Principle of operation

The float-switch industry, indeed, the entire reedswitch industry, has not properly told designers how to get the longest life from reed switches ("Switch Tips," p. 50, Sept. 2). Years ago, I bought a reed switch that did not last. The design was simple: when a tank was full, the float switch engaged a relay which shut off a pump. The relay presented a low load, below the switch's rating. But switch after switch burned out after several months. The manufacturer offered no solution, only telling me the load was too high. But I had chosen their best switch and the load was lower then the switch rating. I was forced to add an intrinsically safe control relay, at a much higher cost.

I later learned that the relay produces a much higher initial load as the magnetic field is filled, then the load drops off. This is death on reed switches. It took me 20 years to trip over the answer: put a 100- resistor on the wire feeding the relay coil. It prevents initial loads from exceeding the limits of the reed switch. So simple, so elegant, so cheap. Why don't those selling such switches include a resistor or at least tell customers of this simple solution when directly operating a relay?