Enhanced Solenoid Controller Minimizes Energy Waste
In electronic control applications, solenoids and relays can consume significant amounts of power, particularly when activated. However, with the implementation of power-saving strategies, it is possible to reduce power dissipation while maintaining functionality. This article explores five strategies for designing power-efficient circuits for solenoids and relays.
Strategies for Power-Saving
1. Pulse Width Modulation (PWM) Technique
By using PWM to control the average power delivered to the solenoid or relay, it is possible to reduce power consumption while maintaining the required functionality. This is achieved by rapidly switching the power on and off, allowing the average power to be reduced.
To implement this strategy, a PWM signal generator (e.g., using a microcontroller or dedicated IC) is used to control a switch (such as a transistor or MOSFET) that modulates the power to the solenoid or relay.
2. Use of Low-Power Holding Circuits
Many relays and solenoids require full power only during initialization but can be held in the active state with significantly less power. A low-power holding circuit can be designed to initially send a high current to activate the relay or solenoid, then reduce the current to a lower holding level using a relay or a circuit that can handle different current levels.
3. Free-Wheeling Diodes
Free-wheeling diodes protect the circuit from damaging voltage spikes when the current to an inductive load (like a solenoid) is interrupted. By placing a free-wheeling diode in parallel with the solenoid or relay, excess energy can be safely dissipated.
4. Optimized Switching Devices
The use of switching devices like MOSFETs or bipolar junction transistors (BJTs) with low on-resistance ((R_{DS(ON)})) can minimize power loss during switching. For the circuit, a transistor with appropriate specifications for the application should be selected and adequately sized for the current and voltage requirements.
5. Fail-Safe Logic
Implementing fail-safe logic ensures that devices return to a safe state if the power or control signal fails. This can be achieved by using normally closed (NC) contacts to ensure that the default state of the system is safe, especially around hazardous processes.
Example Circuit
A basic power-saving circuit using PWM to control a relay or solenoid can be designed as follows:
```circuit // Components - Microcontroller or PWM generator - N-Channel MOSFET or BJT - Power supply - Relay or Solenoid - Free-wheeling diode
// Circuit 1. Connect the microcontroller PWM output to the gate of the MOSFET. 2. Connect the drain of the MOSFET to the relay or solenoid. 3. Connect the source of the MOSFET to ground. 4. Place a free-wheeling diode in parallel with the relay or solenoid. 5. Ensure proper voltage and current ratings for all components. ```
Practical Example: Using a Microcontroller
- Setup:
- Use an Arduino or similar microcontroller to generate the PWM signal.
- Connect the PWM output to the gate of a MOSFET (e.g., IRLB8721).
- Code: ```cpp const int pwmPin = 9; // PWM output pin const int pwmValue = 128; // PWM value for 50% duty cycle
void setup() { pinMode(pwmPin, OUTPUT); }
void loop() { analogWrite(pwmPin, pwmValue); // Set PWM for reduced power delay(1000); } ```
- Adjust the PWM Value: Adjust the variable in the code to control the power level sent to the relay or solenoid.
This approach can significantly reduce power dissipation while maintaining the necessary functionality of the solenoids and relays in electronic control applications. By reducing the driver's operating current after the solenoid or relay is switched ON, power consumption and thermal loading can be reduced.
The circuit later settles to the reduced operating-current mode, operating at 140 mA. The solenoid needs approximately 800 mA to switch on, but requires less than 100 mA to remain ON. The calculated power dissipated in the smart circuit is up to 10 times lower than the activation current for a solenoid operating from a 12-V supply and drawing 800 mA of current. The reduced ON-state current in the solenoid cuts its power dissipation.
The circuit initially turns on the solenoid like a standard circuit, driving 760 mA into the solenoid. After activation, the reference point for the current-sink portion of the circuit is driven high by the comparator output, causing the transistor (TIP120) to enter saturation. Additional power dissipation will exist in the driver transistor and resistor at the emitter of the transistor.
The ON command input signal connects the positive supply voltage at the noninverting input of the comparator formed by part of the LM324. The power dissipated in the solenoid is reduced by the square of the current reduction (P = IR). The R1-C1 time constant determines a predefined time after which the reference point to the current sink settles to the final steady-state value.
The circuit is designed for a solenoid operated from a 12-V supply. The reference voltage for the current sink is shaped to drop the current within tens of milliseconds after the solenoid is switched ON. The power dissipated in the driver transistor follows a linear relationship: P = VI. The power transistor in the circuit drives the solenoid (as shown in Figure 1).
The article was originally published in the October 13, 2005 print edition of Electronic Design magazine. It is important to note that solenoids and relays used in electronic control applications consume more power to activate than to hold in the ON position. The steady-state value of the current sink is determined by the R/R ratio and the current-sense resistor (1Ω). The total power dissipated in the circuit is substantially lower than a standard circuit.
The waveform for the solenoid current is illustrated in Figure 2. The circuit presented in the article employs an op amp to generate a constant-current sink.
These strategies can help designers create more power-efficient circuits for solenoids and relays in electronic control applications, reducing power consumption and thermal loading while maintaining functionality.
Technology plays a crucial role in implementing power-saving strategies for solenoids and relays. For instance, Pulse Width Modulation (PWM) technique is a technology that helps reduce power consumption by rapidly switching power on and off, while maintaining the desired functionality.
Low-Power Holding Circuits is another technology used to minimize power consumption. By designing a circuit that initially sends a high current to activate the relay or solenoid and then reduces the current to a lower holding level, power consumption can be significantly reduced during the holding phase.
