if my transformer provides 100V to 250V range on the secondary with 5-6A current. But I will make larger number of taps so that I can vary the voltage in small steps.

Delta2
Homework Helper
Gold Member
I am not sure cause I am not familiar with this type of transformer but I believe the mathematical explanation is that when two or more coils with impendances $$Z_1=R_1+i\omega L_1,Z_2=R_2+i\omega L_2,...,Z_n=R_n+i\omega L_n$$ are connected in series then the total impendance is $$Z=R+i\omega L=Z_1+Z_2+...+Z_n=\sum_{k=1}^nR_k+i\omega (\sum_{k=1}^nL_k)$$ and the voltages at each coil are $$V_i=V\frac{|Z_i|}{|Z|}=V\frac{\sqrt{R_i^2+\omega^2L_i^2}}{\sqrt{ ( \sum_{k=1}^nR_k)^2+\omega^2( \sum_{k=1}^{n} L_k )^2}}$$ where ##V## ,##\omega## the RMS value and frequency of the supply voltage.

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No, I do not intent to exactly build the transformer shown in the video. I am using the video for learning purposes. I will be happy if my transformer provides 100V to 250V range on the secondary with 5-6A current. But I will make larger number of taps so that I can vary the voltage in small steps.
That sounds promising, and experimenting with an autotransformer can be interesting. I have an infinitely variable version of that, commonly called a Variac.

Please note that the neutral is connected at A and is common to both ‘primary’ and ‘secondary’. So you have a mains reference at the output, and a shock from there will be every bit as dangerous as a mains shock.

The lower voltages you propose are still potentially unsafe for amateur experimentation with such a set-up. I suggest you look up ‘isolation transformer’ and tell us what you find.

anorlunda
Staff Emeritus
No, I do not intent to build exactly that transformer shown in the video. I am using the video for learning purposes. I will be happy if my transformer provides 100V to 250V range on the secondary with 5-6A current. But I will make larger number of taps so that I can vary the voltage in small steps.
That sounds like inadequate awareness of all the safety hazards. If you are just using it for education purposes, why is it necessary to have anything higher than 10 volts?

tech99
Gold Member
I am not sure cause I am not familiar with this type of transformer but I believe the mathematical explanation is that when two or more coils with impendances $$Z_1=R_1+i\omega L_1,Z_2=R_2+i\omega L_2,...,Z_n=R_n+i\omega L_n$$ are connected in series then the total impendance is $$Z=R+i\omega L=Z_1+Z_2+...+Z_n=\sum_{k=1}^nR_k+i\omega (\sum_{k=1}^nL_k)$$ and the voltages at each coil are $$V_i=V\frac{|Z_i|}{|Z|}=V\frac{\sqrt{R_i^2+\omega^2L_i^2}}{\sqrt{ ( \sum_{k=1}^nR_k)^2+\omega^2( \sum_{k=1}^{n} L_k )^2}}$$ where ##V## ,##\omega## the RMS value and frequency of the supply voltage.
These windings share the same fields as they are on the same core, so we cannot add the impedances in series.

Delta2
Homework Helper
Gold Member
These windings share the same fields as they are on the same core, so we cannot add the impedances in series.
Hmmm, maybe you are right, I said I am not sure and now even more not sure :S.

yes it seems to me you are right after all I tried to workout the whole thing with two coils that have different turns but same cross section area and same length and it turns out that the total ##L## when they are sharing the same core is quite different from ##L_1+L_2,## the algebra just doesnt work out as it should. Mathematically it boils down to the fact that ##(n_1+n_2)^2\neq n_1^2+n_2^2##.

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I think the most uncertain thing in the transformer diagram is the lack of polarity marking points to show the phase relation between the windings of the transformer.

Gold Member
I have an infinitely variable version of that, commonly called a Variac.
I would have bought that; it would have saved me from the trouble of running to stores to buy things. But the ones on the net are asking for incredibly high prices.
So you have a mains reference at the output, and a shock from there will be every bit as dangerous as a mains shock.

The lower voltages you propose are still potentially unsafe for amateur experimentation with such a set-up.
That sounds like inadequate awareness of all the safety hazards.
It is good to talk about safety hazards, but let's not get drawn into that. I know these are dangerous voltages (I mark any voltage > 30 V as "dangerous"), and there are associated risks. I always show my plans physically to a learned person before executing them. And I carry out such experiments only where adequate safety measures are present.
Please note that the neutral is connected at A and is common to both ‘primary’ and ‘secondary’.
That is what I wanted to know - how will the person in the video connect the transformer in the circuit.

anorlunda
Staff Emeritus
It is good to talk about safety hazards, but let's not get drawn into that. I know these are dangerous voltages (I mark any voltage > 30 V as "dangerous"), and there are associated risks. I always show my plans physically to a learned person before executing them. And I carry out such experiments only where adequate safety measures are present.
No. You know how PF works. These threads are searchable and readable by all including children. We do not allow public discussion of dangerous topics, regardless of the qualifications of the OP.

I repeat. Why do you need high voltages to demonstrate the effects?

Gold Member
No. You know how PF works. These threads are searchable and readable by all including children. We do not allow public discussion of dangerous topics, regardless of the qualifications of the OP.

I repeat. Why do you need high voltages to demonstrate the effects?
I have a long-time plan of making an automatic voltage stabilizer. I am fed up with the ones I have seen. The ones available in the market all have a fixed tap on the transformer, i.e. if the mains goes down to 200V, they will step it up to 230V, but if the mains goes down to 150V, they will use the same length of secondary coil to step it up to 190V. I have plans to make a stabilizer that will have multiple taps in the secondary. There will be a chip (ATmega 328p as of my previous plans) that will read the mains voltage, and then select the appropriate tap of the secondary via relays. This way, 200V will go up to 220V, and 150V will also go up to 220V.

This variable voltage generator would be helpful in testing the stabilizer. My project has been standing still only because I did not have tools to make a custom transformer. Now that I know what is required, I can at least go to shops and ask for it.

This is the explanation. If you are still unhappy, feel free to lock the thread/delete it entirely.