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How do 120 vac circuits drop to 5v, 12v with NO transformer???

gary350

Well-Known Member
I tried to buy a 12v transformer 2 years ago know one had a clue what I was talking about, now transformers are called converters.

TV and other electronics no longer have transformers, how do they drop 120VAC to 5V, 9V, 12V ?

My 2 amp 12vdc car battery charger has no transformer?
 
Last edited:

Nigel Goodwin

Super Moderator
Most Helpful Member
I tried to buy a 12v transformer 2 years ago know one had a clue what I was talking about, now transformers are called converters.

TV and other electronics no longer have transformers, how do they drop 120VAC to 5V, 9V, 12V ?
Sorry Garry, but you're posting complete rubbish :D

Transformers aren't called 'converters', you're obviously shopping at other than electronics stores (McDonalds perhaps?), and TV's and other electronics almost all still use transformers. The change is simply the frequency of the transformers, instead of been 50/60Hz they are in the hundreds of KHz, so need electronics to drive them known as a SMPSU (switch mode power supply unit).

It's been this way for decades, cheaper, MUCH more efficient, MUCH smaller, MUCH lighter, and regulated as well. First domestic use was the Thorn 3000 series colour TV back in about 1969? - like I said decades.
 

gary350

Well-Known Member
Sorry Garry, but you're posting complete rubbish :D

Transformers aren't called 'converters', you're obviously shopping at other than electronics stores (McDonalds perhaps?), and TV's and other electronics almost all still use transformers. The change is simply the frequency of the transformers, instead of been 50/60Hz they are in the hundreds of KHz, so need electronics to drive them known as a SMPSU (switch mode power supply unit).

It's been this way for decades, cheaper, MUCH more efficient, MUCH smaller, MUCH lighter, and regulated as well. First domestic use was the Thorn 3000 series colour TV back in about 1969? - like I said decades.

I graduated college 1970. I worked at TV repair shop I remember 1 of the last TVs I worked on had 4 circuit boards and no transformer. I wondered how can that be, no transformer. We never learned about switch mode power supplies in college.

I worked industry 40 years 480vac was dropped to 120vac with a transformer but all the robot controllers are 120vac. I never needed to take a controller apart to see if it had a transformer.

Old milling machine digital read outs have small transformers but new units had no transformers.

Hospital equipment has switch mode power supplies I have always wondered what that is?

I found this.

Industrial supply still calls transformers, transformers buy not Walmart or hardware stores. Motor repair shops still know what transformers are.

How does switch mode power supply work?

In switching power supply designs, the input voltage is no longer reduced; instead, it's rectified and filtered at the input. Then the voltage goes through a chopper, which converts it into a high-frequency pulse train. Before the voltage reaches the output, it's filtered and rectified once again.

Google says, 120vac is rectified to 170vdc converted to 100KHz then pulsed so ON time is very short then regulated to produce lower voltage. OK I understand that, it sounds easy, I don't understand the park where 100KHz 170vdc is regulated down to lower voltage?
 
Last edited:

Nigel Goodwin

Super Moderator
Most Helpful Member
I graduated college 1970. I worked at TV repair shop I remember 1 of the last TVs I worked on had 4 circuit boards and no transformer. I wondered how can that be, no transformer. We never learned about switch mode power supplies in college.

I worked industry 40 years 480vac was dropped to 120vac with a transformer but all the robot controllers are 120vac. I never needed to take a controller apart to see if it had a transformer.

Old milling machine digital read outs have small transformers but new units had no transformers.

Hospital equipment has switch mode power supplies I have always wondered what that is?

I found this.

In switching power supply designs, the input voltage is no longer reduced; instead, it's rectified and filtered at the input. Then the voltage goes through a chopper, which converts it into a high-frequency pulse train. Before the voltage reaches the output, it's filtered and rectified once again.

Industrial supply still calls transformers, transformers buy not Walmart or hardware stores. Motor repair shops still know what transformers are.

How does switch mode power supply work? I did google search I found circuit drawing but no info how they work? My books are all 50 years old nothing there either.

Ancient TV's were live chassis and worked on either AC (various frequencies) or DC mains (historically both were used in the UK, as well as various different voltages) - so no transformers, and no chance of one as they had to be able to work from DC.

Switch-mode essentially converts the mains to DC, then converts it back to AC again at a MUCH higher frequency, this allows much smaller transformers, and allows easy regulation as you can use feedback to control the mark/space ratio of the high frequency AC. A later Thorn set, the 9000, was specified to work from 90V mains to about 270V mains, and many modern PC PSU's cover 110V or 240V with no change required.
 

JimB

Super Moderator
Most Helpful Member
In switching power supply designs, the input voltage is no longer reduced; instead, it's rectified and filtered at the input. Then the voltage goes through a chopper, which converts it into a high-frequency pulse train, which passes through a small high frequency transformer.
Before the voltage reaches the output, it's filtered and rectified once again.

There fixed that for you.

JimB
 

gary350

Well-Known Member
I understand SMPS now. This will make a good project for me to build. Now I know what all these odd little ferrite transformers on circuit boards do.

120vac rectified to 170vdc then 100KHz then step down 17:1 and filtered = 12vdc

I have about 100 circuit board transformers I will look for something that will make a good project. I don't know how to determine power ratings for ferrite transformers but for a good lab educational learning project this will be fun.. I can rewind these transformers if i need to.

But wait, osc needs to be 170vdc. I need transistors or mosfet rated about 250v. ?

I need an easy circuit drawing?


100_3532.JPG
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
Only a small number of those are transformers, most are filters - the only one that looks like a transformer is the top right one.

I would VERY much suggest you don't play with them, it's fairly hazardous and pretty complicated - as you can't seem to cope with simple basic battery powered multi-vibrators I would suggest SMPSU's are well beyond your abilities.

If you really, really, want to - then take the rest of the parts out of the circuit you got the transformer from, and re-build it. SMPSU transformers are custom made for every application, as far as I'm aware you can't just go out and buy them ready made.

You might also download datasheets for suitable IC's, here's a simple circuit from an old satellite receiver:

 

MaxHeadRoom78

Well-Known Member
Most Helpful Member
Very similar of what happened to the typical Wall-Wart supplies, the originals were mainly simple transformers and rectifier type, for a while now they have all been of the SMPS types.
There are some applications that do not hold up well using SMPS type supplies, DC motors and servo's is one such application,.
Many of the DIY CNC community use them, but it often turns out to be false economy!
 

Beau Schwabe

Active Member
If you are absolutely certain that a person will not be able to interact with the circuitry, you can use a "reverse chopping" approach with the AC. IOW if the rectified AC is "below" a certain voltage then turn on a transistor to allow the mains voltage to enter. This method produces an "M" signature on the oscilloscope where the width of the M changes depending on if you are using 120V or 220V. I have a proven circuit that we deployed in 100's of industrial machines that used this approach for regulating voltage without a transformer.
 

gary350

Well-Known Member
Only a small number of those are transformers, most are filters - the only one that looks like a transformer is the top right one.

I would VERY much suggest you don't play with them, it's fairly hazardous and pretty complicated - as you can't seem to cope with simple basic battery powered multi-vibrators I would suggest SMPSU's are well beyond your abilities.

If you really, really, want to - then take the rest of the parts out of the circuit you got the transformer from, and re-build it. SMPSU transformers are custom made for every application, as far as I'm aware you can't just go out and buy them ready made.

You might also download datasheets for suitable IC's, here's a simple circuit from an old satellite receiver:


I looked at several circuits online I don't have many of the parts to build a SMPS for 120vac. I was thinking the save thing you mentioned, remove SMPS from a TV circuit board. I have trashed a lot of stuff I don't think I have a SMPS on a PC Board. I have a bucket full of chokes & transformers. I can probably use a 6vac transformer to build a SMPS to produce 15vdc with parts I already have. SMPS is not something I actually need. It is nice to know & understand how SMPS works.

100_3535.JPG
 

ChrisP58

Well-Known Member
Most Helpful Member
Very similar of what happened to the typical Wall-Wart supplies, the originals were mainly simple transformers and rectifier type, for a while now they have all been of the SMPS types.
There are some applications that do not hold up well using SMPS type supplies, DC motors and servo's is one such application,.
Many of the DIY CNC community use them, but it often turns out to be false economy!
One industry where 50/60Hz transformers are still common is lawn sprinkler control systems. That is because they use the line frequency to synchronize the time and day clock.
 

ChrisP58

Well-Known Member
Most Helpful Member
I looked at several circuits online I don't have many of the parts to build a SMPS for 120vac. I was thinking the save thing you mentioned, remove SMPS from a TV circuit board. I have trashed a lot of stuff I don't think I have a SMPS on a PC Board. I have a bucket full of chokes & transformers. I can probably use a 6vac transformer to build a SMPS to produce 15vdc with parts I already have. SMPS is not something I actually need. It is nice to know & understand how SMPS works.

View attachment 135661
The inductive furnace circuits that you build are fundamentally switch mode power supplies. If you wound the load coil on a core and added a secondary winding, you would have an isolated power supply.
 

gary350

Well-Known Member
The inductive furnace circuits that you build are fundamentally switch mode power supplies. If you wound the load coil on a core and added a secondary winding, you would have an isolated power supply.

Yes I noticed that. I was looking through my parts and I have some new 1500v mosfets that I never used. I could use them to build a SMPS power supply for my induction heater but I already have a 1500w transformer it is very simple circuit but heavy about 8 lbs. I could also use 1500v mosfets in the induction heater circuit with 170VDC RF but that RF voltage is probably too dangerous.
 

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