When a solenoid needs to be actuated based on a signal input condition, a major concern is how to obtain the best performance from the solenoid while at the same time minimizing heat dissipation. Solenoids are inductive loads, so when a voltage is first applied no current flows through the solenoid coil, and the voltage across the coil sits at the peak driving voltage. Then the voltage across the coil drops off exponentially while the current rises exponentially.
If the same voltage is applied during this time, an extremely high current would eventually flow which would cause unnecessary heat build-up. Initially, the voltage or current source driving a solenoid should be at the maximum to ensure that the solenoid’s armature is actuated. Once the armature is pulled in on the solenoid, voltage or current from the driving source should be dropped to a lower value that is just sufficient to hold the armature in place.
There are many ways that this can be accomplished by analog circuitry, where the output of the voltage source or current source is controlled by feedback from the solenoid coil or just designed to drop off as a function of time. This article discusses an alternative scheme, where the driving source voltage is either full “on” or full “off”, with no in between levels. The driver circuit output is digital instead of analog, and the voltage driving the solenoid is in a pulse-width modulated (PWM) format.
This technique is used in the DRV solenoid driver series from Texas Instruments (TI) - the DRV101, in particular, is discussed here. When a signal is sent the DRV101 to actuate a solenoid (pin 1), full power is applied to the solenoid initially. This full power signal is programmed to last only for a set time which is controlled by a timing capacitor, CD, connected to pin 2. This applied pulse should be long enough to make sure the solenoid is activated. After this pulse, the signal changes to a PWM signal with a programmed duty cycle that is set by a resistor, RPWM, connected to pin 3.
It is possible to connect pin 3 to a voltage or current source rather than a resistor. This source can be controlled by a feedback loop to maintain an optimum duty cycle time to keep the solenoid on while minimizing power requirements. The duty cycle is low enough so the current through the solenoid doesn’t reach a high value during the part of the cycle when full voltage is applied. Increasing the resistor value or voltage connected to pin 3 always results in a decreased duty cycle (100% duty cycle is full “on”, 0% is full “off”).
With the set duty cycle, the average voltage and average current output are set to some controlled value lower than the maximum, which reduces the power dissipation (P = VI) in the solenoid. The power dissipated in the DRV101 itself is also reduced to a very low level by using this PWM scheme, whereas in analog drivers, power dissipation is a problem. In the “off” state of the duty cycle, the current through the output driver is zero while its voltage is at the full supply voltage. In the “on” state, the driver current becomes high, but its voltage drop is almost zero. Therefore, over the full cycle, the power dissipated in the device is very small (according to P = VI). Because of the relatively low power dissipation in the DRV101, a heat sink is not usually required - but a heat sink should be considered anyway to increase reliability and to make sure that the maximum operating junction temperature (125°C) is not exceeded.
A unique feature of the DRV101 is that it can drive solenoids with a much higher pull-in current requirement than hold requirement. A constant DC voltage is applied initially for a set time period (100% duty cycle), and then switched to PWM mode with a set duty cycle (10% to 100%) to save power. The period (cycle time) of the PWM mode in DRV101 is constant, and is set by an internal 24kHz oscillator. This frequency is not externally adjustable.
The voltage supply and ground connections are made to pins 5 and 4 respectively on the DRV101. Pin 5 is the output that actually drives the solenoid. There is one pin left (pin 7), and this is used as a status flag, and provides a fault indication for over-current, under-current, and thermal shutdown conditions. An over-current fault condition occurs when the output current is greater than about 2.3 A. An under-current fault is when the output current is below the under-scale current threshold (about 23mA). These two fault indications would detect both open and short circuits in the drive circuit. A thermal fault occurs when the device reaches a temperature of about 165°C. The device is shut off during a thermal or over-current fault.
The DRV101 is suitable not only for driving electromechanical loads such as relays, actuators, and solenoids, but it can also work as a linear driver for valves, positioners, heaters, and lamps. The DRV101 comes in a 7-lead staggered TO-220 package and a 7-lead surface-mount DDPAK plastic power package. It has an output drive capability of 2.3 A, and a voltage supply range of 9V to 60V. It is rated to operate over the extended industrial temperature range of –40°C to +85°C. Other solenoid drivers in this family include the DRV102, DRV103, and DRV104. The DRV102 has a 2.7 A output current drive capability, while the DRV103 and DRV104 have adjustable internal oscillators and a lower 8V to 32V voltage supply range.
Daniel is a recent graduate of the City University in London. His Masters thesis looked at how companies use social media for marketing purposes. In his first degree, he focused on electric engineering.