This page has been put together to show the simple yet highly effective modification that can be made to an AT/ATX PSU when used in conjunction with a 12VDC/240VAC inverter for a mp3car computer system.

The inverter, as most of you know performs the task of converting the 12VDC from the car's electrical system to 240VAC (or 115VAC in the US) to operate mains powered devices or in our situation a computer PSU. All is fine and well, except AC power oscillates at 50/60Hz on a sinusoid wave and in an attempt to recreate this, most cheaper inverters create a 50/60Hz square wave output since its easier to do.

This plays havoc with the PSU's internal filter circuits, which are designed to stop voltage transients entering the power supply and also stop possible noise entering back into the mains supply. When faced with the inverters square wave output the filter circuit 'breaks down' and in effect fails. As a result the PSU draws more current, produces audible ticking/buzzing from the PSU and can introduce noise into the computer system (bad for sound cards!).

Thankfully the method to correct this problem is rather simple - remove the filter circuit. It will typically consist of several capacitors and/or a ferrite coil wired in the circuit just after the 240V/115V input and before the PSU's bridge rectifier which converts the AC voltage to DC. In the case of my 300W ATX PSU a single ferrite coil and two small high voltage ceramic capacitors made up the filter circuit. The image to the right shows the filter circuit at the bottom right hand corner of the circuit board.
The capacitors are wired in parallel with the AC input across both power rails and just needed to be removed. The ferrite coil, which actually contained two coils on the one former was wired in series with one coil on each rail. After removal of the ferrite coil, wire links were installed in its place to complete the circuit. Note that wire links were NOT put in place of the capacitors, doing so would cause a dead short. After re-assembling the PSU unit and re-installing into the mp3car player I was pleased to find the system still worked and now had no buzzing from the PSU, speakers and a lower power consumption. In addition, the PSU still functions 100% running off the standard mains, making system testing outside of the car still possible.

The PSU modification was without doubt a great change to make to the system and one I would recommend for anyone else running an inverter/ATX power supply, most importantly for the removal of buzzing from the speaker output. Finally, for anyone wishing to attempt this modification, remember that the internals of a computer PSU does contain high voltages so be sure to know what you are doing before attempting this. Also be sure to check and double check modifications made before powering up the power supply.