The proportional-control solenoids used in some industrial
hydraulic systems are usually driven by microcontrollers
or programmable logic controllers (PLCs). Such complex
drivers typically require several different supply voltages for logic
and control. (The purpose of proportional control is to move
the solenoid plunger to an arbitrary position and leave it there.)
A set-and-leave solenoid driver, however, should not require a
costly PLC or the processing power of a microcontroller. Ideally,
it should operate from the solenoid’s own supply voltage.
The circuit in Figure 1 meets these requirements while occupying
only a small surface-mount footprint. U2 is a 350-mA
driver with built-in analog and pulse-width-modulation
(PWM) dimming control. It’s typically used to drive highbrightness
LEDs, but in this application, its open-drain output
(OUT) and current-sense terminal (CS+) connect directly to
the solenoid terminals. R6 sets the maximum solenoid current.
The circuit drives solenoids ranging from 6 to 40 V dc,
using only the solenoid’s 6- to 40-V dc source. The circuit
was tested using a LEDex 24-V pull-solenoid rated for currents
up to 290 mA.
U1 is a 32-tap, nonvolatile, linear-taper digital potentiometer.
Connected as a variable resistor, its internal 100-k? variable
resistance forms a divider with R5 to produce a 0- to 3.17-V
analog voltage at U2’s DIM input. After activating the potentiometer
by depressing SW1 and setting the direction of change
using SW2 (Open for up, Closed for down), you can increment
this DIM voltage with each toggle of SW3 (with SW1 closed).
Thirty-two steps are available, so 32 press-and-release cycles of
SW3 traverse the range from 0 to 3.17 V. In terms of wiper position,
the approximate voltage at DIM (VDIM) is:
VDIM ≈ 5 V\[(N - 1)3225.8 Ω\]/\[(N -1)3225.8 Ω + R5\]
where R5 = 56.2 kΩ and N is the Nth wiper
step (between 0 and 32). R3, R4, and C2
debounce the SW3 pulses that increment/
decrement the wiper. U2 switches the solenoid
at 200 Hz, with a PWM duty cycle
that varies with the voltage at the DIM pin.
U2 is powered by the solenoid’s power
supply (24 V in this case). U1 is powered
by U2’s V5 pin, a 5-V supply that can
source as much as 2 mA. Capacitors C1,
C4, and C5 bypass the supply voltages at
their respective IC pins. A bulk-holdup
capacitor (C3) may be needed if the circuit
is located at a distance from the dc source.
To accommodate the PWM action applied
by U2, freewheeling diode D1 allows
current to circulate through the solenoid
coil each time its power switches off (200
times per second).
U2 provides indirect short-circuit and
thermal protection to guard its output
against overcurrent and short-circuit conditions
created by a defective solenoid or
shorted leads. Jumper JU1 controls U2
by connecting its EN input (pin 19) to
VIN (enable) or GND (disable). The table
summarizes the circuit’s conditions for
various positions of the solenoid.
Figure 2a shows the solenoid-driver voltage
and current pulses produced when the
circuit operates at a 26% duty cycle. That
duty cycle represents the solenoid voltage
and the 32.4-mA rms current required to
energize the solenoid coil and pull in the
solenoid plunger a distance of 0.0312 in.
Figure 2b shows the solenoid-driver voltage
and current pulses at a 76% duty cycle,
which represents the solenoid voltage and
211-mA rms current needed to energize
the solenoid coil and pull in the solenoid
plunger a distance of 0.312 in.
See associated table
