2.5.1 Voltage Sensor

Summary Description


The NEXSYS Voltage Sensor (VSD1, VSD2) is a Series A (4 Pin) solid-state, low-side direct current (DC) voltage sensor, developed to enhance avionics design flexibility by offering undervoltage and overvoltage detection. The Voltage Sensor compares DC voltage in a linear relationship between the specified setpoint and the voltage that is being monitored by the SENSE input. The fast-response output provides a discrete signal that transitions from High-Z (Open) to Low (Ground) when the input detects voltage that is above or below the specified setpoint, see Figure

The NEXSYS Voltage Sensor features fault-tolerant circuitry and architecture that provides low-drift voltage sensing over temperature. The Voltage Sensor also offers a range of setpoint options from +50 mVDC to +48 VDC.

The NEXSYS Voltage Sensor (VSD1, VSD2) is a commercial-off-the-shelf (COTS) item. This standard catalog hardware includes features to allow for flexible interface design solutions while also meeting aircraft performance standards. The VSD is designed, tested, and qualified to MIL-PRF-22885/116, MIL-PRF-22885/117, DO-160 and MIL-STD-461 requirements. Specifications are available in our MIL-PRF and DO-160 Documents page, and the qualification table is available in Appendix A.

There are two configuration types:

  • VSD1 performs overvoltage and undervoltage detection from +1 to 48 VDC.
  • VSD2 performs overvoltage and undervoltage detection from +50 mVDC to 1000 mVDC (1 VDC).
Input Characteristics

The NEXSYS Voltage Sensor inputs are Power (28V), Ground (GND) and Sense (SENSE). The inputs include the following characteristics for custom voltage sensing and output control. Voltage Sensor input circuity is diode isolated, buffered, and debounced for reliable operation. See additional sections for specific details concerning the Input/Output (I/O) interface.

28V (PIN 2): Operating Voltage (Nom.), VSD1 = Current Draw 2 mA (Max) and VSD2 = Current Draw 4 mA (Max).
GND (PIN 4): Continuous Ground required, otherwise, inadvertent operation could occur.
SENSE (PIN 3): Detects DC input voltage and includes a hysteresis band to prevent signal chatter.

Output Characteristics

The NEXSYS Voltage Sensor features a conditioned Output Z (OUT Z) that is open-drain High-Z (Open) when not active, and surge-protected against transients and overload conditions. The output load capacity is 2.0 A for VSD1 and 0.5 A for VSD2, see Figure (Operating Parameters) for electrical load rating types.

OUT Z (PIN 1): Active = Low (Ground), Not Active = High-Z (Open)

Configuration Options




The NEXSYS Voltage Sensor features an extensive range of setpoints to provide an Active Low (Ground) output when the specified undervoltage or overvoltage condition is detected, as detailed below. The Voltage Sensor can also be configured to perform wide hysteresis and current sensing.

Overvoltage: OUT Z (PIN 1) = Low (Ground), when the voltage monitored by SENSE (PIN 3) is ABOVE the specified setpoint, see Figure

Undervoltage: OUT Z (PIN 1) = Low (Ground), when the voltage monitored by SENSE (PIN 3) is BELOW the specified setpoint, seeFigure

Wide Hysteresis: When combined with a NEXSYS Electronic Latch, the Voltage Sensor can perform wide hysteresis with different pull-in and drop-out levels for enhanced system noise tolerance.

Current Sensing1,2: When combined with an external shunt resistor (not included), the NEXSYS Voltage Sensor (VSD2) can perform as a current sensor by wiring an external resistor to a VSD2, see Figure The VSD2 will measure the voltage drop across the external resistor as compared to the Ground supplied to the device. Common industry shunt resistors and standard VSD2 configurations are listed below as an example of how a current sense solution can be achieved, see Figure


Current Sense Target Customer Shunt Resistor Example NEXSYS VSD2 Configuration
10 A 0.01 ohms (> 1 W) VSD2/0100/A (100 mV)
20 A 0.01 ohms (> 4 W) VSD2/0200/A (200 mV)
30 A 0.01 ohms (> 9 W) VSD2/0300/A (300 mV)
40 A 0.01 ohms (> 16 W) VSD2/0400/A (400 mV)

Voltage Setpoints3: two configurations (VSD1, VSD2) that offer an extensive range of voltage setpoints in increments between +50 mVDC to +48 VDC.

VSD1, performs overvoltage and undervoltage detection from +1 to 48 VDC, at the specified setpoint increments:

+0.5 VDC increments: +1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5. 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5 VDC.
+1.0 VDC increments: +10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 VDC.
+2.0 VDC increments: +32, 34, 36, 38, 40 VDC.
+4.0 VDC increments: +44, 48 VDC.

VSD2, performs overvoltage and undervoltage detection from +50 mVDC to 1000 mVDC (1 VDC), at the specified setpoint increments:

+10 mVDC increments: +50, 60, 70, 80, 90 mVDC.
+50 mVDC increments: +100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000 mVDC.

1. The NEXSYS Current Sensor (CS1) features an integrated resistor (vs. external shunt) for undercurrent and overcurrent sensing, with setpoint options ranging from 10mA to 1000mA (1A), see Current Sensor.
2. Because the VSD2 SENSE Input and GND share the same Ground lead, current flowing through the Ground lead should be limited to 10mA to minimize measurement error. Ground of VSD2 and sensed device must be the same to achieve maximum accuracy.
3. The VSD1 Nominal Rising Voltage setpoint tolerance of SENSE (PIN 3) is +/- 5%. Falling Voltage Hysteresis -3% Max (-2% Typical), measured from the setpoint after the inclusion of the Rising Voltage setpoint. The VSD2 setpoint tolerance of SENSE (PIN 3) is +/- 5%. The VSD2 has a hysteresis band (+/- 0.16%) that will prevent a state change until the sensed voltage exceeds hysteresis. Tolerance at EMC and Environmental extremes is +/- 10%. Voltage Sensor, Operating Parameters


Description Parameters
Operating Parameters
Operating Voltage (Max./Nom./ Min.) +32 VDC /+28 VDC/+18 VDC
Power Supply Input Current VSD1: 2 mA (Maximum)
VSD2: 4 mA (Maximum)
Reset From Power Loss at 25°C 5 sec (Minimum) at +25° C
Hold Up On Power Loss at 25°C VSD1: 200 ms (Minimum)
VSD2: 50 ms (Minimum)
Input Parameters
Sense Voltage VSD1: +1 to 48 VDC
VSD2: +50 to 1000 mVDC
Rising Voltage Setpoint Tolerance (VSD1) +/- 5%
Falling Voltage Hysteresis (VSD1)
Note: Measured from the Setpoint after inclusion of the Rising Voltage Setpoint Tolerance above.
-3% (Maximum) (-2% Typical)
Setpoint Tolerance (VSD2) +/- 5%
Hysteresis Band (VSD2) +/- 0.16%
Typical Input Impedance (TSENSE, Voltage Drop) VSD1: 200 kiloohms
VSD2: 100 kiloohms to Ground
Transition Time Sense to Out (TSENSE) VSD1: 5 ms (Maximum)
VSD2: 10 ms (Maximum)
All Pulled-up inputs are diode isolated
Output Parameters
Output Load Capacity VSD1: Fused MOSFET 2.0 A (Resistive), 0.8 A (Inductive)
VSD2: IntelliFET® 0.5 A (Resistive / Motor / Inductive / Lamp)
On-State Resistance at 25°C VSD1: Typical 0.07 ohms / Maximum 0.09 ohms
VSD2: Typical 0.4 ohms / Maximum 0.6 ohms
Off-State Resistance at 25°C VSD1: Open Drain (High-Z),
Fuse-Protected MOSFET, +32 VDC Maximum
VSD2: Open Drain (High-Z),
Self-Protected IntelliFET®, +32 VDC Maximum
Operating -55° C to +85° C
Non-operating -55° C to +85° C
MIL-HDBK-217F, Notice 2
Airborne Inhabited Cargo (AIC)
at +40°C Continuous Operation
  VSD1: MTBF = 432,460 hrs
VSD2: MTBF = 422,347 hrs

For Qualification Summary, see Appendix A