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.