OCC ATX Power Supply Testing Methodology

Testing:

All tests will be conducted using the UK mains supply voltage of 230V at a frequency of 50Hz. I do appreciate that the main audience of this review will be in the USA, but I don’t think this will be a problem. It will have a small effect on efficiency and users on 117VAC 60Hz mains supply can expect a drop in the region of 3% to 5% on the figures quoted in the review.

(I will at some time in the near future purchase a variac, or varible mains transformer, to enable me to perform tests at 117VAC and 230VAC. The difference in mains frequency of 50Hz or 60Hz will make little or no difference to the results).

Test Equipment

The following test equipment is available and will be used for the reviews:

The following test equipment is required:

My Custom Built ATX Power Supply Load is constructed from several networks of aluminium clad wirewound resistors mounted to two large heatsinks with forced air cooling. (An 80mm fan at each end but forced air cooling sounds better). The following loads are available:

The actual current flowing through the various wirewound resistors, is very much dependent on the power dissipation of the load and hence temperature. The values given above are simply derived from Ohm's law and take no account of temperature. The resistance of pure metals and alloys increases as the temperature increases. The actual currents and voltages are all verified with a digital multimeter, only when the temperature has stabilised. Nothing is presumed, and I leave the power supply running for up to four hours before making any measurements. The actual currents drawn by the load are in reality lower than those stated above. Heat is my biggest problem and I have had to limit the maximum power dissipation of the load to 750W for continuous periods. Loads of up to 1200W are possible for short time periods only. This does however, have its advantages, as I can keep my cup of tea warm and I'm sure it will make an excellent fan heater in the winter months. In the future, I plan to add futher resistors to increase the loading available as more power hungry supplies reach the market.

Electrical Safety

Electrical safety is hardly, if ever, given a thought in power supply reviews, but I think it’s important. Why shouldn’t a power supply meet the requirements for electrical safety and why is it never tested? Most if not all power supplies sold in the UK will carry the CE mark, amongst others, which implies it was designed to meet the requirements of IEC60950 (Safety of Information Technology Equipment). I will say however, that failures are very, very rare and some will say that this test is unnecessary.

The basic requirements of the standard are as follows:

The resistance between the earth connection point at the AC input and the power supply casing should not exceed 0.1 ohm. Because I will be testing the power supply with the IEC mains lead connected, I will be looking for a resistance no greater than 0.2 ohm at a test current of 10A between the earth pin on the power plug and any external point on the power supply casing.

The earth conductor is a safety device and it is essential that it is present. It provides a return path for normal leakage currents and a fault current path which will hopefully blow the fuse before any serious harm is caused. Having a properly earthed power supply enclosure and hence case, also provides a certain degree of RFI (Radio Frequency Interference) screening.

The insulation resistance between the live and neutral conductors connected together and earth, must exceed a certain level when subjected to a high AC or DC voltage. The exact standard gets a little complicated but needless to say, I will be looking for an insulation resistance of greater than 2M ohm at 500VAC.

This test is basically checking the resistance between the live/neutral conductors and the earth conductor and checking that no low resistance path is present.

The earth leakage or touch current, which simply put, is the current flowing down the earth conductor during normal operation, should not exceed 3.5mA. This is quite important because if the power supply earth conductor should become disconnected due to some fault condition, this current will flow through a user who touches the computer case.

The perception of an electric shock will occur at a current of between 1 -10mA at 50/60Hz. If your power supply is not earthed, it is quite likely you will feel a shock when touching your power supply or computer case. The next step, although very unlikely, is at 60mA or above when this current flowing across the chest will cause ventricular fibrillation.

These three tests are by no means the entire standard, but I believe they are the most important and the only ones I will consider. I am not going to go into too much detail here and just give a simple pass/fail result. If the result in my opinion is a failure I will explain why.

The review will just include the table below.

This section will also list any safety features claimed by the manufacturer like current limiting, short circuit protection, overheat protection, etc.

(You may have noticed the inclusion of an RCD circuit breaker in the test equipment list. I would never recommend anybody running a power supply with the cover removed but on occasion I find it necessary and, if doing so, the use of this device is mandatory. It may save your life).

DC Output Voltage Load Regulation

The power supply under test will be loaded at three or maybe four power levels, as evenly spaced as possible, up to the maximum if possible and checked against the ATX12V V2.2 standard for conformity.

The voltage and current will be measured at all stages and the results presented in tabular form for each load level. I will also give the loading as a percentage of the rated maximum. An example is shown below.

Total Load = 569.24W (94.79% of rated maximum)