High-Current Industrial Automation Applications Connect Reliably with Modular Spring Contacts

Spring interconnect devices provide a reliable, repeatable connection between an electronic device and a power source or another device. Modular spring contacts like those developed by Bourns have been used for many years to recharge portable radio handsets and other mobile electronic devices. The modular and repeatable nature of these interconnects makes them suitable for board-to-battery, board-to-board, and board-to-device connections.

The emergence of modular spring contacts capable of handling higher currents has expanded their utility into applications like industrial automation, precision instrumentation, and more. This article will explore the details of modular spring contacts, including how versions with greater current-carrying capacity expand the utility of these interconnects into new applications.

The case for spring contacts

A spring contact creates an electrical connection between one device and another by touching a conductive metal—often copper coated with gold—on one device with a similar conductive metal on the second device (Figure 1). The spring aspect of the contact provides mechanical force that maintains the contact, even with vibration or misalignment. The use of paired male and female contacts designed to fit together ensures a secure connection.

Figure 1: Male and female modular spring contacts from Bourns have a variety of sizes, contact pitches and counts, and electrical capabilities. (Image source: Bourns)

The connection between a portable radio handset and its charging base is a common example of a spring contact. Because the main purpose of this connection is to recharge the radio’s battery, it is usually referred to as a board-to-battery connection, referring to the connection between the charging base’s printed circuit board (PCB) with charging circuitry and the radio’s rechargeable battery pack. Charging docks for tablets, mobile phones, and gaming controllers could also fall into the board-to-battery category.

Board-to-device connections might also transmit data and electrical power via modular spring contacts. For instance, a laptop docking station may connect it with a secondary display or other peripheral. In commercial and industrial settings, applications such as mobile electronic funds transfer point-of-service (EFT-POS) terminals, cordless barcode scanners, devices with removable displays, and position sensors also require board-to-device connections. These devices have hardware interfaces and embedded systems that use lower-speed communication protocols such as general-purpose input/output (GPIO), universal asynchronous receiver/transmitter (UART), inter-integrated circuit (I²C), serial peripheral interface (SPI), and controller area network (CAN) signals.

Spring contacts can also provide board-to-board connections within a single product. In space- and weight-sensitive applications, spring contacts eliminate the need for cabling and mating connectors. They also tolerate vibration and misalignment. Board-to-board spring connections may connect a main control board to a module that manages radio frequency (RF) signals or sensor data, among other applications.

Modularity and mechanics

Spring contacts from Bourns are designed to be modular and reversible. Their modular nature allows them to be stacked end-to-end while maintaining consistent pitch across units. Because contact pitch is constant within a Bourns spring contact series, engineers can mix and match part numbers from within the series to achieve the desired number of contacts.

For instance, modular spring contacts in the 75AB series (Figure 2) have a pitch of 1.27 mm and come with two, three, or four contacts. A design utilizing six male surface-mount (SMT) contacts could use three 75ABJ-2-M0Es, each with two contacts; two 75ABJ-3-M0Es, each with three contacts; or one 75ABJ-2-M0E and one 75ABJ-4-M0E with four contacts.

Figure 2: The 75ABJ-4-M0E (left) and the 75ABJ-4-F0E (right) spring contacts have a contact pitch of 1.27 mm, which is preserved across units stacked end to end. (Image source: Bourns)

The spring contacts’ reversibility also provides design flexibility and simplifies manufacturing. Connection points on a spring contact unit still match up with their counterparts on a fixed PCB after a 180° rotation. Pick-and-place compatibility further streamlines assembly.

Reversibility allows engineers to set the spring contacts at the optimum angle to accept the mating device. Individual contacts, which are broad, slanted leaf springs, are designed to make an electrical connection vertically or horizontally. Vertical contacts are often called compression contacts, while horizontal contact is achieved by sliding one part into the other.

The springs on the male versions of Bourns’ modular contacts are rated for a minimum of 50,000 mating cycles. A tab at the end of each spring is tucked into the part’s inner housing wall. These captured spring ends eliminate inadvertent damage caused by unsecured contact ends catching on items in the environment.

Elementary electrical properties

The mechanical capabilities of modular spring contacts make them reliable and robust, but engineers most often select spring contacts for their electrical properties, such as the current and voltage they are able to handle, contact resistance, and safety. Manufacturing considerations—such as how spring contacts are connected to a PCB or the wires they terminate, and how electrical signals are routed—are also top of mind for design engineers.

Bourns modular spring contacts have a resistance at or below 30 mΩ. With little electrical resistance, the contacts form an efficient electrical connection that protects data integrity and generates little heat. Safety is further enhanced by the 500 V_AC dielectric voltage of the UL 94 V-0 listed, black, glass-reinforced thermoplastic housing. The housings for the modular spring contacts have an insulating resistance of at least 500 MΩ, and over 1,000 MΩ for some models.

With these safety measures in place, modular spring contacts can safely manage up to 60 V and 3 to 8 A of direct current per contact. While 3 A is standard for many consumer applications like hand-held radios, some industrial applications require higher currents.

High-current compatibility

Industrial automation applications may use microcontrollers or edge processing to take in and interpret sensor data and send out appropriate commands to high-powered equipment. As such, these applications need interconnects that can handle higher current. Likewise, precision instrumentation, test equipment, handheld industrial equipment, and docking systems for computers and tablets with advanced processors need more power per spring contact. Engineers working with such applications turn to Bourns 75AB, 76AA, and 78AD series of modular spring contacts.

For example, the 75AB series carries 5 A per contact up to 60 V_DC in a two-, three-, or four-contact package. The male components are 1.98 to 4.52 mm wide x 5.00 mm deep x 3.33 mm thick when uncompressed, while mating female contacts are 1.27 to 3.81 mm wide x 5.08 mm deep x 0.94 mm thick.

The 75AB series features in-line terminations, also known as twin hot contacts. This configuration gives each contact two symmetrical terminations on the connector housing; designers can connect circuitry on the PCB to either termination or to both. Twin hot contacts provide more flexibility in routing conductor paths to provide parallel connections, redundancy, or for track-crossing efficiency that avoids shorting or signal interference, an important consideration as current levels increase.

Twin hot contacts are also available in the 76AA series (Figure 3), which can handle 6 A per contact in an SMT package with two to six contacts on a 2.54 mm pitch.

Figure 3: 76AA series spring contacts (left: male, right: female) can carry 6 A per contact up to 60 V_DC. They have two to six twin hot contacts on a 2.54 mm pitch. (Image source: Bourns)

Male components in the 76AA series measure 5.00 mm to 15.16 mm wide x 8.48 mm deep x 3.81 mm thick when uncompressed. Their female counterparts are 4.98 mm to 15.14 mm wide x 10 mm deep x 1.27 mm thick.

For up to 8 A of current-carrying capacity per contact at 30 V_DC, designers turn to the 78AD series of modular spring contacts (Figure 4).

Figure 4: 78AD series spring contacts (left: male, right: female) are rated for 8 A per contact and 30 V_DC. They have two to six contacts on a 4 mm pitch. (Image source: Bourns)

Like the 76AA series, 78AD spring contacts have between two and six contacts spaced on a 4 mm pitch. Male units are 7.82 to 23.83 mm wide x 11.00 mm deep x 5.55 mm thick when uncompressed. Female units are 9.0 to 25.0 mm wide x 11.00 mm deep x 2.3 mm thick. 78AD series interconnects use a tin-alloy termination per MIL-STD-202, Method 208, rather than having twin hot contacts.

Conclusion

Modular spring contacts are interconnects that provide reliable board-to-board and board-to-device connections, even under challenging conditions such as frequent mate-unmate cycles, vibration, and misalignment. The higher current capability, low-profile design, and routing options available in modular spring contacts such as those in Bourns’ 75AB, 76AA, and 78AD series make these interconnects a reliable option for designers of industrial automation applications, precision instrumentation, portable industrial equipment, and more.