Sourcing and Sinking

The terms sourcing and sinking describe power flow into and out of a device. Devices that are sourcing power are delivering power into a load, while devices that are sinking power behave like a load, absorbing power that is being driven into them and providing a return path for current.

A battery is one example of a device that is capable of both sourcing and sinking power. During the charging process the battery acts as a power sink by drawing current from the charging circuit. After it has been removed from the charger and installed into an electronic device, the battery begins to act as a source that delivers power to a load.

The following quadrant diagram graphically represents whether a particular channel is sourcing or sinking power. Quadrants consist of the various combinations of positive and negative currents and voltages. Quadrants I and III represent sourcing power, while quadrants II and IV represent sinking power.

For example, when you have a positive voltage and current flowing out of the positive terminal (that is, a positive current), the output operation falls within Quadrant I and is sourcing power. When you have a positive voltage and a current flowing into the positive terminal (that is, a negative current), the output operation falls within Quadrant II, and is sinking power.

A single-quadrant channel on a power supply can operate only in one quadrant. For example, while the NI PXI-4110 has multiple channels capable of sourcing power in either Quadrant I or Quadrant III, individually, each channel on the NI PXI-4110 can operate only within one quadrant (channels 0 and 1 operate only within Quadrant I, and channel 2 operates only within Quadrant III). Thus, all channels on the NI PXI-4110 are single-quadrant supplies.

Devices that are capable of sourcing power in both Quadrant I and III are sometimes referred to as bipolar because they can generate both positive and negative voltages and currents. Bipolar output channels may or may not have current sinking capabilities (Quadrants II and IV).

An output channel on a four-quadrant power supply or SMU can both source and sink power with a positive or negative voltage and current. For example, the NI PXI-4130 SMU channel (channel 1) is capable of both sourcing power in Quadrant I or Quadrant III and sinking power in Quadrant II or Quadrant IV. Thus, the NI PXI-4130 SMU channel is a bipolar, four-quadrant device. For NI four-quadrant devices, it is important to remember that an auxiliary power supply is required for both sourcing and sinking power to reach the full current capability of the device. For more information about auxiliary power, refer to the Internal and Auxiliary Power topic for your device.

Because of the required power dissipation, sourcing and sinking capabilities for a channel are not always identical. Refer to NI PXI-4110 or NI PXI-4130 for more information about the sourcing and sinking capabilities of your device.

The following table summarizes the power capabilities per channel for each NI power supply and SMU device.