The following blog post describes and recommends the key features an engineer should consider when choosing a Reed Relay.
Signal Voltage, Current and Power Specification
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.
SMD or Thru Hole Mounting
For applications where relays may have to be serviced Pickering Electronics recommend thru hole components are used. Outside of this the choice is driven by user manufacturing preferences and component choices such as footprint area, relay ratings and relay height.
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.
If relays are to be closely packed together choose a relay with an integrated magnetic shield.
Electrostatically Screened Relays
A reed switch is enclosed by a coil but is otherwise open to pick up from adjacent circuits. For a screened relay a foil layer is added between the coil and the glass body of the reed switch and a contact to the foil is brought out to the outside of the package. If this screen is earthed it can reduce the amount of signal picked up on the signal lines from the coil itself or from other external signals.
An electrostatic screen does not provide protection against an external magnetic field inducing signals into the signal path.
RF relays are designed to operate to defined transmission line impedance, usually 50 or 75Ω. The glass body is wrapped in a conductive tube and the two ends provided as an earth contact. Provided the glass and blade dimensions are correct they form a coaxial transmission line.
RF relay applications have to be designed with care since the prime issue is about overall system performance and not just relay performance. If an application requires the disconnection of the signal path and no alternative routing (for example for a protection system) then reed relays can be a very good choice because of their fast response.
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