Page:Gametronics Proceedings.djvu/87

 All games' transformers should have an electrostatic (Faraday) shield between primary and secondary to reduce capacitance and line noise feed through. This shield lead should be grounded to the transformer frame and the third wire ground from the line cord should be connected to the transformer frame. Adequate insulation must be employed between the primary and shield (as well as primary to core) to allow a 1-minute hypot test of 1500 VAC (2,500 Volts if used in Europe).

If it is anticipated that more than 25% of your games may be sold outside of the United States, it is generally desirable to employ parallel primary windings that can be connected in parallel or series for either 115V or 230V operation as well as enough steel for 50Hz operation. This will usually result in lower overall cost since stocking of two separate (115V & 220V.) transformers would not be necessary and changing them for specific applications would not be required.

Remember to allow adequate margin in your transformer design for the drop due to high ambient temperatures but also remember that too much margin adds even more heat to the system.

There are other reliability considerations for specific applications that should be considered:

1.)Inductive Load Protection: If the power supply is to operate inductive loads such as solenoids, relays or motors, inductive voltage spikes can raise the DC buss well above it normal level often momentarily reversing the voltage across the series pass transistor and causing destruction. This can be prevented by connecting a diode in reverse polarity across the pass transistor. The voltage spike is then directly bypassed and absorbed by the main filter capacitor.

2.)Reverse Polarity Protection: If multiple output supplies of opposite polarity are used, there is a definite danger of accidentally shorting the two opposing supply lines together during test and servicing. In many cases certain external circuit failures can tie the busses together. While this is almost guaranteed to cause at least one of the supplies to fail catastrophically, it can readily be prevented from damaging either supply merely by connecting a reverse polarity diode (of adequate capacity to handle the short circuit current of the opposite supply) across each supply.

3.)Overvoltage Protection: In the early days of transistorized supplies, failures were frequent due to the low quality of components available and the crude circuitry involved. Also the IC's and transistors used in the circuits being supplies by the power supply were very expensive. Overvoltage protection was nearly a