Choosing a reed relay
All reed relays have specified voltage and current ratings that need to be kept within if the reed relay is to have a long service life. It important to be clear if the application envisages hot switch or cold switching, it can have a substantial impact on the cost and size of relay used. If hot switching is likely to occur note the power rating of the reed relay, the fact a particular relay may be capable of 100V and 1A does not mean it can hot switch a signal with these extremes. At 100V a 10W reed relay will only switch 100mA reliably.
If hot switching is not expected then the user can rely on the carry current rating and the rated withstand voltage across the contacts.
Diode or No Diode
Reed relays often have a choice to include an internal protection diode.
The purpose of this diode is primarily to protect the device that is driving the relay coil from the Back EMF that is generated when the current flow is interrupted.
Reed relays are supplied with a variety of coil voltage options. For logic driving 3V and 5V drives are preferred since these voltages are directly compatible with common logic families.
However, all the coils for a given reed switch have to have a certain number of Ampere Turns, so as coil voltage is dropped the coil current required is increased.
LED drivers can directly support either 5V or 12V coils, open collector drivers can support even higher voltages. However, as coil voltage increases the wire used to create the relay coil becomes finer and harder to wind without breakages. Ultimately this limits the highest voltage coils that can be offered.
One factor often ignored by users is the impact of temperature on coil current. The copper winding wire has a resistance coefficient of 0.39 percent per centigrade. As the temperature increases, so will the coil resistance with a corresponding fall in current and therefore the level of magnetic field generated by the coil.
Consequently reed relays should have a reasonable operating margin to ensure reliable operation in all conditions.
- Reduce the effect of magnetic interaction between closely packed reed relays.
- Reduce injection of noise into the signal path by external magnetic fields.
- Increase the magnetic efficiency resulting in a reduction of the coil power needed.