# Why transformer secondary only have 3 turns?

#### Ian0

Joined Aug 7, 2020
3,760
Impossible to read the component values on your diagram, but a turns ratio of 35:3 and a flyback voltage of 125V would suggest an output of around 9V. Is that correct?
You have 12 turns altogether on the secondary (4 windings of 3 turns each). Make sure that you spread them out to cover the entire bobbin.

#### MisterBill2

Joined Jan 23, 2018
9,824
There is nothing wrong with a transformer having only three turns, if that meets the design requirements for that application. I was once involved with a project that had a large transformer with only one turn on the secondary. It was a 3000 watt transformer and so the secondary produced a whole lot of current that was used to heat a copper pipe fitting. So the turns must match the application. Of course, if the result does not seem reasonable then it means to check the calculations again. Sometimes errors happen.

#### BobTPH

Joined Jun 5, 2013
4,046
People repurpose microwave oven transformers with one turn to make spot welders. Produces a lot of current.

Bob

#### Externet

Joined Nov 29, 2005
1,706
Why ? Because 3 turns on secondary may produce the needed voltage of about 12V or more from the rectified unknown 120 or 240 VAC mains.

#### MisterBill2

Joined Jan 23, 2018
9,824
Why ? Because 3 turns on secondary may produce the needed voltage of about 12V or more from the rectified unknown 120 or 240 VAC mains.
Exactly!! The number of secondary turns, and the number of primary turns, both depend on the application and the application circuit. That is the way good designs work out.

#### strantor

Joined Oct 3, 2010
5,695
3 turns on the output may result in the correct ratio (35:3) and thus the desired output voltage, but my concern would be about the impedance of the primary, having only 35 turns. The inrush current may be very high. I have had this problem with custom-order toroidal transformers before; could not find an appropriately rated breaker that wouldn't immediately trip when turning it on.

I know nothing about the softwareyou used, maybe it already took that into consideration and my concern is not valid. But I would confirm that.

People repurpose microwave oven transformers with one turn to make spot welders. Produces a lot of current.

Bob
Yep. My spot welder uses 3 microwave transformers with 1/2 turn each, and outputs several thousand amps at very low voltage.

#### Ian0

Joined Aug 7, 2020
3,760
The inrush current may be very high.
It's a current-mode flyback circuit - you can tell from the circuit configuration even though you can't see the component values.
There won't be inrush current.
The software is Texas Instruments' WeBench, you can access it from their website - the designs it produces are reasonably sensible.

#### MisterBill2

Joined Jan 23, 2018
9,824
It's a current-mode flyback circuit - you can tell from the circuit configuration even though you can't see the component values.
There won't be inrush current.
The software is Texas Instruments' WeBench, you can access it from their website - the designs it produces are reasonably sensible.
Indeed!! look at the transformers in the switchmode power supply of an older desktop computer and see that the transformer there, stepping down from the rectified mains voltage, has only a fairly small number of turns. And certainly there is "an inrush" of current. But as it is switching DC on and off that is OK, that is how switchers work.

#### Ramussons

Joined May 3, 2013
1,100
View attachment 250432
View attachment 250433

Here is my We Bench Design=https://webench.ti.com/power-design...ated=Y&Topology=Flyback&source=AC&line_fsw=60

Question: Is it even possible to just have 3 turns in secondary of the transformer or we bench is wrong?
The 35:3 is the turns ratio. A transformer with a physical 35 turns primary is not meant for 50 or 60 Hz mains operation. It is for a High Frequency operation, a few 10's of Kilohertz. And the core will be of Ferrite material to prevent eddy current losses.

#### Ian0

Joined Aug 7, 2020
3,760
And certainly there is "an inrush" of current.
I wouldn't call it an "inrush" - it ramps up from zero to a point where the controller switches it off.
$$I=\frac {V \cdot t}{L}$$
Or am I just being too pedantic about that?

#### BobTPH

Joined Jun 5, 2013
4,046
Or am I just being too pedantic about that?
Probably , since I was about to say the same thing.

Bob

#### Ian0

Joined Aug 7, 2020
3,760
Probably , since I was about to say the same thing.

Bob
All right, I'm a pedant.

Joined Jul 18, 2013
23,923
Another typical example of a transformer with a one turn secondary producing in excess of 100a is a Weller solder gun!

#### MisterBill2

Joined Jan 23, 2018
9,824
The transformer soldering gun has a primary with a lot more than 35 turns. AND it is not ment for continuous operation, by the way.