Product reliability, as commonly referred to, usually means operational reliability, defined as the product's ability to perform its intended function under specified conditions and within a specified time. It consists of inherent reliability and operational reliability. The inherent reliability is determined by the product's design and manufacturing process, while operational reliability is related to proper user operation and the manufacturer's pre-sales and after-sales service. Users should pay attention to the following points when using the product.
I. Coil Operating Voltage
The coil operating voltage should ideally be selected according to the rated voltage during design. If this is not possible, the temperature rise curve can be used as a reference. Using any coil voltage lower than the rated operating voltage will affect the relay's operation. Note that the coil operating voltage refers to the voltage applied between the coil leads, especially when using an amplifier circuit to excite the coil; ensure the voltage value between the two coil leads is correct. Conversely, exceeding the maximum rated operating voltage will also affect product performance. Excessive operating voltage will cause the coil temperature to rise too high, especially at high temperatures. Excessive temperature rise can damage the insulation material and affect the relay's operational safety. For magnetic latching relays, the excitation (or reset) pulse width should not be less than three times the pull-in (or reset) time; otherwise, the product will remain in a neutral state. When using solid-state devices to excite the coil, the device's withstand voltage should be at least 80V, and the leakage current should be sufficiently low to ensure the relay's release.
II. Transient Suppression
When a relay coil is de-energized, a reverse peak voltage exceeding 30 times its rated operating voltage can be generated on the coil, posing a significant hazard to electronic circuitry. This is typically suppressed by connecting a transient voltage suppressor (also called a clipping diode) or resistor in parallel, ensuring the reverse peak voltage does not exceed 50V. However, connecting a diode in parallel will extend the relay's release time by 3 to 5 times. When a high release time is required, a suitable resistor can be connected in series with one end of the diode.
Excitation power supply: At 110% of rated current, the power supply regulation rate should be ≤10% (or the output impedance should be <5% of the coil impedance), and the ripple voltage of the DC power supply should be <5%. The AC waveform should be a sine wave with a waveform coefficient between 0.95 and 1.25, waveform distortion within ±10%, and frequency variation within ±1Hz or ±1% of the specified frequency (whichever is greater). Its output power should not be less than the coil power consumption.
III. Parallel and Series Power Supply of Multiple Relays
When multiple relays are connected in parallel for power supply, the relay with a high reverse peak voltage (i.e., a large inductance) will discharge to the relay with a low reverse peak voltage, and its release time will be prolonged. Therefore, it is best to control each relay separately before connecting them in parallel to eliminate mutual interference.
Relays with different coil resistances and power consumption should not be connected in series for power supply; otherwise, the relay with a large coil current in the series circuit will not work reliably. Only relays of the same specification and model can be connected in series for power supply, but the reverse peak voltage will increase and should be suppressed. A series resistor can be used according to the voltage divider ratio to withstand the portion of the supply voltage that exceeds the rated coil voltage of the relay.
IV. Contact Load
The load applied to the contacts should conform to the contacts' rated load and characteristics. Applying a load that is not in accordance with the rated load size (or range) and characteristics often leads to problems. Products suitable only for DC loads should not be used in AC applications. A relay capable of reliably switching a 10A load may not work reliably under low-level loads (less than 10mA × 6A) or dry circuits. A relay capable of switching a single-phase AC power supply may not be suitable for switching two asynchronous single-phase AC loads; products specified for switching AC 50Hz (or 60Hz) should not be used to switch 400Hz AC loads.
V. Parallel and Series Connections of Contacts
Parallel connection of contacts cannot increase the load current because the absolute non-synchronous operation of multiple sets of contacts in a relay—meaning it's still a single set of contacts switching the increased load—can easily lead to contact damage, failure to make contact, or welding, preventing disconnection. Parallel connection can reduce the failure rate for "opening" failures, but has the opposite effect on "sticking" failures. Since "opening" failure is the primary failure mode for contacts, parallel connection is commendable for improving reliability and can be used in critical parts of equipment. However, the operating voltage should not exceed the coil's maximum operating voltage, nor should it be lower than 90% of the rated voltage; otherwise, it will jeopardize the coil's lifespan and reliability. Series connection of contacts can increase the load voltage; the increase is proportional to the number of sets of contacts in series. Series connection can improve reliability for "sticking" failures, but has the opposite effect on "opening" failures. In short, when using redundancy techniques to improve contact reliability, it is crucial to pay attention to the nature, size, and failure mode of the load.
VI. Switching Rate
The relay switching rate should not exceed the reciprocal of the sum of its operating time and release time (times/s); otherwise, the relay contacts will not be able to connect stably. Magnetic latching should be used within the pulse width specified in the relay technical standard; otherwise, the coil may be damaged.
