Reed Relays Offer Reliability for EVs, High-Voltage Measurement

Standex Electronics is releasing two high-voltage reed relay products, the BH series for high-power industrial measurement applications and the compact MHV series in surface mount (SMD) packaging for space-constrained, high-voltage control circuit applications. Both models offer low leakage currents and capacitances for accurate signal measurement and are designed for operating system voltages up to 1 kVDC with a minimum breakdown voltage of 3 kVDC. 

 

Reed relays are commonly used in EV systems. Image used courtesy of Standex Electronics

 

The BH series is designed for higher power use cases and can accommodate up to 100 W. Along with its power rating, the BH series also boasts a high insulation resistance of 10 TΩ across its open contacts and between the actuating coil and contacts. Target applications include semiconductor test equipment, test and measurement equipment, and general-purpose high-voltage industrial applications.  

Rated for high-voltage switch applications up to 10 W, the MHV series is available in a compact surface mount package (17.2 mm x 10.5 mm x 8.5 mm) and is designed for use in electric vehicles, solar power, and medical equipment measurement and control applications.

 

Constructing a Reed Relay

Reed relays are electromagnetic switching devices that use an electrically induced magnetic field from a small coil wrapped around the device to actuate (open and close) the internal reed switches.   

 

Reed relays used in EV BMS applications. Image used courtesy of Standex Electronics

 

The most common form of reed relay is the Form A. The default condition for a Form A switch is the open (off) state when no current is applied to the actuating coil. When current is passed through the electromagnetic coil, the induced magnetic field causes the reed switches to close, allowing for an electrical connection across the relay.  

 

Construction and operation of a reed relay. Image used courtesy of Standex Electronics

 

One unique characteristic of reed relays, compared with other switching solutions, is that their electromagnetic actuation mechanism allows them to be hermetically sealed. This means that,  unlike electromechanical relays, they are not exposed to dirt, dust, or other particles that can adversely impact their reliability over time.     
 

Hybrid and Electric Vehicle Applications

For hybrid vehicles, all-electric vehicles, and other e-mobility applications, reed relays can typically be found in the measurement and control circuits of a vehicle’s battery management system (BMS) or onboard charger (OBC).

In the battery measurement applications used to monitor the status and health of an EV battery pack, reed relays are typically placed between the high-voltage battery bus and ground. The switching relay allows for brief and periodic direct measurement of the battery’s voltage while offering a high level of system isolation when measurements are not being taken.  

The low contact resistance of reed relays makes them ideal for these types of measurement applications since they have little impact on the measured signals. The MHV series has a contact resistance of only 150 mΩ with much lower leakage current and capacitance when compared with solid-state or semiconductor relays.  

 

Multi-Switch Applications

Reed relays can be constructed in a single package with multiple switches and switch forms.

 

BH series Form 4A multi-switch operation. Image used courtesy of Standex Electronics

 

The BH series is available with up to four Form A switches in a single package for multi-channel applications in semiconductor tests and other high-voltage measurement equipment.