How to ensure high-voltage safety
Andy Williams, Senior Product Engineer
Michael Hoogstra, Senior Electronic Technician
Motorola, Tempe, AZ
With the incorporation of high-breakdown-voltage device technology (MOS and IGBT) into integrated circuits, 20 to 50V switching devices have climbed to 700V. This change requires new design features to protect against high-voltage shock. Operator and equipment safety must be high priorities.
Designing high-voltage safety and reliability into machines and processes requires a high degree of attentiveness to detail, as well as a level of redundancy not present in standard voltage level products and machines--up to 100V--which have been in production for years. Care must be taken to interlock the unit (test site or socket), the cables (the stimulus), the test handling machines (handlers or probers), etc.
One solution is to connect the high-voltage circuitry to the metal protective covers attached to both sides of all load boards with metal standoffs (see illustration). One metal standoff on each side of the board connects to the high-voltage protection circuitry; all other standoffs connect to ground. Therefore, with the metal covers on, the two special standoffs also connect to ground, causing the high-voltage circuitry to remain low. Removing either metal cover causes the high-voltage circuitry to go high, which disables the high voltage.
Warning LEDs are also used on the test boards to display the interlock signal. With the LEDs on, maintenance, engineering, and operators know that the potential exists for high voltage on the test board.
The Motorola Analog Division has been testing and probing devices for over two years without error or safety problems.
To speak with a Motorola applications engineer, call (602) 413-3957.
Designing stronger plastic gears
Rod Kleiss, Kleiss Engineering
Molded plastic gears offer a multitude of advantages over steel gears: low cost; low weight; greater accuracy; self-lubrication; chemical resistance; and shock tolerance. A fundamental limitation to making plastic gears, however, is low material strength.
One way to minimize this problem is to design the gears for maximum strength at maximum material condition. Any competent CAD designer can do this. The approach is simple: First select the desired number of teeth in both the pinion and gear. Then select the minimum center distance between them. Decide on the pressure angle for engagement. That, in turn, defines the base circles for both gears. Now, select the tooth thickness for one gear and create an involute profile for it. Use CAD to roll that gear around the pitch circle of its mate. This creates the involute outline and root of the mating gear. Trim the outside diameter of the mating gear if desired, then roll its outline back on the first gear. This finishes the root outline of that gear and gives the designer a real visualization of the gear set.
This process creates two gears at their maximum material conditions. The gears can then be thinned or separated for clearance. Gear outlines can be transferred directly from the print for file definition and inspection. The outline can also be sent directly to the molder for direct EDM wire burn of the cavity.
To speak with a Kleiss Gears applications engineer, call 612-483-0461.