Reed relay is a reed switch is operated by electromagnetic field induced by passing current through a coil wounded around reed switch cylinder. Reed Relays always are constructed with reed switch and a copper wire wounded around it.
Reed Relays are available for only small power and signal switching. Coil voltage range is 3V to 24V DC, Due to very low derive current, some of 5V and 3V reed relays can be operate directly from TTL and MOSFET logic output. Some of these reed relays also included with internal flywheel diode. Verity of Reed relays is included with single, double pole, Form-A, Form-B and Form-C types. For example, Coto Technology® 9012-05-11 is Form-A SPST reed relay can carry up to 500mA, coil voltage is 5V and coil resistance is 500Ω. How ever it does not have internal protection diode. Reliability of 9012-05-11 is 1000 million operations without failure. SMD Reed Relays are available in small size
Mostly Reed Relays are used in data acquisition systems, Automatic Test Equipment and Measuring instruments. However some times are used for small power switching and protection circuits. Reed relays simplify and reduces design complexity of data acquisition system rather than using Analog multiplexers. Using analog multiplexers output load balancing and special signal conditioning is required even at low frequency signal. But hermetically metal sealed reed relays offer very small open contact capacitance and very low channel interference at RF and micro wave signals. Reed relays are available in miniature SMD packages and require a space almost equal to an IC.