ATX PSU Conversion

A bench or "lab" power supply is obviously a central piece of equipment required for performing electronics projects. Relatively inexpensive units are available, but generally have limited voltage/current capabilities, and a DIY power supply not only can save $$, but is an excellent learning opportunity. The simplest power supply is just to use batteries. Using a rechargeable 12V battery, and a linear voltage regulator can be a suitable (and safe) alternative.

Instead of the rechargeable 12V battery, using a surplus "wall wart" (laptop rechargers typically give ~20V and 3-4A) is a good option and saves needing to recharge the battery etc. The majority of the projects I have constructed use "wall warts" with LM317 or similar as the voltage supply. Even though a variable voltage supply is relatively easy with linear voltage regulators, and current capacity up to 3-4A can be obtained with only minor extra circuit complexity, having higher current capacity (using transformers etc) starts to get expensive and or circuit complexity increases dramatically if using switched-mode type supplies. This is where converting a surplus ATX supply from old computer equipment comes in. These supplies are generally available for free (scrouged from old equipment), provide up to 30A or more, have short-circuit protection and generally have good voltage regulation (at least on the 5V and 3.3V lines).

The various sections below detail the conversion of a Auriga Model 9806B-300W ATX supply into a simple benchtop power supply providing ± 12V, ± 5V and 3.3V. Even though the ATX supply is capable of 30A on the 5V, 10A on the 12V and 14A on the 3.3V lines, seperate 4A fuses (which can be changed as desired) are installed to limit current and protect test circuits etc. The following information is some general detail about different types of power supply and background material concerning ATX power supply conversion.

Types of "Lab" Power Supply

A bench power supply usually refers to a power supply capable of supplying a variety of output voltages useful for bench testing electronic circuits, possibly with continuous variation of the output voltage, or just some preset voltages (1). A lab power supply normally implies an accurate bench power supply, while a balanced or tracking power supply refers to twin supplies for use when a circuit requires both positive and negative supply rails).

Power supplies are categorized in various ways, including by functional features (2). For example, a regulated power supply is one that maintains constant output voltage or current despite variations in load current or input voltage. Conversely, the output of an unregulated power supply can change significantly when its input voltage or load current changes. Adjustable power supplies allow the output voltage or current to be set by mechanical controls (e.g., knobs on the power supply front panel) or by means of a control input. An adjustable regulated power supply is one that is both adjustable and regulated. An isolated power supply has a power output that is electrically independent of its power input; this is in contrast to other power supplies that share a common connection between power input and output. Power supplies can be broadly divided into linear and switching types. Linear power converters process the input power directly, with all active power conversion components operating in their linear operating regions. In switching power converters, the input power is converted to AC or to DC pulses before processing, by components that operate predominantly in non-linear modes (e.g., transistors that spend most of their time in cutoff or saturation). Power is "lost" (converted to heat) when components operate in their linear regions and, consequently, switching converters are usually more efficient than linear converters because their components spend less time in linear operating regions.

ATX Power Supply Conversion

The are numerable web-sites that detail how to perform a conversion of an ATX supply into a lab bench power supply (see References Section below). Some of the more noteworthy/and or detailed accounts are:

1: http://www.mbeckler.org/powersupply/

2: http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

3: http://majsterkowo.pl/a-lab-bench-psu-from-an-old-atx-power-supply/

ATX supplies are produced according to a specification (7) that includes not only Power Supply Unit, but also the interface to the case and motherboard. In addition to the old AT standard (v1.x with a 20 pin connector) ATX 2.0 has one extra voltage line available (+3.3V), a connector chain-lined to the single 20-pin and a power-on wire that allows Software to turn off the PSU. The ATX specification requires the power supply to produce three main outputs, +3.3 V (±0.165 V), +5 V (±0.25 V) and +12 V (±0.60 V). Low-power −12 V (±1.2 V) and 5 VSB (standby) (±0.25 V) supplies are also required. A −5 V output was originally required because it was supplied on the ISA bus, but it became obsolete with the removal of the ISA bus in modern PCs and has been removed in later versions of the ATX standard. Originally the motherboard was powered by one 20-pin connector. Current version of ATX12V 2.x power supply provides two connectors for the motherboard: a 4-pin auxiliary connector providing additional power to the CPU, and a main 24-pin ATX 2 power supply connector, an extension of the original 20-pin version.

ATX Connector Pinout

The colour coding can vary, with many power supply units having pin-12 as Brown (not Blue), pin-18 may be Blue (not White), and pin-8 may be White (not Gray). However, you need to double check as any individual ATX supply can violate the colour coding of wires. Pin 9 (standby) supply 5V even when PSU is turned off. Pin 14 goes from 0 to 3.7 when PSU switch is turned on. Shorting pin 14 (/PS_ON) to GND (COM) causes power supply to switch ON and PWR_OK to change to +5V.

ATX Conversion Steps

The ATX supply cannot be just plugged into the mains supply to produce the various output voltages. This is because these PC power supply units have two safety mechanisms that prevent the ATX supply operating without the PC motherboard being attached (8). Firstly, the ATX supply requires a “Power-ON” zero voltage signal to start up similar to the “ON-OFF” switch on the front of a PC. Secondly, for the ATX supply to correctly regulate the +5V output voltage it needs to have some sort of load attached, at least 5W to trick the ATX supply into thinking its attached to the motherboard.

The first "safety mechanism" is overcome by connecting together pin 14 – Green (Power-ON) to one of the common black wires (ground) which is how the PC motherboard tells the power supply to turn “ON”. Secondly, to provide a small load on the +5V (red wires) output to trick the ATX supply into thinking its attached to the motherboard and to keep the power supply in the “ON” mode. To do this connect a large resistor of 10 Ohms or less, with a standard power rating of 5W to 10W across the +5V output using just one set of the red and black wires.

Optionally, use pin 8 – Grey (Pwr_Ok) as a visual indication that the ATX supply has started up correctly and is ready to operate. The Pwr_Ok signal goes high (+5V) when the power supply has settled down after its initial startup, and all the voltages are within their proper tolerance ranges. A red LED in series with a 220 Ohm current limiting resistor or similar.

The ATX supply is a switched-mode power supply. Since this is a conversion to a bench-top supply, we are really only interested in knowing sufficient details about "tricking" the ATX supply into thinking its attached to the motherboard and to keep the power supply in the “ON” mode (see Background Section for why cannot simply use the output voltages after plugging into mains power). Note that the particular ATX supply being modified (an Auriga Model 9806B-300W) does not follow the standard cabling colour coding discussed in the Background Section

Circuit Operation

The first "safety mechanism" is overcome by connecting together pin 14 – "Power-ON" (grey coloured wire for the particular ATX supply in question) to one of the common black wires (ground) which is how the PC motherboard tells the power supply to turn “ON”. Since the ATX supply already had a "on/off" toggle switch on the back of the casing, a further switch (between pin 14 and common [ground]) was not included, just pin 14 wired permanently to ground. Secondly, to provide a small load on the +5V (red wires) output to trick the ATX supply into thinking its attached to the motherboard and to keep the power supply in the “ON” mode. To do this connect a large resistor of 10 Ohms or less, with a standard power rating of 5W to 10W across the +5V output using just one set of the red and black wires.

Optionally, use pin 8 – "Pwr_Ok" (orange coloured wire for the particular ATX supply in question)as a visual indication that the ATX supply has started up correctly and is ready to operate. The Pwr_Ok signal goes high (+5V) when the power supply has settled down after its initial startup, and all the voltages are within their proper tolerance ranges. A red LED in series with a 220 Ohm current limiting resistor or similar.

The other major alteration is the addition of seperate fuses to the 3.3V, 5V and 12V outputs. The ATX supply has its own short-circuit and over-current protection (there is already a fuse on the 250V AC input). The additional fuses on the output voltages are there to protect connected test circuity/load from over-current draw due to possibly problems with the connected test circuity/load, rather than anything to do with the ATX supply per se. Since the 5V stand-by supply (the brown wire, pin 9 on the particular ATX supply being modified) is already limited to 2A by the ATX supply (and similarily with the -5V and -12V being limited to 0.8A) no additional fuse protection was included.

Finally, I used screw terminals to provide external connectivity for the various output voltages. Any convenient method can be used, e.g. terminal poles, banana-plug connections etc.

Calibration

No calibration is necessary. The ATX specification requires the power supply to produce three main outputs, +3.3 V (±0.165 V), +5 V (±0.25 V) and +12 V (±0.60 V). Low-power −12 V (±1.2 V) and 5 VSB (standby) (±0.25 V) supplies are also required. On the particular ATX supply being modified (Auriga Model 9806B-300W) I found the "positive" voltages to be at the higher end of the specified range, whereas, the "negative" voltages were at the lower end of the allowable range for that particular voltage.

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  ATX Conversion Schematic

Connection/conversion as per recommendations

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  ATX Actual Conversion Schematic

  Connection/conversion actual

Testing of the converted ATX supply was performed using a constructed electronic dummy load. The construction of the electronic dummy load is documented in the Electronics Projects/DIY Tools section of the site.

The following graph shows the results of applied load current versus ATX supply voltage (a number of other PSU's were also tested and compared).

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  Converted ATX PSU Testing

The most important thing to keep in mind is that 240V circuity is being modified and high value capacitors on the ATX supply PCB can be dangerous, even after mains power has been removed. Any modifications are done at your own risk.

The ATX specification has had a number of modifications and various physical models that you are likely to encounter will give pecularities (and possibly adhere to the requirements of the standard to a greater or lesser degree (for example, the Auriga Model 9806B-300W supply modified in this instance did not follow the standard wiring colour coding for the Molex connector).

Information from the various web-sites detailing conversions of ATX supplies indicate that the majority of the "trouble" stems from the value of the load resistor to provide a small load on the +5V (red wires) output to trick the ATX supply into thinking its attached to the motherboard and to keep the power supply in the “ON” mode. For the Auriga Model 9806B-300W supply a load resistor was not required! It is reported (8) that the newer type ATX12V PSU’s are a little more tricky to convert as they use a ‘soft’ power switch function and require a much larger external load resistance. To get them to start-up, or switch-ON, the supply must be loaded to at least 20W or 10% of the rated power for the larger 600W+ PSU’s. Anything below this the power supply may run, but regulation will be very poor less than 50%.

The type of physical terminals for enabling connection to the various output voltages, indicator LED, on/off switch etc depend upon the layout of the particular ATX supply in question and what your requirements entail. The Auriga Model 9806B-300W supply already had an on/off switch on the external case and had sufficient internal space to enable inclusion of a terminal block and additional fuses. This enabled a compact, portable "build" that required only drilling a few holes in the existing ATX supply case and some prototyping board to locate fuses and wire connections, and therefore was a quick modification only requiring a couple of hours.

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ref001: http://en.wikipedia.org/wiki/Regulated_power_supply

ref002: http://en.wikipedia.org/wiki/Power_supply

ref003: http://www.ka6wke.net/finished-projects/bench-power-supply

ref004: http://www.mbeckler.org/powersupply/

ref005: http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

ref006: http://majsterkowo.pl/a-lab-bench-psu-from-an-old-atx-power-supply/

ref007: http://pinouts.ru/Power/atxpower_pinout.shtml

ref008: http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html