Regarding the selection method of relays, the principles of relay first use, and common sense of relay selection, including key elements such as modeling according to the use environment, determining the type of relay according to different input signals, and selecting the type and capacity of relay contacts according to load conditions.
1. According to the use environment
The operating environment conditions mainly refer to temperature (maximum and minimum), humidity (generally refers to the maximum relative humidity at 40 degrees Celsius), low air pressure (can not be considered below the use altitude of 1000 meters), vibration and shock. In addition, there are still requirements for packaging methods, installation methods, external dimensions and insulation. Due to different materials and structures, the relays are subjected to different environmental mechanical conditions. If the relays are used under the environmental mechanical conditions specified in the product standard, the relay may be damaged. It can be selected according to the environmental mechanical conditions of the whole machine or the higher-level conditions.
It is best not to use AC-excited relays around devices that are sensitive to electromagnetic interference or radio frequency interference. When selecting a DC relay, a product with a coil transient suppression circuit should be selected. Those that use solid-state devices or circuits to provide excitation and are sensitive to spikes should also choose products with transient suppression circuits.
2. Determine the type of relay according to the input signal
According to the input signal is electricity, temperature, time, optical signal to determine the selection of electromagnetic, temperature, time, photoelectric relay, this is no problem.
In particular, the selection of voltage and current relays is explained here. If the relay coil supplied by the whole machine is a constant current, the current relay should be selected, and the voltage relay should be selected if the voltage value is constant.
3. Selection of input parameters
The input quantity closely related to the user is the coil working voltage (or current), and the pull-in voltage (or current) is the parameter for the relay manufacturer to control the sensitivity of the relay and to judge and evaluate it.
For the user, it is just a working lower limit parameter value. The control safety factor is the working voltage (current) / pull-in voltage (current). If the relay is used at the pull-in value, it is unreliable and unsafe. The increase in ambient temperature or under vibration and impact conditions will make the relay Unreliable work.
When designing the whole machine, the no-load voltage cannot be used as the basis for the working voltage of the relay, but the actual voltage should be calculated by connecting the coil as the load, especially when the internal resistance of the power supply is large.
When a triode is used as a switching element to control the on-off of the coil, the triode must be in the switching state. For the relay with a working voltage below 6VDC, the saturation voltage drop of the triode should also be deducted.
Of course, it is not that the higher the working value, the better. If the working value exceeds the rated working value, it will increase the impact wear of the armature, increase the number of contact bounces, and shorten the electrical life. Generally, the working value is 1.5 times the pull-in value, and the working value The error is generally ±10%.
4. Select the type and capacity of the relay contacts according to the load conditions
Long-term practice at home and abroad has proved that about 70% of the faults occur on the contacts, which shows that the correct selection and use of relay contacts is very important.
The contact combination form and the number of contact groups should be determined according to the actual situation of the controlled circuit. Commonly used contact combinations are shown in Table 6. Due to the small number of contact bounces and the large amount of compensation after contact ablation, the load capacity and contact reliability of the moving contact group and the moving contact pair in the changeover contact group are higher than those of the moving contact. The pair of moving and breaking contacts in the group and the changeover contact group should be high, and the circuit of the whole machine can be adjusted properly by adjusting the position of the contacts, and the moving contacts should be used as much as possible.
It is very important to determine the parameters according to the load capacity and the nature of the load (resistive, inductive, capacitive, lamp load and motor load). It is not correct to think that the small switching load of the contact must be more reliable than the large switching load. Generally speaking, the relay switching load is under the rated voltage, and the current is greater than 100mA and less than 75% of the reted current . If the current is less than 100mA, the contact carbon deposition will increase and the reliability will decrease. Therefore, 100mA is called the test current, which is the assessment content of the process conditions and level of the relay factory by professional standards at home and abroad. Since general relays do not have low-level switching capability, they are used to switch orders for relays with loads below 50mV and 50μA. The user should specify, and the relay manufacturer should assist in the selection if necessary.
The contact rated load and life of the relay refers to the number of actions that the load is resistive under the reated voltage and current . When the reated voltage is exceeded, it can be selected by referring to the contact load curve.When the nature of the load changes, the load capacity of its contacts will change. Users can refer to Table 8 to change the load current of the contacts.
Table 8
Resistive Current Resistive Current Motor Current Lamp Current Minimum Current
100% 30% 20% 15% 100MA
1. Coil voltage
The voltage used by the coil should be selected according to the rated voltage in the design. If not, it can be selected with reference to the temperature rise curve. Using 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 coil terminals, especially when the amplifier circuit is used to excite the coil, the voltage value between the two terminals of the coil must be guaranteed. Conversely, when the maximum rated working voltage is exceeded, the performance of the product will also be affected. Excessive working voltage will cause the coil temperature to rise too high, especially at high temperature, the high temperature rise will damage the insulating material and affect the operation of the relay. Safety.
For magnetic latching relays, the excitation (or reset) pulse width should not be less than 3 times 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, the withstand voltage of the device should be at least 80V, and the leakage current should be small enough to ensure the release of the relay.
2. Transient suppression
When the relay coil is powered off, an inverse peak voltage that is more than 30 times higher than the rated working voltage of the coil can be generated on the coil, which is very 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 inverse peak voltage does not exceed 50V, but the parallel diode will prolong the release time of the relay by 3 to 5 times. When the release time is required to be high, a suitable resistor can be connected in series with one end of the diode.
Excitation power supply: At 110% rated current, the power supply regulation rate is less than or equal to 10% (or coil impedance with output impedance <5%), and 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 coil power consumption.
3. Parallel and series power supply of multiple relays
When multiple relays are powered in parallel, the relay with high inverse peak voltage (that is, the inductance is large) will discharge to the relay with low inverse peak voltage, and its release time will be prolonged, so it is best to control each relay separately and then connect in parallel to eliminate mutual influence.
Relays with different coil resistances and power consumption should not be used in series for power supply, otherwise the 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 supply voltage that is higher than the rated voltage of the relay's coil can be tolerated by the voltage divider ratio in series with the resistance.
4. Contact load
The load added to the contact should conform to the rated load and nature of the contact, and it is often prone to problems if the load is not applied according to the size (or range) and nature of the rated load. 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×6A) or dry circuits. A relay that can switch single-phase AC power is not necessarily suitable for switching two asynchronous single-phase AC loads; products that are only specified to switch AC 50Hz (or 60Hz) should not be used to switch 400Hz AC loads.
5. Contacts in parallel and in series
The use of contacts in parallel cannot increase the load current, because the action of multiple sets of contacts of the relay is absolutely asynchronous, that is, it is still the load of a set of contacts after switching and increasing, and it is easy to damage the contacts without contacting or welding. cannot be disconnected.
Contacts in parallel can reduce the failure rate for "off" errors, but the opposite is true for "stick" errors. Since the main failure mode of contact failure is "broken" failure, parallel connection should be affirmed to improve reliability and can be used in key parts of equipment.
However, the operating 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 life and reliability of the coil. Contacts connected in series can increase the load voltage, and the increase is the number of sets of contacts in series. Contacts in series can improve reliability for "stick" faults, but the opposite is true for "broken" faults. In short, when using redundancy technology to improve the reliability of contact operation, it is necessary to pay attention to the nature, size and failure mode of the load.
6. Switching rate
The relay switching rate should not be higher than the reciprocal (times/s) of the sum of its 10 times the action time and the release time, otherwise the relay contacts cannot be stably connected. The magnetic latch should be used under the pulse width specified in the technical standard of the relay, otherwise the coil may be damaged.