# AC Transformers on circuits boards

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#### gary350

##### Well-Known Member
Transformers these days are different than 50 years ago some how they manage to operate fine with a few turns of wire in the primary coil on 120 VAC.

I don't understand the circuits most TV circuit boards are low voltage. First picture transformer steps 120vac down to 100vac ?

Many of the 4 wire transformers are step down or 1 to 1 ratio, 5 wires 1 coil has center tap on secondary coil.

Multi wire transformers have 4 to 7 coils and I am not sure why I removed these from PC board with out examining the circuit to see what they do.

I have experimented with these newer transformers in the past I think it is interesting the 120 vac primary coil has about 75% less primary coil turns than transformers had 50 years ago and they do not over heat.

Today I needed a 120vac to 12vac transformer the 120vac transformer in my Bridgeport mill burned up in the digital read out. Physical size of old transformer is about 100 watts so I used a variac and amp meter to put 120 vac on each transformer 1 by 1 looking for a 12vac output. As far at the physical size of the newer transformers I have no clue what physical size is 100 watts? I wonder since new transformers have less wire maybe physical size can be smaller to = 100 watts.

It was a challenge to find the correct transformer and to play it safe once I got it figured out I left it ON all day to make sure it did not over heat and burn up. This morning I connected it to the digital read out and it works good. I soldered several windings in series & in phase to finally get 1 output winding almost 12vac.

Some transformers have a copper strap around them, I read once what that does but now I have forgotten?

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#### cowboybob

##### Well-Known Member
Some transformers have a copper strap around them, I read once what that does but now I have forgotten?
The copper strap presents a shorted return to ground for the leakage magnetic flux.

#### Nigel Goodwin

##### Super Moderator
Most of those transformers are switch-mode ones, which is why they have many fewer windings, and TOTALLY unsuitable for use as a normal transformer (there would be a big BANG, and fuses would blow to pieces). Some aren't transformers at all, but are mains chokes, designed to reduce interference both IN and OUT of the power supply.

#### rjenkinsgb

##### Well-Known Member
Many of the top ones (with two similar winding & four pins) appear to be mains filter chokes and not even transformers...

#### ronsimpson

##### Well-Known Member
These transformers are "common mode line filters". The ones I underlined in red are filters. The rest might be filters or "base drive" transformers.

These transformers are all from switching power supplies. They will not work at 60hz! They do work at 60khz or more. The smaller size and less wire is because of the higher frequency.

Nice Bridgeport!

#### AnalogKid

##### Well-Known Member
Transformers these days are different than 50 years ago some how they manage to operate fine with a few turns of wire in the primary coil on 120 VAC.
They don't. Those are not transformers.
I don't understand the circuits most TV circuit boards are low voltage. First picture transformer steps 120vac down to 100vac ?
No, it doesn't. In the first photo, that is not a transformer. It is a common-mode choke, part of the powerline EMI filter to reduce conducted emissions (outgoing noise) and conducted susceptibility (incoming noise). In the second photo, eight of the items are variations of the same thing.
I have experimented with these newer transformers in the past I think it is interesting the 120 vac primary coil has about 75% less primary coil turns than transformers had 50 years ago and they do not over heat.
Many of the transformers you show are for switching, not linear, supplies. Very different circuits, and very different magnetics.

ak

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#### shortbus=

##### Well-Known Member
I'm starting a pool for this thread. How many pages and how many topics do you think it will end at?

#### gary350

##### Well-Known Member
Circuit boards are powered by 120vac how does that voltage goes reduced to lower voltage. I see a 5v regulator and 9v regulator.

#### ronsimpson

##### Well-Known Member
There are two different approached to making a power supply.

1) 50/60 hz transformer is likely laminated. Works at low frequency but not good at high frequency.
The picture shows the primary and secondary side by side but they can be on top of eachother.
The transformer might be 120:7 turns to get 120 volts down to 7 volts and get isolation from the power line.

2) The power line is full wave rectified and energy is stored on 200 or 400 volt capacitors. (120V rms is rectified and filtered to get 160 volts DC)
Then a IC and transistor (MOSFET) chops the 160Vdc to make AC at 60khz to 200khz. ( because the frequency is 1000x faster a smaller transformer can be used) This switching transformer must have a turn ratio to get 160V down to 7V. The IC is smart. It looks at the 7 volts. If the voltage is low it increases the duty cycle to bring the voltage up. If the power line is too high or low it changes the "on time" to keep the output voltage constant.
In the bottom picture you can see the transformer core is ferrite not iron sheets.

#### ronsimpson

##### Well-Known Member
Here is a silly little video of a switching power supply.
switcher

#### AnalogKid

##### Well-Known Member
Circuit boards are powered by 120vac how does that voltage goes reduced to lower voltage. I see a 5v regulator and 9v regulator.
If you are asking how an off-line, AC-to-DC power supply works, there are dozens (hundreds?) of websites that explain it and take you through a design step-by-step. Again, note - linear and switching power supplies are two very different animals. Both rely on alternating current through a transformer for galvanic isolation and voltage/current conversion, but there are significant differences among the various switcher topologies.

ak

#### gary350

##### Well-Known Member
I know iron core transformers are for 60Hz AC step down or step up. I did not know ferrite core is high frequency only.

Problem is, some are easy to tell if they are iron core some are not and a magnet sticks to both so that is no help.

Both transformers on the right look like iron core. Top right is iron core. Not sure about bottom right.

Both transformers on left appear to be ferrite by size & shape but can't tell for sure until plastic case is removed from bottom left transformer.

I see PC boards with 120vac cords that have low voltage DC parts on the boards. I need to start looking closer to see how they are getting low voltage. All the TV PC boards have already been stripped of lots of part so that is no help. I still have 2 old TVs that I have been putting off stripping because I don't need the parts so maybe I take them apart soon to see how 120vac gets stepped down to lower voltage. I never notice obvious step down transformers.

#### gary350

##### Well-Known Member
I have about 100 transformers I have never paid very close attention to them I just save them in case I need one some day. I sorted through the transformers this even there are several EI transformers that I did not know I had. Tomorrow will be a project to ohm each transformer then connect the primary coil to 120 vac to see what voltage I get on the secondary coil. All the transformers need to be marked with voltage if I need one they will be easy to find.

I decided to test the 2 EI iron transformers on the right side of this picture.
Right transformer is 120 vac primary and 14.4 vac secondary.
Left transformer is 120 vac primary and 6.3 vac secondary.

#### shortbus=

##### Well-Known Member
I'm having a hard time believing that you actually built the Tesla coil you keep showing here. Just saying....

#### gary350

##### Well-Known Member
I'm having a hard time believing that you actually built the Tesla coil you keep showing here. Just saying....
Used TV parts are new technology to me. When I was in high school & college we learned about vacuum tubes. I did TV repair part time in high school. After high school I went to electronic technical school got a 2 year degree. Worked at a transformer company $1 per hr for a year. Then went 4 yr college took mechanical engineering. After college I built projects with ICs not transistors. I have never done anything with transistors. Tesla coil have no transistors & no ICs. After college there were 10 graduates for every job 9 out of 10 people did not get a job. I worked factory maintenance for 1 year$3.75 per hour, beginning electronics job offers were only \$1 per hour I had college loan to pay off. Then I took a job servicing industrial equipment for a well known company I traveled the USA servicing industrial equipment in a different factory every day for 2 years. Then I learned to be a tool & die maker and we built special equipment for industry I did all the electronics on the new equipment. I worked as electronic technician repairing circuit boards for 1 year. Then I started my own machine shop did tool & die plus built proto type industrial equipment for 5 years I worked my butt of 16 hrs every day 7 days a week decided money can't be beat but I need a permanent vacation. Family took 3 week Florida vacation when we returned home I could never get into working 16 hrs every day again. Then I took a job as design engineer for a big company for 6 years. Then I took a job programming industrial computers that make machines go & robots go so they build parts on factory assembly lines. Then I was offered a job as Plant Engineer in a factory that makes parts for, Ford, Chrysler, GM then retired at age 55. So you see I never learned anything about modern day electronics like you know that is why I am trying to learn now. I have been striping parts from old TVs but only keep, resistors, capacitors, wire, few other things most of the other stuff was not needed to build projects that I build until I learn about mosfets, voltage regulators, and other new things.

Today I learned new technology about transformers at first grade level. I can see iron core lamination in some of the transformers. All the transformers inside plastic case are iron core. Rust spots on iron core is a give away it is not ferrite. Weight is a give away too iron core is heaver than ferrite. I have 23 iron core transformers turns out most are 120v primary, secondary voltages are 11v to 23v and every thing in between. One transformer has a tag that says, 120v primary, output 11.4v 140ma but digital meter shows voltage to be 16.3v.?

I found another 11v, 12v, 13v iron core transformer I will use 1 of them for the other transformer I rigged up. It doesn't take much power to run a digital read out. I might do something with that tomorrow I don't need it anytime soon. OH wait tomorrow is the Strawberry Festival I think we go there it is a 75 mile drive.

Tesla coil was a challange. I could not get one to work I built in high school I was not smart enough yet. Then I learned about TC forum online learned how to build TC so I built 6 each one larger than the last. Been there done that, time for a new challenge.

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#### ChrisP58

##### Well-Known Member
One transformer has a tag that says, 120v primary, output 11.4v 140ma but digital meter shows voltage to be 16.3v.?
The output voltage of many iron transformers will be high when unloaded, but will be fairly close to their rated voltage when loaded to their rated current. And the smaller they are, the more common the this problem is.

#### BobW

##### Active Member
The copper strap presents a shorted return to ground for the leakage magnetic flux.
No. It's just a shield. Notice that the copper strap doesn't pass through the aperture of the core like the winding, but goes around the outside. If it did pass through the core, then it would indeed be a shorted turn, and it would fry the transformer.

#### tvtech

##### Well-Known Member
The small grey ones above look like CRT line driver transformers. DONT put 220VAC anywhere near them.