Hack the PC SMPS

Modern personal computers (PC) use well designed, rich capacity and reliable switch mode power supplies (SMPS), which ought to be ideal for regular use in electronics hobby labs as dependable bench-top lab power supplies. Since pc smps provide +3.3V, +5V, and +12V at high ampere (A) ratings as well as some low current negative voltages (blended with plentiful over voltage and short circuit protection) a standard pc smps fit the lab requirements very well.
First of all, you have to learn which voltage is on which line of the 20/24-pin ATX smps output connector. ATX 2.2 and later (ATX12V 2) have common ATX standard 24-pin connector, and old ATX v 1.x power supply have 20-pin output connector. In the best of cases shorting the green “Power Supply On” wire to Ground connection will be enough to turn on the “stand-alone” smps. However, note that,to run outside of a computer most ATX smps require a minimum output load.

Next is to extend required power supply lines from the 20/24-pin smps output connector to the addon board, as shown in the circuit diagram. The add-on board can be constructed on a small piece of veroboard/custom-built circuit board, using a handful of inexpensive components. The push button switch (S1) in the circuit is a “luxury” power on/off switch. Besides, two LEDs (LED1&LED2) are used to indicate the power supply status.

 Advanced Hacking Clues
Often, dc output voltage higher than 12V (for instance 13.8V) is required for some lab experiments. Fortunately, it is possible to increase the dc output levels of the smps with some advanced tricks. The smps I’ve hacked is built around a TL494 IC for the PWM but IC KA7500 is a drop in equivalent of the TL494 IC, so if your smps uses one of these in its pulse-width modulation (PWM) circuitry, you are in good luck.
First, remove the circuit board from the smps cabinet. The cooling fan may also have to be removed or disconnected. The smps can be safely run without the cooling fan for low power tests. Carefully observe the circuit board and replace concerned output filter capaciors with same value electrolytic capacitors, but have near-double dc voltage rating. For instance, replace old C03 and C28 (2200uF/16V) with new 2200uF/25V (low ESR) capacitors. And, 1500uF/16V in place of C27. Next, examine the bottom of the circuit board and look for two narrow traces coming from the +5v and the +12V output. This is the feedback path,and hence adding some resistance to this path will raise the output voltage level.

For this, cut the feedback path (routed to pin 1 of TL494) and add a multi-turn potentiometer (10K) as indicated here. Note that, in the smps circuit the output of the 5V and the 12V is sampled, compared to a reference and used to control the PWM generator (TL494) to provide feedback to control the voltages. Because of this, in some cases, all of the related components in both the +5V and +12V feedback line may need to be modified independently.

After modification, switch on the smps and slowly turn the potentiometer from zero resistance, while monitoring the 12V output line through a DVM. If the smps shuts down at +14V, the over voltage protection circuit may be activated. If so, the over voltage protection mechanism should be disabled to proceed with the trickery. In TL494 IC based circuits, overvoltage protection -OVPworks by setting pin 4 of the TL494 high. Removal of certain components in the circuit will defeat the protection mechanism. For example, diode D17 in the circuit diagram shown here. Since disabling the over voltage protection mechanism at all is not safe, applicable hacks are intentionally omitted!

Warning! This project is merely presented as a source of inspiration, and should only be conducted by well-experienced hobbyists, after thorough homeworks. We do not take responsibility for any injury, or other forms of damage which may result from performing this experiment.