The design of a generator system requires many hours of detailed planning with the goal of creating an extremely reliable backup power source. Properly installed, the system will deliver the intended level of reliability. However, if incorrectly wired, the system can become a problem for both the owner and the manufacturer.
While the generator installation can be handled by a range of people, from a trained technician to the typical homeowner, wiring mistakes can occur. Installation includes working with 120 VAC split phase, 240 V line voltage, along with low-voltage signals below 50 V. A small and easily made mistake, such as miswiring high voltage to low voltage, will destroy sensitive electronics quickly and may render the equipment inoperable.
Thus, a resettable overcurrent and overvoltage solution capable of handling line voltage, electrostatic discharge (ESD), electrical fast transients (EFT), and current surge is required to protect low voltage interface circuits against this problem.
Typically, a circuit consisting of a relay, resettable fuse, and metal oxide varistor (MOV) would protect against such threats. To improve on the circuit's time response, current-handling capability, and satisfy demanding space constraints, an improved circuit design is presented.
Additionally, a solution is provided to effectively ease the difficulty of coordinating primary- and secondary-circuit protection. The proposed solution incorporates a fast-acting resettable fuse to alleviate the challenges mentioned above, thus providing scalable, universal circuit protection for use at all exposed low-voltage generator terminals.
Generator Installation Environment
An overview of the generator installation and its interfaces is provided in Figure 1. Terminals on the interface strip can be miswired during installation: 120 V split phase, engine interlock, and general I/O. A controller internal to the generator monitors AC line voltage.
Typically, a 10-second interruption of AC mains power will turn on the low-voltage relay-driver circuit, energizing the transfer-switch relay. The transfer-switch relay is connected to the generator with a 500 mA cable, ten to fifty feet long, which is exposed to inductive transients from close proximity to the mains-circuit panel wiring.
The cable length is indeterminate during the installation, and protection for this transient environment must be anticipated. To protect the mains supply, a Transient Voltage Surge Suppressor (TVSS) is required at the breaker panel to ensure adequate line surge suppression and a primary level of networked protection for the generator system.
Figure 1: Generator Interface Block Diagram
(Click on image to enlarge)
The combination of high and low voltages coexisting at the generator interface creates an obvious concern for the equipment manufacturer. While every effort is made to clearly mark and segregate these connections, the reality of all-too-common miswiring prompts a phone call to the manufacturer for a replacement PCB assembly.
Thus, the designer's goal is to make the generator interface as "bullet proof" as possible, minimizing the implications of layout and ambient environment on the circuit's response to potential threats. Lastly, it should be resettable and consume as little PCB space as possible so it can be applied at all exposed low-voltage circuit nodes in the system.
To realize a minimal footprint, every component must be properly sized along with an impedance for interstage coordination. Coordination of primary and secondary circuit protection elements is accomplished when the primary protector is triggered, protecting the secondary element before it is destroyed. The design of interstage coordination is complicated, demanding a longer test-evaluation period based typically on a large set of test vectors to represent various voltage, current, and frequency variables resembling the expected threat environment.
Since the interstage coordination is dependent on the variation of the above vectors (voltage, current, frequency, temperature), a solution which can provide a repeatable circuit response, regardless of these parameters, would be desirable and provide easier qualification and characterization with the proposed threat environment.
A universal protection scheme is desired to protect the low-voltage generator interface circuits. To meet this requirement, the first-pass solution is to insert a resettable fuse as overcurrent protection.
While the resettable fuse package size is adequate, the interstage coordination required for various combinations of threat levels demands more characterization. The proposed fast-acting semiconductor-based resettable fuse, such as the Bourns® Transient Blocking Unit (TBU™) device, can work as an optimal solution for universal application across multiple threats expected in the low voltage generator interface. Because this device can provide a repeatable, reduced system response time by three orders of magnitude (from 1 ms down to1 μs), scalability to meet various current requirements, and a package size to meet demanding space constraints, a universal circuit-protection solution is needed.
The final design incorporates a fast-acting resettable fuse, MOV, and zener diode. Power cross and overcurrent protection are provided by the Bourns® TBU™ device. Two TBU™ devices, rated to 260 mA each, are used in parallel (demonstrates scalability) to provide the 500 mA cable drive current. The zener diode provides low voltage clamping to protect sensitive circuits and creates an avalanche current to trigger the TBU™ device. The MOV protects the TBU™ device from excessive common mode voltages when exposed to ESD and EFT.
Line-to-line peak voltage, which should be allowed to pass through unclamped, determines the MOV voltage level:
120 Vrms × 2 × 1.414 = 340 VAC.
Thus a 360 V MOV is chosen. Any voltage exceeding the peak-to-peak line voltage will be clamped to stay within the TBU™ device's maximum common mode voltage of 650 V. The complete solution is presented in Figure 2.
Figure 1: Universal Circuit-Protection Solution
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A significant advantage of the proposed design is that no complicated coordination between primary and secondary protection elements is required to protect the low-voltage circuitry. The timing and layout considerations associated with coordinating circuit protection become straightforwad with the fast-acting resettable fuse. The fast-acting resettable fuse provides a reliable, scalable electronic current limiting solution in a compact package. The bidirectional 'C' series TBU™ device chosen for this application has a 4 mm by 8.25 mm package, which even the tightest printed circuit board layout should be able to accommodate.
This solution is capable of protecting low-voltage generator interface circuits from line voltage (up to 650 V impulse), electrostatic discharge (ESD), electrical fast transients (EFT), and current surge as detailed in Table 1.
Table 1: Protection Provided for Various Threats
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The design truly provides a simple drop-in solution for circuit protection of generator interface circuits and could be deployed throughout a complete line of generators.
About the author
Michael Smith is a field applications engineer with Bourns, Inc. He is a technical expert on the subject of circuit protection for consumer, industrial, and hi-reliability electronics and has over 28 years experience in the design, test, and manufacturing of electronic products. Smith has a Bachelor of Science in Electrical Engineering from New York Institute of Technology and an MBA from American InterContinental University.