supplies. To avoid this problem be sure your rectifiers are rated for a ½ cycle surge of 20 x the DC current at the expected diode max. junction temperature. The P.I.V. of the rectifiers must be at least 3 x the V in D.C. value to assure safe operation.
Most filter capacitor failures are caused by inadequate consideration of ripple current rating. The ripple current on a well designed power supply will generally be 1.3 to 1.5 x the DC current. Using an underated capacitor for economy or due to ignorance will result in excess heating of the capacitors and premature failure. Always be sure your main filter capacitors have a RMS ripple current rating of at least 1.5 x the DC current at the lowest frequency of operation and the maximum operating temperature expected. The continuous operating voltage rating of the filter capacitor should be at least 1.5 x (V in nom. x 1.1) to allow for no load peak charging and should not exceed 2 x V in. nom. since a higher rated electrolytic would eventually reform to the operating voltage in any event.
The capacitance of the main filter is determined by the allowable ripple. For a worst case ripple of 5% RMS ωCRL for full wave rectification would be 15, therefore:
C = ω15RL
C in farads
On supplies with current levels below 3 amps a higher ripple can be tolerated and offset by a higher DC voltage since low dissipation is not as important. In these cases, capacitance as low as 2,000 MFD per Amp is permissible providing the capacitor has adequate ripple current rating.
TRANSFORMER
Transformer design follows the usual basic design calculations but certain considerations must be given for game applications. Many video games operate in areas that are relatively open to the outside air. In parts of the country, ambient temperatures of 35°C. (96°F.) are not unusual for extended periods with occasional temperatures to 40°C. (105°F.) or higher. As mentioned previously, internal ambient temperature rises of 30 to 35°C. are quite common in video games. With a total ambient at the transformer of 70°C., a Class "A" (105°/C.) transformer would only be allowed to have a 35oC. maximum rise; with 80°C. only a 25°C. rise would be allowed. This would result in a large and costly transformer. It is strongly recommended that games' transformers be Class "B" (130°C.) impregnated. This permits designing to a maximum temperature rise of 60°C. for maximum life and lowest transformer cost. Short term rises above the rating will not affect life if of f set by equivalent lower temperature operation for similar periods.
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