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Bridge rectifier, transformer

mw52563

Member
Hi
I'm a total beginner at electronics. I haven't got a schematic for the following circuit and I'm trying to understand what's going on. I thought you put a bridge rectifier after a transformer. But it looks to me like the bridge rectifier is before the transformer. I'm probably wrong but hoping someone can explain it.
Thanks
MikeIMG_20200422_110115.jpg
 

Nigel Goodwin

Super Moderator
Most Helpful Member
It is before the transformer - because it's a switch-mode power supply.

These work by converting the mains to DC (hence the bridge) then converting it to high frequency AC which is FAR more efficient, this makes everything smaller, cooler, cheaper, and uses far less power. As an added bonus, you can apply feedback round the high frequency part and make an efficient regulated power supply. Almost everything has been this way for many years now.

In fact the first domestic example was the Thorn 3000 colour TV back around 1970?.
 

Diver300

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Most Helpful Member
There is a rectifier after the transformer as well. The rectifier and capacitor make the mains into high voltage DC. The 8 pin IC (to the right of the transformer, under your orange line) turns the high voltage DC on and off very quickly, so that the transformer can be much smaller than if it ran on 50 Hz. The output of the transformer is still AC, as transformers don't work on DC and there is a diode somewhere to the left of the transformer.

The input to the transformer isn't a sine wave. Most small power supplies like that work on the "flyback" principle, which is much like a car ignition coil. Like the car ignition, the current only flows one way in the primary an in the secondary. Unlike the car ignition this transformer transforms down in voltage not up, and this transformer is isolated to keep the mains away from the output, where an ignition coil has the same ground for input and output. In the power supply, the IC turns on, current and energy build up in the transformer. Then the IC turns off, and the energy goes out of the transformer, though second rectifier, into the low voltage output. That cycle is repeated around 20,000 to 500,000 times a second.
 

mw52563

Member
Hi
Thanks for the detailed explanations... So the rectifier I've pointed out changes it from AC to DC then to high frequency AC? So the power will be AC leaving that rectifier?
 

dr pepper

Well-Known Member
Most Helpful Member
Nige, couldnt you say that the lopty in a tv is a smps, therefore going back 70 years?
 

Nigel Goodwin

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Most Helpful Member
Hi
Thanks for the detailed explanations... So the rectifier I've pointed out changes it from AC to DC then to high frequency AC? So the power will be AC leaving that rectifier?
No, it's DC after the mains rectifier - that DC is then converted to high frequency AC by the switching element - then converted back to DC by another rectifier after the transformer.
 

dr pepper

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Most Helpful Member
Does a smps only qualify if it has feedback, if so i have built a few whatever they would be called and called them smps's.
Push pull convertors work well without feedback.
 

Diver300

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Is that basically what's happening
As Nigel said, that is a very clear explanation.
I like the waveforms which give a clear idea of what each stage / component does.

There is one typo on the page. Mains is 50 or 60 Hz, not 50 or 60 kHz.
 

mw52563

Member
Hi
Thanks, this has all been a great help. Just one last question before I have a go at seeing what's wrong with it. Theres 4 connections on one side of the transformer and 4 on the other. Ive had a look online and I can't find a schematic or anything... What are the connections likely to be?
Thanks
Mike
 

Nigel Goodwin

Super Moderator
Most Helpful Member
SMPSU's aren't good things to repair, for a start they are full of high voltages and high frequencies, so quite hazardous, the primary side is also live to the mains, so an extra shock hazard, and it causes problems for using an oscilloscope.

A few common issues:

1) Switch-modes supplies are sort of 'magic', they generate their own power to run themselves. But they can't do this without power in the first place, so they require some kind of startup circuit - years back this was often an electrolytic capacitor which gave a pulse of energy to 'kick it' in to life (these go faulty, and the PSU won't start up). But in more modern times a high value startup resistor is commonly used, again these fail (they go high or O/C) and the PSU won't start.

2) You can get short circuits on the outputs, often the rectifiers go short - this causes the PSU to shutdown to protect itself.

3) Electrolytics going high ESR, this is by far the most common fault in all electronic failures, 'sometimes' you can see they are swollen or leaking, but often there's no physical sign - the ones in the picture 'look' OK.

For a start see if you can read the number off U10, and if you can then try and download a datasheet for it - that will give you an example circuit, which should be near enough to what you have.
 

mw52563

Member
Ok thanks I'll give it a go now.. you were right about it being hazardous. The 2A fuse literally exploded on me
 

dr pepper

Well-Known Member
Most Helpful Member
The switching trans, input rectifier or input cap sound like they are shorted.
Might be time to call it a day before you have an accident.
 

JimB

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I have just recently seen a comment that SMPS are binary...

1 = working perfectly
0 = scrap

JimB
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I have just recently seen a comment that SMPS are binary...

1 = working perfectly
0 = scrap
Having spent a great deal of my previous career repairing SMPSU's I know exactly what you mean :D

It also doesn't help when you have obscure designs and no schematics available - most of mine had full service manuals and spares availability.

For a 'one off' you could spend an awful lot of time trying to sort it, most of mine were multiple examples of a few dozen types, so you got to understand what failed, and for the most popular I made up kits of parts for repairing them.

As with most repairs, the trick it to know your opponent, and what generally goes wrong.

For example, the reservoir capacitor occasionally dies in SMPSU's - and on a range of Grundig sets if it did the SMPSU 'self destructed'. Yet on a Tatung series of the same era, using the same IC, and almost identical circuit, it all stayed intact, and in fact continued to work (with issues). So a Tatung repair would be one capacitor (100uF 400V or so), a Grundig repair would be the same, plus IC, transistor, multiple diodes, multiple resistor, more capacitors - a lot more expensive repair. So when you came to repair a Grundig, you always had to check the reservoir - otherwise - BOOM! it all blows again.
 

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