The SMPS - switched mode power supply
Computer power supplies are inherently fast switching devices and not 50Hz like the conventional power supply with iron core transformer, rectifier and reservoir capacitors. Hence the smaller mains transformer – not the linear type which is inefficient but serves as a great heater for winter months. Can these power supplies be used elsewhere – for instance to drive a car stereo system? This is a very good question and yes, it can be done.
Modifying Computer Power Supplies – modern ATX with lid off. Creative Commons, Wikipedia.
To answer this one needs to look at the loading on the power supply and what it was designed for. A computer power supply will almost definitely run a car stereo but the disadvantages are over current tripping on peak loads. A car stereo will not draw a constant current because the amplifier requires changes in current to drive the loudspeaker and unfortunately these currents can vary from a few amperes to anything up to 50 to 100 Amperes as drawn from the battery. Here we are looking at a 1kW 12V system.
The power supply will stall under heavy load conditions – it’s designed to do this. Unfortunately car stereos are often driven to their maximum potential and when hitting transients the power supply may not be able to cope. Typically a 12V rail feeding an amplifier with push pull output and a 4 Ohm load will have a maximum output of about 4.5W, if it has been bridged (4 x 4.5W or max 20W into 4 Ohm load) means that the user requires about 2A per amplifier. So your minimum current draw on the commonly advertised 4 x 25W amplifier is 100W/12V, about 10A if the amplifier was 100% efficient, which it is not. But yes, in my experience the 20A 12V rail from an ATx PSU will supply the power required without tripping. (one can also add beefer reservoir capacitors).
Modifying Computer Power Supplies for home stereo use
What about modifying the power supply to run your home stereo system which requires higher voltages. Now this is something that only the experienced electronics buffs should try and definitely have a great deal of knowledge in the design and construction of switched mode power supplies. There are circuits shown on the web, one of which I link to but please be cautious. If you have had no experience with switched mode power supplies I would not advise you to attempt any project where you are looking at changing the output voltages.
(Note: as of the late 2000s boost circuits are available cheaply from local and overseas suppliers. Connecting them to a computer ATx supply will give you the necessary voltages required. These are usually single rail output devices).
Changes and Limitations
In all of the circuits you will come across, the heart of the switching supply consists of a controller and switching IC, the formidable TL494 or KA7500, including these popular devices as well: UC3842, 3843, 3844, 3845. This is where the action is, by controlling a DC threshold one can up the output voltage but in most cases, this includes as rewound switching transformer (or modified).
As the output voltages are going to be higher one needs to change the reservoir capacitors to work safely as well as the Schottky rectifier diodes. (fast switching and low voltage drop).
Remember that you will be powering an amplifier so this should form part of the slow switch on circuit. ATX power supplies do need some sort of load initially to start up – I do this via the slow switch-on circuit as well. Load 500mA until the rails are stable and then switchover to the power amplifier.
The output supply voltage
There are two very important aspects of your design you need to look at a) is this split rail or single rail you require b) what is the voltage you require and c) your SMPSU has an over voltage protection circuit – is this on the 5V rail or 12V rail, in most cases it is on both. You will need a schematic – modern power supplies are cramped and can take you ages to trace.
Split rail power supplies
Regarding (a), most power amplifiers are directly coupled and need a split rail supply so you will need to start looking at rebuilding the secondary windings. See below. (b) Regarding your voltages, remember that the amplifier you are delivering power into most probably did NOT have a regulated power supply which means under heavy load conditions the voltage rails drop reducing power dissipation. Regulated supplies usually have excellent results but are also brilliant at destroying output devices if they cannot handle the voltages AND current across them. (Bipolar transistors are a case in point). Consult the specification sheet and check the load line graphs otherwise you may end up with an expensive mess.
(I found this out with the older 2N3055 MJ2955 amplifier designed for 80V single rail. I regulated this amplifier to 80V and although I believe there was an awesome improvement to bass response it was also followed by fuses vapourising along with meltdown of the collector-emitter junction. Although the amplifier used 2 x of each transistor, the 4 Ohm load and regulated supply rail was definitely not what the designer had in mind).
Supply controller and switching IC reference voltage
The TL494 pinout
Big thanks to the author of this website, although he mentions that he is not responsible for the designs, here we have a tremendous list of power supply schematics used in computers which will give the experimenter a great deal to work on. But again, be cautioned – mains voltages are lethal and as one can see, these power supplies are complicated. This schematic quality (TL494) is very good and will give the reader a very strong idea as to where one needs to apply changes in the components to get the necessary output voltages.
Regarding the final point (c), your power supply needs a reference voltage – it will hunt for that voltage and lock on. In most power supplies this reference is from both the 5V and 12V rails so you will need to trace and remove/modify these feedback lines. Get the pdf data sheet on the pulse with modulator from the manufacturer. Texas Instruments and International Rectifier readily submit their sheets on the web, it’s easy to trace back from the control circuit case you don’t have the schematic. Go to this web site http://www.smps.us/computer-power-supply.html for some details – thanks guys. The 200W power supply is a great learning tool, uses the ever popular TL494 (spec sheet readily available as you can see). I’m not going to bore you with the details but experimenting with these supplies you must remember that…
The output rails are low voltage. You will need to change the output rectifiers to fast switching devices with the proper PIV rating.
Your reservoir capacitors need to be changed as well.
You should modify the over current circuitry to suit your needs.
Possibly use the +5V standby circuit to power up a 110/220V fan.
Use the soft start circuit to power up the main switching circuit (the 200W power supply gives a very good indication of what one would normally find) and not hard wire the main switcher because in most cases this circuit also forms part of the over voltage circuit. i.e. look at the base of Q1.
Secondary windings in ATx supplies
The secondaries in PCs are normally centre tapped where the centre tap is grounded (and earthed). You will need to isolate this part and full wave rectify the output (remember that these diodes must be fast switching, Schottky types). I mentioned split rail power supplies earlier on – this is where you will need to start looking at rewinding the secondary part of the main switcher. I believe the more professional experimenters normally opt for two power supplies in series – obviously you will then need to start looking at the polarities of the output. Myself, although I no longer have the project running any more – I rewound the secondary winding and with a little bit of assistance from an engineer whom specialises in UPS design we managed to get a running +/-40V 8A from an older 400W power supply. There was a lot more to this though – I had issues with the main switchers blowing because of a saturation issue. I’ll try to find the plans, should have it still. I will still be redoing this project within the next month or two and give full plans. Power supplies are so much cheaper these days. BTW – do add a choke / pi filter to the output.
I came across this very interesting forum http://4hv.org/e107_plugins/forum/forum_viewtopic.php?19632 which essentially covers what has been said here but please remember that you are working with mains voltages and very importantly remember to filter out noise. Unnecessary EMI is illegal and you can be prosecuted. You may not worry about it but your neighbours and authorities will. To the budding experimenters out there always use fuses that are not going to explode and leave glass fragments all over the place, normally your arms and face.
Proper fusing
I have repaired hundreds of PC power supplies over the years and more often than not taking short cuts have their implications. You can end up losing an eye if you are not careful. Do use HRC or high rupture fuses – they are there also for your own safety. They are usually ceramic or sand filled.
Many of the circuits shown on the internet are designed by passionate people, often engineers and very experienced hobbyists – they expect you to respect electricity.
Further Reading – April 2025 – Switched Mode Power Supplies