The main purpose of all power systems is to maintain a high level of continuous power supply, and to minimize the scope of influence and power outage time when an unbearable state occurs. Power loss, voltage drop, overcurrent and overvoltage will always occur because we cannot avoid natural events, physical accidents, equipment failures, or human misoperations. Some devices are used in combination to protect electrical equipment from damage from these events, which is also known as “connector.” Solenoids and relays are an indispensable part of all connected devices. They are energized and contacted through the coil, connecting/disconnecting the power supply of the protected equipment. This article introduces you to some characteristics of solenoid coils commonly found in relays, current contactors and valves. In addition, the article also introduces some methods to drive them, and explains the development trend of effective drive. This article also lists some examples of application circuits for electrical connection devices.
Overcurrent protection devices (such as circuit breakers, etc.) are used to protect conductors from overcurrent damage. The purpose of designing these protection devices is to keep the current in the circuit at a safe level to prevent the circuit conductor from overheating. The current contactor is mainly used to connect or disconnect the conductor contact current. They are used for frequent or long-term constant conduction-disconnection.
In order to protect the circuit from the damage of the strong current, the protective device must know when the fault condition appears, and can automatically disconnect the electrical equipment from the power supply. Overcurrent protection devices must be able to distinguish the difference between overcurrent and short circuit and react in the correct way. A small overcurrent can be allowed for a certain period of time, but as the amount of current increases, the protection device must be able to respond more quickly, for example, to prevent a short circuit immediately.
Solenoid coil characteristics
The electromechanical solenoid consists of an electromagnetic induction coil winding around a movable steel or iron core (called the “armature”). The shape of the coil allows the armature to move in or out of its center, thereby changing the inductance of the coil and ultimately forming an electromagnetic (see Figure 1). The armature is used to provide mechanical force to some mechanical devices.
picture1Working principle of solenoid
The main electrical characteristic of a solenoid is that it is an inductor with inductance, which is a characteristic that resists changes in current. This is why the current does not reach the maximum level immediately when the solenoid is energized. Instead, the current increases at a steady rate until it is limited by the solenoid’s DC resistance. Inductors (such as solenoids) store energy in a concentrated magnetic field. As long as there is current in the line or conductor, a magnetic field (albeit small) will be formed around the line. After winding the circuit into a coil (for example: the coil in a solenoid), the magnetic field becomes very concentrated. Through electrical signals, electromagnetics can be used to control mechanical valves. As soon as the solenoid is energized, the current increases, which causes the magnetic field to expand until it is strong enough to move the armature. The movement of the armature increases the concentration of the magnetic field, because the armature’s own magnetic mass moves farther into the magnetic field. Remember that the direction of the magnetic field changes is the same as the direction of the current that makes it form, causing a reverse voltage in the winding. Since the magnetic field expands rapidly when the armature moves, it will cause the current through the solenoid winding to drop briefly. After the armature moves, the current continues to rise along its normal path to the maximum level. The result is shown in the current waveform in Figure 2. Pay attention to the obvious dip point in the rising process of the current waveform.
Solenoid coil drive: voltage or current drive?
As mentioned earlier, the armature of the solenoid is used to provide mechanical force for the mechanical device. The force applied to the armature is proportional to the change in the inductance of the coil when the position of the armature changes. In addition, it is proportional to the current flowing through the coil (according to Faraday’s law of inductance). Equation 1 calculates the force exerted by the solenoid electromagnetic on a certain passing charge:
Force = Q × V × (magnetic constant × N × I), (1)
Among them, Q is the charge passing through a point charge; V is the speed of the point charge; the magnetic constant is 4π×10–7; N is the number of turns of the solenoid coil; I is the current through the solenoid. This shows that the electromagnetic force of the solenoid is directly related to the current.
Traditionally, voltage drive is used to drive solenoid coils; therefore, power is continuously consumed in the coils. An adverse effect of this power consumption is the heating of the coil, which then spreads to the entire relay. The coil temperature is determined by the ambient temperature, the self-heating caused by the power consumption of the V×I coil, the heating caused by the contact system, the magnetization loss caused by the eddy current, and other heat sources (such as some components near the relay). As the coil heats up, the coil resistance increases. The calculation method of high temperature resistance is shown in Equation 2:
Where RCoil_20°CIs the resistance value at 20°C, and kR_TThen is the thermal coefficient of copper, which is equal to 0.0034 per degree Celsius. According to RCoil_20°C(Generally can be found in the solenoid coil product manual), the extreme coil resistance at high temperature can be calculated. During circuit design, it is necessary to pay attention to relevant calculations under extreme conditions, such as the highest possible coil temperature of the working pick-up voltage.
picture2 Solenoid current
Another point to note is that as far as a particular coil is concerned, the pickup current remains the same under any conditions. The pickup current depends on the pickup voltage and the coil resistance (IPick-up= VPick-up/RCoil). Most relays are made of copper wire. According to Equation 2, as the temperature of the coil rises, the resistance of the coil increases. Therefore, the pickup voltage of the hot coil should be higher to generate the required pickup current. For example, if the pickup voltage of a 12VDC relay is 9.6VDC, and the coil resistance is 400 Ω at 20°C, then IPick-up= 24 mA. When the coil temperature rises to 40°C, the coil resistance increases to 432 Ω. Therefore, the pickup voltage is 10.36 VDC. (The pickup current remains the same.) In other words, the temperature increases by 20°C and the pickup voltage rises by 0.76 VDC. When the relay uses a higher duty cycle, the pickup voltage of each successive cycle may increase slightly due to the increase in the temperature of the coil. Figure 3 shows that if voltage drive is used, the user may have to design the coil with excess margin.
picture3 Super-margin design of solenoid voltage drive
In short, because the current changes with changes in coil resistance, temperature, power supply voltage, etc., voltage drive forces us to design with excess margins. Therefore, for many solenoid devices, current drive is the best way.
Closing a relay or valve requires a lot of energy. The instantaneous current that activates the solenoid actuator (called “peak current”, Ipeak) Will be very high. However, once the relay or valve is closed, the current required to maintain it in this state (called “hold current”, IHold) Is much smaller than the peak current. Generally speaking, the holding current is less than the peak current: IHold<< IPeak.
When using voltage drive, the solenoid coil current continues and is higher than when using current drive (Figure 4). Unlike voltage drive, current drive does not need to leave room for parameter changes caused by temperature or solenoid differences. This design requires the use of a single peak current value (which may be several amperes in size) and the use of solid-state holding current (which may be only 1/20 of the peak current value).
picture4 Voltage-driven and current-driven solenoid current
Implementation of current control for solenoid coil drive
Traditionally, we directly drive the solenoid coil through the general-purpose input/output (GPIO) of the microcontroller (MCU) (Figure 5a). Through a switch controlled by the GPIO of the MCU, the coil is activated. A new drive system was developed that uses pulse width modulation (PWM) of the waveform (Figure 5b). The coil is activated via a switch controlled by the MCU’s PWM, and then the duty cycle determines the average current through the coil. We used Texas Instruments DRV110, which is an energy-saving solenoid controller with integrated power regulation (Figure 5c). This DRV110-based system is designed to adjust the current through a well-controlled waveform to reduce power consumption. After the initial rise, the solenoid current is maintained at the peak value to ensure normal operation, and then drops to a lower holding level in order to avoid heating problems and reduce power consumption. The graph in Figure 6 compares the working conditions of a traditional drive and DRV110. Note that some other methods can also reduce the voltage, but they require a certain amount of overhead to ensure that the current remains constant at various temperatures.
picture5 Coil driving method
picture6 Traditional drives andDRV110Comparison of working principle
Figure 7 shows a typical application circuit based on DRV110. DRV110 controls the current through the solenoid (LS), as shown in Figure 7. When the EN pin voltage is pulled high (internal or external driver), activation starts. At the beginning of activation, DRV110 allows the load current to rise to the peak value (IPeak), and then reduced to IHoldIt was previously tKeepTime adjustment. As long as the EN pin maintains a high level, the load current is adjusted to the maintained value. The initial current rise time depends on the inductance and resistance of the solenoid. Once the EN pin is driven to GND, DRV110 allows the solenoid current to drop to zero.
picture7 DRV110And the typical application circuit of solenoid current waveform
The activation (peak) current of the DRV110 is determined by the “on” resistance of the coil and the pickup voltage required by the relay. The highest temperature resistance value (RCoil_T(max)) And the rated working voltage of the relay (Vnom) Can be used to calculate the required I at the highest temperaturePeakvalue:
The holding current of the DRV110 is determined by the “on” resistance of the coil and the voltage required to avoid a voltage drop in the relay. In order to prevent the voltage drop of the relay, manufacturers have listed the recommended voltage values in their product manuals; however, a certain margin should be left for vibration and other unexpected conditions. Many relay manufacturers use 35% of the rated voltage as a safety limit. Assuming that this limit is sufficient, you can use RCoil_T(max)Value and the rated working voltage of the relay (Vnom) To calculate I for different operating temperaturesHoldvalue:
Application examples of power connection device
If the load exceeds the rated current of the device at the specified time, the overload protection will cause the device to disconnect the circuit. The protection circuit shown in Figure 8 generates an activation (EN) signal by measuring current and voltage. (In order to simplify Figure 8-10, the DRV110 pin connections of OSC, PEAK, HOLD and KEEP are not shown.)
picture8 Overload protection
Magnetic contactors require a current to pass through the coil to move the contactor into the closed or open position. Figure 9 shows the RMS voltage detection circuit implementation of a contactor system using DRV110.
picture9 RMSVoltage detection magnetic contactor system
Utilizing DRV110 can also realize undervoltage and overvoltage protection (Figure 10). Two comparators are used to measure the high and low threshold voltages. According to the output of each comparator, the SR flip-flop sends an activation (EN) signal to the DRV110.
picture10 Undervoltage and overvoltage protection
There are many advantages to using energy-saving solenoid controllers with integrated power regulation. In order to achieve energy saving, current regulation is the most accurate method of actuator force control. Since this system is not affected by coil resistance, power supply voltage and temperature changes, there is no need to increase the margin. In addition, system reliability has also been improved because the solenoid behavior has been repeatedly optimized. Finally, the system cost is also reduced. Because the energy is precisely controlled, using smaller and cheaper coils can easily achieve acceptable drive performance.
1. “Energy-saving solenoid controller with integrated power regulation”, DRV110 product manual, website: www.ti.com/slvsba8-aaj.