Recently I wrote a column for a site which specialises in audio repair and did some research on randomly selected forums which covered same. Sadly, much of the information is misleading and for a novice, this could cause more damage in the long run. This article hopefully will set the record straight. I’m prone to rambling but let’s hope I can keep on track.
The computer power supply is generally a switching device called a SMPSU or switched mode power supply. The a.c. is rectified and then chopped, the pulse width driving the circuitry being longer when the load is higher. These power supplies are more efficient than linear power supplies, cheaper to build in large scale and with modern technology very reliable. The fundis here again are our Asian friends whom have made smaller, better and more reliable. Not all of them though – you get what you pay for. If you have been in the industry for a while it’s easy to single out the more useless items from the gems. Here I speak about quality of build and advertised current handling capabilities. Don’t always be fooled by a board having fewer components meaning it is less sophisticated. Some PSU manufacturers spend a lot of time on the layout and the PCBs are also of better design than the cheaper units – of course compare PSU units from the mid 90’s to modern units – the AT, ATX or Micro ATX form has not changed but there are some power supplies which are very neat inside and are just begging to be modified. Then we also get the others….
Why would one want to modify a PSU?
The main reasons one can think of is bench or lab power supplies, higher voltages or a dedicated voltage other than the 12 and 5V for which it was designed. Power supplies used to be expensive but for a good 200W unit these days one would rarely pay more than $20.00. Some of the better makes to look out for are the Sparkle, Seven Team, Seasonic and FSP. There are thousands of different brands, believed to come mainly from the industrial city of Shenzhen, Guangdong Province, known as SHIP or Shenzheng Hi-Tech Industrial Park.
What to look out for?
I repaired power supplies for about 15 years. To be honest, it’s not really worth it – the bigger problem here is that once repaired if anything else blows afterwards in your customer’s machine will be blamed on your repair. In certain cases the customer may be right – cheap power supplies have some design issues all to do with the low costing and taking short cuts. There are three things that your power supply cannot do without – over-voltage, over-current protection and the power good line. (the better supplies also have over temp sensors). We’ll become clever now and take a short cut – the quality of the capacitors. If the capacitors are from an unknown source then don’t go there. Gigabyte motherboards are expensive because they use very expensive capacitors – power supplies don’t necessarily. Is the circuitry / mains transformer double insulated? This is a requirement in the USA, Australia, UK – in fact in most countries. This requirement is there in case a non-earthed cable is used. Again, the more expensive the components, the more thought went into the design and the safer you are.
For a great read on the brand wars and dodgy power supplies – Tom’s Hardware to the rescue.
Getting back to repairing power supplies – the most common problem found on the ATX supplies was the 5V standby circuit. Rated initially at a few hundred milliamperes they didn’t last long – now the 2A units have proven to be more reliable and in the long run, so has the mains chopper circuitry. Intel may have changed the specification but I never looked it up. The four power supply types mentioned above were predominantly the units I worked on, the cheap and nasty units went into the bin. I really didn’t feel like changing all the electrolytics and if you felt bored and did this you would have still come across other problems like dodgy MOVs, poor soldering and well, the transformers may have been whacked as well or just not safe for our use. I won’t paint all manufacturers with the same brush though – there are some small timers out there that did a good job. Not all though.
Lastly, the switching components often came up short of manufacturer specification. Datasheets are aplenty on the net so a quick check will reveal any shortcomings. Most PC technicians should bench test the power supply under the recommended load – the power supply should run warm but never hot.
Once you have made your selection of PSU and you are in the mood for making some modifications then be warned: The voltages inside a power supply are lethal. You will need to discharge the reservoir capacitors. And discharge them again after a few seconds – check the voltages across them. Never work with a unit plugged in even if it is switched off. If you do go against our wishes and decide to modify your power supply stand by for a loud cracking sound – always anticipate the worst. I’m being negative here but SMPSU in most cases are an engineering marvel – design teams are made up of engineers and not usually DIYers.
The most common request for design modification is to up the voltage of the power supply. The switching circuit of the main chopper is essentially the chopper driver, a PWM device, a comparator, the switching transformer and power transistors. Out of the circuits I have seen on the web where a modification was done I think the most logical approach was to rewind the main transformer. I don’t like the idea of removing the ground wiring to put two power supplies in series but that’s a matter of preference.
- The schematic for a 200W ATX can be found here, thanks to Pavel Ruzicka. The heart of the device is the Texas Instruments TL494, the comparator LM393 acts as the guard, monitoring the power supply status.
- First things first: a look at the ace electronic’s website ePanorama to get more information.
- Secondly look at Homepage of VK5JST/VK5TR – this is a 13.8V supply modified/built from a Seven Team 250W power supply. What I like about this author’s work is the safety precautions taken – the work is very detailed and meticulously recorded.
- For anyone interested in getting more information on the TL494 go here. Another chip I like is the LM3812.
What to do and what not to do
- It is often recommended to purchase an autotransformer / variac when working with SMPS. This is a must for any electronics workshop. The one thing that really I cannot understand is why in South Africa there is either limited information available through websites or when there is information available the pricing is not shown. So now, 8-5 workers through the week cannot get prices over a week-end. Are they frightened to show customers their pricing because they feel they are ripping the public off? I noticed a 3KVA variac going for about $135.00. In South Africa this goes for about $350.00.
- Never attempt to modify a power supply which is not working. Fix it first.
- Read up about SMPSUs through ePanorama or your favourite website – don’t attempt modifying a mains driven device without knowing exactly how it works.
- Don’t allow distractions. I have 3 dogs, 2 cats and four birds. I don’t work on anything. Have you ever worked on a CRT monitor or television set and the phone rings. Chances are you may have forgotten what you were doing before the phone rang. Chances are when you switch on you will only then realise that the potential divider chain used in the protection circuitry has a resistor missing.
- Don’t work on mains equipment after a few drinks or with a hangover. Don’t ask me why.
- If all else fails go linear. This is the tried and tested method – most workshops have a transformer or two lying around which can be used for making a power supply. A comprehensive list can be found here.
The dreaded surface mount device has been around for ages, love it or hate it it’s here to stay. I actually learnt some tricks from a very large computer motherboard repair company which actually relied more on your skill with a soldering iron than expensive tools to work on these boards. Like all craftsmen, one can blame the tools but a good craftsman makes a plan. Where to start?
Like most laymen I never got round to bothering about the intricacies of soldering – solder types, temperature, compounds, tip angle and the importance of flux. Until a friend of mine did a soldering course – it was all top secret until he told me rather shamefacedly that it was part of his training. In fact it wasn’t any soldering course – it was to NASA standard. It was then that I acquired a book about soldering and I then rather ashamedly realised how arrogant I was – I really knew nothing about soldering techniques and more so, that what holds for us as mere consumer product repairmen does not hold the same for people developing and building a satellite or military spec radio receiver. Yes, we knew about the different skill-set required but we rarely if every worried about it. Solder was solder wasn’t it? There is an industry standard requirement for solder and soldering – it’s out of the scope of this article. This is a very generalised article only because the web has it’s own set of professional writers and more to the point (pun intended) expert “how to do” pages.
Tag Strip and PC Boards
If you have been fortunate enough to have gone through the transition from tag strips, to vero board to PC board, then to multi-layered boards and SMD you’ll note that there is always one common factor, when conducting a repair aften you need to remove a device to make measurements, most probably either just one pin as in a resistor or diode or in some cases just the input or output of a device such as a transistor or digital chip. The technique of fault-finding doesn’t change but in many ways with modern equipment it does become simpler. Yes, there will be disagreements to this but I’m following the rationale that more often than not it will be a power device that fails first or possibly a component that normally runs hot. Static electricity is also a big killer and in motherboards (computers) that flaky power supply can cause more damage than what it’s worth repairing. Always use good power supplies from a trusted source.
Where to start? What tools to use? Your workshop has to have a very good soldering iron, these budget units don’t work. Get a heat gun with temperature control and fashion a set of tips for air concentration if you cannot buy. Make a box where the heat gun can stand upright, the wooden pieces (yes, wood) on either side of the heat gun should be fairly narrow – in most cases 10 cm is sufficient – allow sufficient cooling for the heat gun. With the heat gun facing upwards and the faulty board resting on top you can heat up areas of the board very quickly, removing faulty chips or loose components with a pair of tweezers. Be careful not to remove or move components you don’t want moved. Be cautious of a double sided board where components can drop off – most computer motherboards don’t have components on both sides. This method described above works very well for very general repair but not BGA or Ball Grid Array repair work. BGA machines heat the top and bottom of the board equally, there is a top nozzle blowing hot air onto the component, possibly a chipset or processor. The fitting has to be precise so the nozzle should be made to run as close as possible to the perimeter of the chip – standard masking tape is attached to the outside components to prevent these components from moving. To get a good idea of this we have The Instructables website to the rescue: http://www.instructables.com/id/BGA-rework/
There are quite a few companies supplying BGA Machines, the cheapest one I saw which I know worked very well cost in the vicinity of U$3500. This was the most basic machine. The next one up which also used hot air cost in the vicinity of 10 000 U$. The BIG difference between the two was that the more costly unit was totally automatic – the only real work you had to do was align the chip. With a magnified view this becomes a simple task. The cheaper units are for the more experienced or rather for those that feel comfortable in working quickly and not being shy of precise alignment work. I am surprised to see that there are very few experimenters building their own BGA machines. Modern technology is also veering away from hot air to IR.
Building a BGA machine should not be difficult although I have not built one myself. It is important to have a (preferably) digital temperature readout of top and bottom air flow temperatures. Being too impatient can cost you the job entirely – it’s not unheard of for operators to remove a piece of work too soon and components, including your replacement work shift. Worse still if a component drops off and you do not have the type or value.
Main difficulty here is working with RHOS which is another beast entirely. I started working with BGA reflow when RHOS started becoming popular and the biggest threat was the higher melting temperatures of RHOS solder over your conventional types. Why? Board damage. Even seasoned operators found themselves burning boards whist being as careful as possible. This is where the more advanced BGA machines showed their worth. Some chipsets can be extremely expensive – look no further than high end graphics cards. In companies conducting repairs of this type the graphics cards are normally repaired by the more seasoned operators – a chipset could cost in the vicinity of a 1 000 U$ and upwards so they do not take chances. A northbridge for a very popular older board cost about U$20.00.
Fortunately for most experimenters and repair personnel the parts most prone to failure are the power components – usually FET voltage control circuits, the VCORE supply rail controllers and smoothing capacitors. On some motherboards your VCORE is controlled by a three phase supply so all the switching power components get changed – not just the one you deem to be faulty. Fortunately they normally sit on a very accessible part of the PCB – the copper acting as a heat shunt. BIOS or IO chips can be removed fairly easily – get an EPROM reader/programmer. More often than not the BIOS chip loses its instruction set – rewriting solves the problem. However it has to be done off the board. Not always, but mostly.
Here things can become tricky again – when removing connectors off a board the procedure is to run the connector over a bath of solder, with experience this becomes a very simple practice but agin here RHOS has it’s own behaviour because of it’s higher temperature. Inexperienced operators will find that RHOS is very stubborn and all the small components rather dislodge first, any rapid movement can cause the smaller components to dislodge and you end up with a heap of mangled components and solder. Patience is the key.
The introduction to Restriction of Hazardous Substances Directive or (RoHS) soldering is nothing new – essentially lead free soldering. The higher than normal operating temperatures to melt solder can leave us confused, causing damage to adjacent components on your work. As mentioned previously, I have seen seasoned operators pulling their hair out when first working with RHOS. RHOS has changed over the last 10 years and there are lower melting point solders available. The main thing to watch out for is not to combine RHOS solder baths and stations with non-RHOS. Mostly this has to do with compliance and ISO certified companies take this very seriously.
An article covering this can be found at Hardware Secrets.
In order not to plagiarise a very common topic I’ll get straight to the point – patience, patience and more patience. In my days doing repair work the two most common problems that stand out a mile is a) first and foremost, lack of patience. If you don’t have this virtue don’t do this kind of repair work. In fact don’t get involved in any form of repair because all that will happen is that you, the reapirman, will become more frustrated and cause more damage. It’s nice to learn everyhting through Google but there is nothing better than seeing someone actually walk the talk. This can be seen on You Tube time and time again. So not to beat this subject to death we move on to number two, (b) poor eyesight. Poor eyesight is not usually a hindrance, in fact I have seen some old – timers doing better work than our 20/20 vision workers, only because they apply a lot more patience to their work through of course, the best virtue of all ‘Passion”. It may take a little bit longer but it will get you there.
For many people starting out in the electronics industry, possibly as a hobby or later on full-time engineering there is still nothing better than gaping at a well constructed piece of electronic equipment. I have seen some cheap computer power supplies from Asia which boggles the mind when it comes to quality – very good quality at a next to nothing price. There are expensive consumer products which look as if they were put together in someone’s backyard – yes, a NAD amplifier purchased in South Africa. I have seen television sets, the last of the CRT variety, with wire wrapped power supply components and a PCB for the main circuits. Marine and Aviation equipment is all but cheap and their construction shows it. I haven’t seen NASA equipment, I have seen military and medical spec and again you pay for what you get. Soldering can be done by all ages but good soldering is only done by someone whom shows the ability and passion to want to do a good job.
Steam Engines – Part One: Links to models and other Trivia
The steam engine has always been a marvel to most people. Incredible torque from standstill but also pulling Goliath locomotives to well over 100 m.p.h. The first engine powered locomotive was developed well over 200 years ago. What makes them so special? What is the secret to their success and more importantly why do modelers the world over always seem to lean towards building their own engine?
Although that simplistic looking reciprocating engine is always looked upon in awe there are two things that us newbies forget, the first steam engines were not built with modern tooling but yet their parts were machined to perfection – the engineers of the time, many just craftsmen were every bit as good as today – in fact maybe better. The physics involved in building a practical machine, as a prime mover or show piece is not for the feint hearted. Ask any engineer about boiler safety, boiler design and the inevitable accident when we forget to utilise proper safety margins. The Mississippi River can bear testament to the many steam powered boats and ships that had catastrophic boiler failure and subsequent loss of life. So what makes a steam engine so special, why are we prompted to build something which most of the technical geeks today feel is nothing better than an archaic relic of a bygone era?
Your basic steam engine design would consist of nothing more than a pressurised vessel, a piston, valve or port and a flywheel. This engine may be a single acting or double acting steam engine, the difference being in that a double acting steam engine has alternating pressure on both sides of the piston, whereas single acting would normally be only on the crown of the piston with the momentum of the flywheel pushing the piston through to the next power stroke. The models of these engines often needed to be manually started. Single acting engines were most often seen in the form of a Beam engine which was used as a water pump to drain mines and canals. There are lots of informative articles based on this type of engine.
Along came the oscillating steam engine which is a double acting engine. A simple engine yet brilliant in design. A great place to see an animated engine is the ultimate in animated websites – http://www.animatedengines.com – the engine has two ports to allow pressurised steam to create the downward or upward stroke. This article however is not about how steam engines work, there’s enough information on the web covering this – more so we want to cover companies that supply working plans, kits and parts. We will move from pressurised boilers which every single model maker will warn you against making to mono-tube boilers or flashboilers.
As an aside, my first model steam engine was bought as is, passed down through two generations. It had a safety valve which we now know was never tested and we now know could have been pretty dangerous. As a six year old I was not allowed to run this engine if my father wasn’t supervising. Anyone believe a 6 year old would be a responsible adult at any time and heed this advice? Of course we tried different heating experiments and luck more than being informative played a role in never having any major disaster while playing with this engine. These reciprocating engines now have one very common problem – absurdly costly. I saw one recently, albeit a far more modern version costing about U$300.00.
Early reciprocating steam engines were notoriously inefficient. Geoff Wolfe has a very informative website and an article which is excellent in that it is thought provoking to the layman is his post on Compression Ratio and Break Mean Effective Pressure. We all know that diesel engines are far more efficient than your normally aspirated gas or petrol engine but why is this? There are some decently written articles about compression ratio etc but it’s in the context of monotube / flashboiler engine design that makes this article thought provoking. More on this later.
The flashboiler supplies an almost immediate source of pressure – it was used in motor cars and subsequently model hydroplanes. They are very efficient and currently there are many designs been used which are supposedly green. I use supposed because it all depends on the heat source and how the exhaust emission is controlled. One such design is the Cyclone. I am going to plagiarise here and add the following from the http://www.greencarcongress.com website: “Cyclone Technologies LLP, developer of the Cyclone external combustion engine, received an Automotive Engineering International (SAE’s publication) Tech Award at the SAE World Congress in Detroit.
The Cyclone modifies the traditional Rankin cycle steam engine to deliver the use of super-critical pressure (3,200 psi) and super-heated steam (1,200° F)—normally found in high-efficiency electrical power plants—in an efficient, compact package suitable for a vehicle.”
The Cyclone is an external combustion engine. External combustion engines, like electric motors exhibit extremely high torque at very low RPM. Ever think why an electric motor driven car from pullawaycan spin wheels without a clutch – or gearbox? Straight coupling between engine and wheels. Steam locomotives and ice breakers spring to mind. The Cyclone can run on any fuel, it doesn’t need a clutch as it’s highest torque is at lowest RPM, it is double the efficiency of a gas (petrol) engine and a three cylinder device is self starting. Is it a breakthrough? Somebody had the genius to build this engine, Harry Schoell, surprising enough the information was all there, it took just one person to put the plan together. So from steam to internal combustion petrol and diesel engines back to external combustion. This is not crackpot – read about the gentleman here. I am sure most Parts-Ring readers will not be familiar with Cyclone Technologies, neither was I until last year when searching for something greener, modern bio-fuel run engines and what to do with the waste. Of course elsewhere we wrote about hydrogen fuel cells – I think the “Schoell Cycle Engine” may be just that tad more innovative. Watch their space.
Possibly one of the best websites with regard to their kit range and quality would be Stuart Models. Stuart Models also have a comprehensive catalogue covering gas burning boilers and pumps to make your desktop model work.
Chasteam is a great website. I loved their kit range – I have no idea of the final build quality but their pricing is awesome. A definite visit for the modeller – please visit their website.
The CEDesign website gives a comprehensive list of companies manufacturing model kits. We can only wish that we had so much more money hidden in the piggy bank.
Build your Own
There are three websites which give considerable information,
One of the most popular websites in the universe, Instructables has a few plans – this one is a cheap and simple unit designed to run on compressed air. In fact all the models on these pages, when built from a kit are designed for compressed air – this does not mean you should not build a boiler, it just means that we distance ourselves from your attempts at possibly hurting or God forbid killing yourself.
County College of Morris Engineering – Tech Department, by N.J. Randolph – lots of plans which will get your creative juices flowing.
Over 120 free plans for steam engines at John-Tom’s website. Includes plans from Mr. Elmer Verburg – see also Amazon: http://www.amazon.com/Elmers-Engines-Elmer-Verburg/dp/096216710X
Avery nice forum for the modeller – http://www.homemodelenginemachinist.com/ – some worthy advice from participating members, some great builds and yes, do get yourself a milling machine. At Parts-Ring we do believe that the prices of these will go down as Chinese imports pick up up but be careful – try before you buy. I have heard some really bad stories, you get what you pay for. Read our parent article on CNC.
The electronics industry has in most parts made our internal combustion engine more efficient – I will not say more reliable because diesel engines have always been reliable. We add more and more sensors, more and more feedback systems – all to improve efficiency. The external combustion engine has not had the privilege of been seen as a technological marvel except in the 1700’s – but yet, having said that are we not heading in the wrong direction. Many innovative thinkers do believe so, the Schoell Cyclone engine is a case in point.
Green Steam Website
Interesting fact, most steam engines can be made green – it all depends on what you are using as fuel.
The Green Steam Engine website is an interesting one. Advantages are plenty – the flexible rod transmission system, patented by Robert Green is shown in numerous photos and videos.
There have numerous queries with regards to this motor – it has been around for many years yet interest is pretty miserable.
Below is a tube video of a Sterling Motor.
Recently I was on the hunt for an oxy-acetylene kit. I found most of the hardware stores kept units which were for the DIYer, which is really no problem except I don’t know how long it will last in the cylinders provided, usually propane or MAP/Pro and I couldn’t make out what was actually in the cylinder. An entire kit (without bottles) cost U$199.00 through Harborfreight for Map Pro and Oxygen. This is a great price but what’s the quality like? Scouring the web we come across many such advertisements, much from our Asian exporters. The information through manufacturers or blogs is mainly around how to cut, how to weld, braze and getting the right colour flame but really to find one useful advertisement covering an entire kit with bottles is a difficult catch. In fact it’s like the proverbial needle in the haystack. Continue reading “Oxy Acetylene or MAP/Pro”