6 things you must pay attention to when using relays

- Jul 16, 2020-

6 things you must pay attention to when using relays

 

A relay is an electrical control device. It is an electrical appliance that causes a predetermined step change in the controlled quantity in the electrical output circuit when the change in the input quantity (excitation quantity) reaches the specified requirement.

 

It has an interactive relationship between the control system (aka input loop) and the controlled system (aka output loop). Usually used in automated control circuits, the relay is actually an "automatic switch" that uses a small current to control the operation of a large current. Therefore, it plays the role of automatic adjustment, safety protection and conversion circuit in the circuit.

 

The relay is generally composed of a combination of a relay and a relay base. The relay base can be quickly installed on the rail, and the relay coil and the electric shock contact can be led out to the quick connection column of the base, so that when used and connected Both are very convenient. If the relay is damaged, you can directly pull the relay from the base and replace it directly, saving maintenance time.

 

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When using the relay, the following points should be noted:

1. Coil voltage

It is best to design the voltage of the coil according to the rated voltage. If not, please refer to the temperature rise curve. The use of any coil voltage less than the rated operating voltage will affect the operation of the relay. Note that the working voltage of the coil refers to the voltage applied between the terminals of the coil. Especially when the amplifier circuit is used to excite the coil, the voltage between the two terminals of the coil must be ensured. Conversely, when the maximum rated operating voltage is exceeded, the product performance will be affected. Excessive operating voltage will cause the coil temperature to rise too high, especially at high temperatures, which will damage the insulating material and affect the operation of the relay. Safety.

 

For magnetic holding relays, the pulse width of the excitation (or reset) should not be less than 3 times of the pull-in (or reset) time, otherwise the product will be in the neutral state. When a solid-state device is used to excite the coil, its device withstand voltage is at least 80V and the leakage current should be small enough to ensure the release of the relay.

 

2. Transient suppression

At the moment when the relay coil is powered off, an inverse peak voltage above the coil's rated working voltage value can be generated more than 30 times, which is extremely harmful to the electronic circuit. Usually a parallel transient suppression (also called peak clipping) diode or resistor is used The method is suppressed so that the reverse peak voltage does not exceed 50V, but the parallel diode will extend the release time of the relay by 3 to 5 times. When the release time is high, a suitable resistor can be connected in series with the diode.

 

Excitation power supply: At 110% rated current, the power supply adjustment rate is ≤10% (or the output impedance "5% coil resistance", the ripple voltage of the DC power supply should be "5%. The AC waveform is a sine wave, the form factor should be between 0.95 and 1.25, the waveform distortion should be within ±10%, and the frequency change should be within ±1Hz or ±1% of the specified frequency (whichever is greater). Its output power is not less than the power consumption of the coil.

 

3. Parallel and series power supply of multiple relays

When multiple relays are powered in parallel, a relay with a high inverse peak voltage (that is, a large inductance) will discharge to a relay with a low inverse voltage, and its release time will be extended. Therefore, it is best to control each relay separately and then parallel to eliminate mutual influence. Relays with different coil resistance and power consumption should not be used in series power supply, otherwise relays with large coil current in the series circuit cannot work reliably. Only relays of the same specification and model can be powered in series, but the inverse peak voltage will increase and should be suppressed. The part of the voltage that is higher than the rated voltage of the coil of the relay can be withstood by the series resistance according to the voltage division ratio.

 

4. Contact load

The load applied to the contact should conform to the rated load and nature of the contact. It is often prone to problems if the load is not applied according to the rated load size (or range) and nature. Products that are only suitable for DC loads should not be used in AC applications. Relays that can reliably switch 10A loads may not work reliably under low-level loads (less than 10mA&TImes; 6A) or dry circuits. Relays that can switch single-phase AC power are not necessarily suitable for switching two unsynchronized single-phase AC loads; only products that switch AC 50Hz (or 60Hz) should not be used to switch 400Hz AC loads.

 

5. Contacts in parallel and series

The parallel use of the contacts cannot increase the load current, because the action of the multiple sets of contacts of the relay is absolutely different, that is, the load of the set of contacts after switching is increased, it is easy to damage the contacts without contact or welding. Cannot be disconnected. Contacts connected in parallel to "break" errors can reduce the failure rate, but the opposite is true for "sticky" errors. Since the fault of contact is "break" fault as the main failure mode, the parallel connection should be affirmed to improve the reliability and can be used in key parts of the equipment. But the use voltage should not be higher than the maximum working voltage of the coil, nor lower than 90% of the rated voltage, otherwise it will endanger the coil life and reliability of use.

 

The series connection of contacts can increase its load voltage, and the increase multiple is the number of series contacts. Contacts connected in series can improve their reliability for "sticky" errors, but the opposite is true for "broken" errors. In short, when using redundancy technology to improve the working reliability of contacts, we must pay attention to the nature, size and failure mode of the load.

 

5. Switching rate

The switching rate of the relay should not be higher than the reciprocal (times/s) of the sum of its 10-fold action time and release time, otherwise the relay contact cannot be stably connected. Magnetic retention should be used within the pulse width specified in the relay technical standards, otherwise the coil may be damaged.