Relay Engineering Dept.
Fujitsu Component Ltd.
San Francisco, Calif.
Cars and trucks are carrying more electrical systems than ever, from power windows, seats, doors, and locks, to navigation modules, CD players, and Internet connections. And the trend shows no sign of waning. But if consumer vehicles were to carry more electrical equipment and stay with the industry's current 12-V power system, it would require significantly thicker wiring, thus increasing costs and weight. The solution is an industry-wide transition from the standard battery voltage of 12 to 42 V.
Unfortunately, electric relays used in today's cars and trucks become unusable at 42 V due to arc discharges. To make the new relay more durable under arcing, the relay would need a wider contact gap, but this calls for increased power consumption at the coil. And as the coil consumes more power, it overheats because it cannot shed the heat fast enough. Therefore, the ambient temperature or the current carrying capacity must be lowered.
With all this in mind, our company engineers set to work designing a compact relay with minimal arcing, high current capacity, a wide contact gap, and low power consumption.
Step 1: Determining the basics
Our first step was to determine the relationship between contact gap and electrical life. Apparently, the wider the contact gap, the longer the relay's electrical life. However, as the gap widens, power consumption in the coil increases and the current-carrying capacity decreases. Thus, we had to examine these points to select an optimal gap.
Next, we studied four relay circuit configurations with an eye toward miniaturizing the relay and minimizing power consumption at the coil. By plotting current versus arc duration and studying other variables, we determined arc duration could be reduced by almost half if contacts are connected in series. And the duration of arcing caused by relays breaking contact cannot be reduced in some circuits. This helped choose which configuration to explore further (Circuit 2).
Another arcing characteristic studied was the difference or variations in contact opening times for switches connected in series inside relays. Arc duration shortens as the difference between contact opening times shrinks. Thus, we had to develop a relay structure with consistent opening of the two contacts in series.
Step 2: Choosing the right material
There were two major concerns that governed our choice of material for the contact. We wanted it to contribute to the relay's long life, and we wanted it to be environmentally friendly. On this basis, we chose silver-tin oxide indium, a material free of cadmium and other heavy metals.
Step 3: Selecting the design
Several designs were evaluated, with particular attention on the magnetic pull-in force, power consumption in the coil, spring loads on the contact, and manufacturing efficiencies.
Eventually, we chose a 42-V relay with one transfer structure that uses movable bifurcated springs. Stationary contact terminals are placed to let contacts be connected in series inside the base block. Only one coil suppresses or counteracts the differences between contact opening times inside the relay. And using only a single suppression coil helps keep the relay compact. The design has a wide contact gap and it consumes relatively little power at the coil, both of which improve the relay's current-carrying capacity. The relay also withstood 350,000 operations at 42 Vdc and 10 A (inductive load) under continuous testing.
Step 4: Check resultsAfter testing, we concluded our new 42-V automotive relay had:
Relays with arc resistance will surely not be limited to automotive electronics but spread to other fields, including home electronics. And our company plans to continue research to develop a smaller 42-Vdc relay with contact capacity between 5 and 10 A.