How do PSU's work?

[ICIT2LOL]

FIRST A WARNING!!
As I think Britton30 pointed out any tampering, dismantling, or attempts at repairing these units is potentially fatal as referenced in Brinks post on a teen death. From personal experience I can say that during my uninformed years I have by sheer accident breached some of the terminals while taking these devices apart and have witnessed the destruction of the item that came into contact with the stored energy with the capacitors in thee units.
I might add that this applies to anyone dismantling microwave ovens, old TV sets or ANY device that uses capacitors in any size - even the small ones can BITE - you must treat all capacitors as charged - to ignore this warning is foolhardy and as stated potentially fatal.
Working as an RN I can attest to the power of these electrical components in that they are the actuating "item" / component in a defibrillator machine. Now we have all seen how those things work (or supposedly work - dramatised for viewers) on TV or films but in reality it does stop the heart!

However:-


Seeing there is a debate within the tutorial on PSU's I felt it necessary to start a new thread for any discussion or debate that isn't strictly general information on PSU's for average user.

So given that I do have a fairly limited and self taught knowledge in that I do know that high currents that are needed within a computer can be supplied by a rectified voltage source form a mains transformer.

However to supply the currents that we now need by purely using a dedicated transformer/s requires huge, heavy, and VERY expensive transformers, therefore it takes some very clever switched mode circuitry to be able to use smaller and therefore cheaper transformers to produce the currents that are required.

I did some researching and came up with this ref and some very interesting videos should anyone want to see what goes on in a PSU.

Switching-power-supply « « Electronic Circuits diagrams Schematics Projects*:: Next.gr

AGAIN - WARNING!!
Any tampering, dismantling, or attempts at repairing these units is potentially fatal!!

 

[Britton30]

Or as the labels say, "No user serviceable parts inside". Aside from the very real electrocution hazard capacitors can and do explode. Some have a semi-liquid electrolyte and other components which will be very hot and stick to and severely burn you. There can also be shrapnel which can take an eye out and cause deep cuts in skin.

 

[ICIT2LOL]

Or as the labels say, "No user serviceable parts inside". Aside from the very real electrocution hazard capacitors can and do explode. Some have a semi-liquid electrolyte and other components which will be very hot and stick to and severely burn you. There can also be shrapnel which can take an eye out and cause deep cuts in skin.

Yes I know from my old audio building days I blew a particular vacuum tube amplifier to bits when the two main ripple caps exploded showering me with and the room with snow like debris - the electrolyte you speak of really burns not only from heat but the chemical reaction on your skin.
This was caused for sake of interest by the caps being hooked up in reverse polarity by mistake. Experiments showed me after even small ones can and do explode with some violence. Those old electro's were just really a swiss roll of two thin metal plates separated by a tiny gap with electrolyte in between.
What would happen with the new ones I don't know and am not about to try LOL!!

 

[Dave76]

Nice thread John, hope this discussion on PSUs continues, I find this very interesting.

Let me know if you want any of the posts from the PSU tutorial moved here.

 

[ICIT2LOL]

Well Dave I was hoping westom would see the thread and come over as I find this stuff rather fascinating.

But I do think that some debate and or discussion is pretty relevant given that a lot of us do tinker with these devices and as I said it is not limited to PSU's. I took to taking out those super magnets from microwave oven's magnetrons and they have some REALLY substantial caps in them (not to mention the toxic beryllium cylinder that holds them apart) and I was unaware of the danger until much later. By the way those magnets are the ones I use stuck onto plastic pots for holding screws etc if you wondered why I was getting them.
Instead I use the super magnets out of old hard drives now and some I had for prospecting - bit safer LOL!!

I did find some caps once from an industrial main frame computer at the local dump and they had a capacities of between 1 -4 Farads not micro farads but whole ones and at some unholy voltage ratings if they had been charged when I and my son were handing them I wouldn't be sitting here typing now

 

[Indianatone]

Power supplies

I am an electronics engineer and I have been repairing TV's, VCR's audio systems to component level since 1975. I also build my own computers and have for about the last 15 years or so. I recently repaired the last of my 3 LG monitors to fail by replacing parts in the PSU.
Power supplies in computers should NEVER be opened by anyone. If they have failed simply head to your nearest computer electronics store and purchase a new one. Look at the label on the side to get the relevant knowledge. For tech heads only read on:
When Switched Mode Power Supplies fail there is always a catastrophic failure where the power supply drives it self into overload by failure of the feedback circuits usually due to bad capacitors causing voltages to rise and as the switching transistor is on for longer and longer until it shorts out taking the driver circuits, bridge rectifier diodes and mains fuse. Now if this is a TV you can usually buy a new board these days, in the past it would be repaired by changing 10 to 20 parts and tested, that was cheaper than a new board. To repair a computer power supply is madness as well as being unsafe as it is always more cost effective to replace the power supply. If you get your "repair" wrong you will destroy the computer.
Make sure you buy a quality brand and NEVER OPEN THE POWER SUPPLY !!

 

[Indianatone]

Britton30 I read your tutorial and thought it was excellent, everything we need to know. There will always be those who know more and start to go overboard but since I left work I no longer have a scope just a couple of digital voltmeters.
One suggestion to admins IMHO if you have allowed a tutorial to be created for reference purposes could it not be locked so we don't get them spoiled by people writing a ton of replies of you should do this or you should do that?

 

[bobafetthotmail]

if you are curious about how PSU look inside but don't want to die young, you can easily head for PSU review sites and find a ton of pics and differences between good and crappy ones, with decent explanations on what you are looking at.

They open most PSUs for the sake of it. for crappy ones you see even pictures after they failed in their 100% load tests.

This could be added to the tutorial maybe, just to satiate curious people.

As for powering stuff, I successfully used external 5V power supplies to power most HDDs and DVD drives, but for the rig and especially the graphic cards I don't think it is wise to even try. Too rapidly-shifting loads, anything not specifically designed to do it is likely to fail.

 

[ICIT2LOL]

Britton30 I read your tutorial and thought it was excellent, everything we need to know. There will always be those who know more and start to go overboard but since I left work I no longer have a scope just a couple of digital voltmeters.
One suggestion to admins IMHO if you have allowed a tutorial to be created for reference purposes could it not be locked so we don't get them spoiled by people writing a ton of replies of you should do this or you should do that?

Well Indie I am glad you picked up on that and that you have some in depth knowledge I think locking the tutorial could be done but only for openly critcising the tut.
I ran some things past Gary first as I thought they were important legal if not pertinent PSU tech specs and they were basically aimed at covering his six in these days of legalistic wrangling. I didn't go in like I guess some would and spoil it just to be a pain in the neck.

I did some testing on some old PSU's I have here that I will not elaborate on my methods because you can bet your sweet life that some fool will try what I did and well you know what can happen.

Now before I go any further I have to say I did these tests at my own risk and I am NOT inferring that anyone does their own tests unless they are a qualified technician. As Indiantone has already said these devices should never be touched apart from a qualified person.

 

[Britton30]

I for one would like a simple, understandable of how switched mode circuitry works and what it is/does. I have looked at Wikipedia and electronic sites and have a headache from seeing squiggly lines and diagrams.

The same paper clip test is listed on the Corsair site with no warnings or other unneeded info.

 

[bobafetthotmail]

I for one would like a simple, understandable of how switched mode circuitry works and what it is/does.

Try reading here (one of my favorite sites, btw). They say it's a more efficient way to convert wall socket AC to DC for computer components, that allows the miniaturization of high power PSUs to the point they actually fit in a computer case.

They explain the technicalities in a readable way even for me that I'm not exactly an electrician, and provide dumbed-down diagrams (that I still can't properly understand) but don't rely too much on them to convey the point.



BTW, in another article they talk about how to properly test a PSU the pro way.

 

[ICIT2LOL]

I for one would like a simple, understandable of how switched mode circuitry works and what it is/does. I have looked at Wikipedia and electronic sites and have a headache from seeing squiggly lines and diagrams.

The same paper clip test is listed on the Corsair site with no warnings or other unneeded info.

Hiyya Gary Now this looks complicated and it might take a bit of reading but the pics are good and really it is all down to transforming the mains ac voltage down the dc voltage we need for the motherboard opto drives GPU's etc

Anatomy of Switching Power Supplies | Hardware Secrets

Now to do that with a "normal transformer would take some hideously large and not to mention VERY expensive transformer to get a secondary voltage of around say 25 amps at 15 volts - for the 12v supply (you loose volts in rectification and regulatory circuits)
The switched mode uses circuitry to be able to use small cost effective transformers to turn that high frequency / high voltage back to low volts / high current without the weight bulk and cost
The downside is that it is more costly to make but that is offset by the efficiency.

But if there is anyone who thinks I am being too simplistic I stand to be corrected.

 

[Indianatone]

I for one would like a simple, understandable of how switched mode circuitry works and what it is/does. I have looked at Wikipedia and electronic sites and have a headache from seeing squiggly lines and diagrams.

The same paper clip test is listed on the Corsair site with no warnings or other unneeded info.

Hiyya Gary Now this looks complicated and it might take a bit of reading but the pics are good and really it is all down to transforming the mains ac voltage down the dc voltage we need for the motherboard opto drives GPU's etc

Anatomy of Switching Power Supplies | Hardware Secrets

Now to do that with a "normal transformer would take some hideously large and not to mention VERY expensive transformer to get a secondary voltage of around say 25 amps at 15 volts - for the 12v supply (you loose volts in rectification and regulatory circuits)
The switched mode uses circuitry to be able to use small cost effective transformers to turn that high frequency / high voltage back to low volts / high current without the weight bulk and cost
The downside is that it is more costly to make but that is offset by the efficiency.

But if there is anyone who thinks I am being too simplistic I stand to be corrected.

No you are doing a good job. I'll have a think about a basic explanation of how an Switching Mode Power supply works and what the benefits are and why almost everything including wall warts for phones have them and then post it here after the weekend.

 

[Indianatone]

The same paper clip test is listed on the Corsair site with no warnings or other unneeded info.

That is for safety reasons all Computer Power Supply Manufactureres and vendors regard the PSU as a "closed black box". Test it by turning it on or replace it with another unit. It is perfectly safe to do this.

 

[ICIT2LOL]

Thank you Tony I shall take that as a real compliment and I am glad we have you on board so to speak with this one as there heaps of questions that need to be answered.

As Britton pointed out it needs some explanation of how these things work. I personally think that once folks know what dangers lurk inside they would be more inclined to follow your advice to get the unit to a qualified person for checking.

I did see one ref that testing for voltage too is ok but one needs specialised equipment to test for loading as I know it is one thing to have volts but without current - pretty pointless really eh?
This was apparent just recently when my old tester machine kept dying on me and the volts ca,e up ok but obviously something was failing maybe those large caps because as soon as I stuck in an old recycled Coolermaster it just took off. Before it was getting slower and I suppose in hindsight that might have been a warning sign.

My next fiddle is to make a bench tester out of one because I want to try elector plating some heatsinks to improve heat removal.

 

[Britton30]

Thanks John, but that article is 9 pages to long for me to grasp, "switched mode".

 

[ICIT2LOL]

Thanks John, but that article is 9 pages to long for me to grasp, "switched mode".

Ok mate I'll pick the eyes out of it if I can

 

[Indianatone]

Part One Why, What and who needs it?

Hi,
First of all to be here you must have an interest in computers and to some extent power supplies. This is meant to be a simple layman’s explanation as to how and why we use Switched Mode Power Supplies in modern electronics and in particular computers. I have made this simple by request; there are far more detailed explanations available in your favorite search engine. This is meant to be easy.
First some history.
Many years ago in the 1950’s most electronics in the home was made featuring valves or tubes. There was no regulated power supply. One side of the AC mains was connected directly to the chassis and the supplies to the various stages were made usually by resistors dropping a rectified DC voltage to the required voltage. The rectifier was at one time a valve and needed to warm up. The resistors gave off a large amount of heat and were often called “droppers” in the trade. If the mains voltage went down or up the picture got bigger, brighter, smaller darker and the volume wandered around. This was the only way of doing this until the solid state revolution arrived. It was also lethal, if the plug was wired wrong the chassis was live and killed plenty of people.

Transistors could be used to regulate the supplies which was especially important in the early days of colour TV. The first switching power supplies I came across used SCR’s or thyristors to take a chunk of the positive side of the AC voltage and rectify it. This was a great step to prevent picture sizes and sound levels wandering around all over the place. The problem was that all the power supplies were taking a chunk out of the positive side of the AC only and this led to potential differences on the AC neutral return lines which the power companies were not happy about. Plus when the SCR went short it usually took the plug out the wall and threw a breaker.

Along came Switched mode power supplies.
These first appeared in expensive TV sets. Basically the power supply became a very regulated affair. The brighter the image or the louder the sound the picture did not budge and the audio did not clip (within limits there was always some deaf blind old git that managed it).
They also became safer. TV’s were still dangerous to work with as this kind of power supply was not yet fully isolated.
VCR’s came out in the late 70’s. These at first had huge mains transformers which completely isolated the unit. They were safe to work on and the only dangerous place was the input fuse and the live side of the transformer. When these machines cost many hundreds or even over a thousand (JVC front loader was at one time the most complex home electronic device ever made) a costly mains power transformer was something the manufacturer could afford and since all the power supply rails were low voltage they used what was called series regulators. Usually a couple of transistors and a zener diode and sometimes a power regulating voltage specific IC to set the required voltage. VCR’s as all but the youngest of us will remember got lighter and cheaper with more and more features. TV’s got more and more features, got bigger and heavier. The need to connect your TV to the VCR, camcorder meant that you must have an isolated power supply.
This is where and when Switched Mode power supplies come into their own and surpass all others. Normally the more power you need the bigger and bigger the transformer you need, they get heavy and expensive fast. With a Switched Mode Power supply if you switch at a faster rate (higher frequency) the smaller the transformer and higher the current. This means we can have a powerful power supply in a little box rather than wheel it around in a cabinet. Also the design was tweaked to give us a HOT side (the AC input side and LIVE) and a cold side, perfectly safe to plug in our new camcorders and VCR’s and Amiga computer etc. An important factor at this time was TV’s started to use microprocessors and needed very stable supplies and even a supply when the unit was in standby.
This is why we use Switching power supplies in most electronics. They are efficient, safe (modern designs shut down in the case of a short or no load) and can run in a very low power mode for standby. Although most of us never give them a second thought without this we could not use computers today. I'll write a simple explanation how a switched mode power supply actually works with some simple diagrams next time.

 

[Britton30]

This is a great start Tony! SCR=Silicone Controlled Rectifier or Semi-conductor Rectifier?

I think here in the US we always called vacuum tubes just "tubes" while the English call them "valves". I remember as a boy waiting to see the tiny red glow to come on in the back of the TV or radio.

Let's see if I'm getting it. Switched mode means rapidly turning a large power source on and off so it effectively becomes a smaller power source, is that it?

 

[Indianatone]

SCR silicon controled rectifier or thyristor (common name) It has an anode and cathode like a normal diode with a "GATE" to switch the device on.
The answer to your second comment is basically yes, we switch a transistor on and off at a high frequency (32KHZ) and this creates a back electomotive force (EMF) in the secondary side of a transformer and we rectify that voltage to supply our circuits. Wait for part 2.