# DIY power transformer help

#### quitenoob

Joined Mar 27, 2022
262
WARNING. Playing with mains power is not recommended and can lead to fatal injuries. Proceed with caution.

I am in the process of making a power transformer from 230VAC/16amps to 3.65VAC/140amps

Currently I am able to pass mains voltage through the primary coil and get something out of the secondary while the system is open.

The pvs pipe is filled with iron powder.

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TL DR update;
The transformer works at 36% efficiency. Not something to write home about but for quick and dirty / crisis situations a good option.

Currently investigating a brainwave I had in where the secondary coil should be below the primary.
All in an effort to improve on the 36% base efficiency.

That has seemed to up the efficiency to 39%.
Fear not though. I am not done yet with this one

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How can I cap the output of the secondary coil at 140 amps?

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#### BobTPH

Joined Jun 5, 2013
5,759
If your primary is drawing 16A the secondary should be over 1000A. A transformer does not limit the output current is except via the restance of the coil.

I suspect you don’t have enough turns.

Bob

#### Hymie

Joined Mar 30, 2018
1,090
That’s a novel transformer construction – I’ve never seen anyone attempt making a linear transformer using plastic water pipes filled with iron fillings.

To achieve a secondary output current of 140A at 3.65V, you would only need a primary current of around 2.5A at 230V.

The turns ratio (primary to secondary) should be around 65 to achieve the expected output voltage – but I suspect the whole thing won’t give the desired result due to the iron fillings not giving the required magnetic flux density to induce the required secondary current.

I suggest you short circuit the ends of the secondary windings and measure the resultant current (with a current clamp/shunt) to see if you have any chance of getting anywhere near 140A. But don’t power the transformer in this state for more than a few seconds otherwise you might start a fire.

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#### quitenoob

Joined Mar 27, 2022
262
If your primary is drawing 16A the secondary should be over 1000A. A transformer does not limit the output current is except via the restance of the coil.

I suspect you don’t have enough turns.

Bob
Primary, while the system is open, draws 8.26 amps.
The secondary output 6.23V so I should reduce the secondary winds

I suggest you short circuit the ends of the secondary windings and measure the resultant current (with a current clamp/shunt) to see if you have any chance of getting anywhere near 140A. But don’t power the transformer in this state for more than a few seconds otherwise you might start a fire.
Can you please elaborate a bit?
I connect both ends of the secondary coil and put an amp clamp around it and see what happens?

#### Hymie

Joined Mar 30, 2018
1,090
Based on a current draw of 6.8 amps with the secondary not loaded, you definitely need more primary windings. The no-load primary current should be less than 0.5A.

// Can you please elaborate a bit?
I connect both ends of the secondary coil and put an amp clamp around it and see what happens?// - Yes, but first sort out the primary current draw and do not power the transformer with the secondary winding short circuit for more than a few seconds (only long enough to obtain a current reading).

#### BobTPH

Joined Jun 5, 2013
5,759
Primary, while the system is open, draws 8.26 amps.
The secondary output 6.23V so I should reduce the secondary winds
How many windings in your primary? I suspect it is too low by a factor of 10.

Bob

#### quitenoob

Joined Mar 27, 2022
262
How many windings in your primary? I suspect it is too low by a factor of 10.

Bob
+-1234

Yes, but first sort out the primary current draw
How do I "sort out" the primary current draw?

I'll admit I am quite new

I had about 284 meters of 1mm enameled copper wire and wound it around a 75mm diam pvs pipe.

How do I "sort out" the primary current draw?

I'll admit I am quite new
never mind, I think I understood now. You meant first get more windings as to lower the amp draw on the primary yes?

Sorry guys, I have been mistaken.

I meant that the variac draws around 8 amps while the primary coil draws 1 amp.

So should I still increase the primary windings or am I ready to rock?

#### Hymie

Joined Mar 30, 2018
1,090
I suggest you use enamel wire with a CSA of 0.5mm2 or less for the primary winding, with sufficient turns to get the no-load current at under 1A at 230V. Then consider the secondary turns required to get the desired output current.

#### quitenoob

Joined Mar 27, 2022
262
I suggest you use enamel wire with a CSA of 0.5mm2 or less for the primary winding, with sufficient turns to get the no-load current at under 1A at 230V
Please forgive my ignorance. I am new to the field and a slow learner.

Would your suggestion have been met, other than wire size, if I add a bit more windings to the current primary coil and when it draws 0.99999999 amps?

I am trying to narrow it down to what it is your are aiming at.

#### Hymie

Joined Mar 30, 2018
1,090
It may be that your design concept will not work. Linear transformers using an iron/steel core use laminations (rather than a solid core) to reduce eddy currents induced within the core structure – which would otherwise significantly reduce the efficiency of the transformer.

The iron fillings within the plastic water pipe may be behaving as a solid core such that you will never achieve what you want from the transformer.

Personally, I’d recommend you experiment with a much smaller construction (say outputting 12Vac at 1A) and see if such a design concept has any chance of working.

#### quitenoob

Joined Mar 27, 2022
262
It may be that your design concept will not work. Linear transformers using an iron/steel core use laminations (rather than a solid core) to reduce eddy currents induced within the core structure – which would otherwise significantly reduce the efficiency of the transformer.

The iron fillings within the plastic water pipe may be behaving as a solid core such that you will never achieve what you want from the transformer.

Personally, I’d recommend you experiment with a much smaller construction (say outputting 12Vac at 1A) and see if such a design concept has any chance of working.
Thank you for adding this. However can you please elaborate more on your suggestion before that? The one with the less than 1 amp goal.

Regarding the iron powder. Since it is not electrically conductive I assume to not have issues with eddy current losses.

I think it is important for me if we take it step by step.
So based on the feed back from you guys I am now planning to add a bit more winds to the primary coil to reduce the amps drawn by it from 1 amp to <1 amps

#### Hymie

Joined Mar 30, 2018
1,090
If the primary of your transformer is drawing around 1A (no-secondary load), try shorting the secondary winding (while measuring the current in both the primary & secondary) and see what you get.

#### quitenoob

Joined Mar 27, 2022
262
yes sir! Will do so. Might take a moment or 2 but I'll keep one updated if I do not burn the house down

#### quitenoob

Joined Mar 27, 2022
262
ok, with the secondary shorted things look a whole lot different

Secondary is at 131 amps. primary is around 3.3 amps.
VAC in variac is around 30

I did not dare going any higher.

Any ideas on how to interpret these findings?

#### Hymie

Joined Mar 30, 2018
1,090
Well, you wanted 140A and got 131A, so not bad.

The question is what load were you planning to supply with the secondary output?

Given that the short circuit output is at 131A, with some additional resistive load this will be reduced slightly. 140A at 3.65V will require a 26mΩ impedance. So I suggest you connect the transformer output to whatever you wanted the 140A for, and increase the primary input voltage to achieve this.

I’d keep a close eye on the input power and the temperature of the transformer – but at 30V, 3.3A the input power is only 100W, given the size of your transformer, I’d expect it to handle any likely internal power losses/dissipation.

#### Hymie

Joined Mar 30, 2018
1,090
For the purposes of protection against electric shock you should treat the enamel wire as being bare – and providing no protection.

Therefore your final construction should include some insulation/sleeving over the accessible section of the enamel wire.

#### strantor

Joined Oct 3, 2010
6,112
I am confused, you mentioned more than once about there being a difference in amps going into the variac and coming out of the variac, and indeed in your picture I see 8.26A in and 1.0A out, yet I see the variac needle fully to the right, indicating (on most variacs) that output voltage = input voltage (or input × ~1.2). So, why don't your meters agree? They're not even close. What is your input voltage and output voltage of the variac?

#### quitenoob

Joined Mar 27, 2022
262
The question is what load were you planning to supply with the secondary output?
I'd like to stress test a LifePO4 battery (you can see some in one in some of my photos) and charge it at it's rated capacity which is .5C (140 amps)

I know it will involve recitfying and such but that is the next hurdle. Now I'd like to focus on getting the first part right

I have available a 3 phase power source to help smoothen the ripple but lets first focus on getting part 1 right

So, why don't your meters agree?

For the purposes of protection against electric shock you should treat the enamel wire as being bare – and providing no protection.

Therefore your final construction should include some insulation/sleeving over the accessible section of the enamel wire.
yes sir, understood

#### strantor

Joined Oct 3, 2010
6,112