Solution
To determine the power budget for your PXI(e) system there are three components that must be considered: the power supply, the controller and modules, and the chassis.
- Analyze the power supply to determine how much power can be provided.
Every PXI(e) backplane has four voltage rails that are available to all controllers and modules. The specifications per rail of the power supply can be found in the user manual for your chassis.
For example, the specifications per fail of the power supply contained in the PXIe-1085 are:
| PXIe-1085 | 3.3 V | 5 V | +12 V | -12 V | 5 VAUX |
|---|
| Maximum Current | 60 A | 44 A | 62 A | 4 A | 2 A |
- Determine which controller and modules will be used and calculate the power consumed by each.
To determine if the specifications of the power supply are sufficient, you must now calculate the total current draw for the controller and each module in the chassis. The maximum current draw values can be found in the User Manual / Device Specifications of each controller and module. Although it is unlikely that all modules will be drawing the maximum current at the same time, these values are used to account for all current draw conditions.
For example, consider a system with a PXIe-1085 chassis, a PXIe-8880 embedded controller, a PXIe-4304 SC Express module, a PXIe-6376 Multifunction I/O module, and a PXI-5154 Oscilloscope module. Find the power requirements for each card and calculate the total maximum current for each voltage rail.
| | 3.3 V | 5 V | +12 V | -12 V | 5 VAUX |
|---|
| PXIe-8880 | 2.96 A | 3.11 A | 8.7 A | | 0.23 A |
|---|
| PXIe-4304 | 1.1 A | | 1.1 A | | |
|---|
| PXIe-6376 | 1.4 A | | 1.3 A | | |
|---|
| PXI-5154 | 1.7 A | 1.8 A | 0.52 A | 0.2 A | |
|---|
| Total Current | 7.16 A | 4.91 A | 11.62 A | 0.2 A | 0.23 A |
|---|
If any voltage rail exceeds the specifications of what the power supply can handle from part 1, then the performance of a particular module or the chassis as a whole would be unpredictable. This unpredictability could range from unexpected system shutdowns to decreased input and output measurement accuracy. Exceeding the power supply specifications may also result in potential damage to the power supply or modules in the chassis.
To find the total power, multiply the current on each rail by it's voltage value and then sum the power.
Total Power (W) = 7.16A(3.3V) + 4.91A(5V) + 11.62A(12V) + 0.2A(12V) + 0.23A(5V) = 191.12W
- Determine which chassis will be used and find the power consumed by the chassis.
Use
this article to determine the empty power draw of your chassis. National Instruments measures the power consumption independent of the power supply; therefore efficiency is built into the state values.
For example, the empty chassis power draw of the PXIe-1085 is 95W.
- Calculate the overall power consumption of your PXI(e) System
Look in the user manual of your chassis to determine the efficiency of your power supply. Power supply efficiency is the ratio of output power to input power. This specification denotes how well the power supply can handle different amounts of power draw.
Calculate the overall power consumption using the following equation:
Power (W) = [Total Power from part 2 (W) / Power Supply Efficiency] + Empty Chassis Power Draw from part 3 (W)
For example, overall power consumption of the above system is:
Power(W) = [191.12W / 0.7] + 95W = 368W