Load Consideration for Programmable DC Power Supplies and Precision DC Sources

This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series, will teach you a specific topic of common measurement applications, by explaining the theory and giving practical examples. This tutorial contains information you may find useful as you connect specific types of loads to a power supply.

For additional power supply only concepts, refer to the Power Supply Fundamentals main page.
For the complete list of tutorials, return to the NI Measurement Fundamentals Main page.

Capacitive Loads

Generally, a capacitive load connected to a power supply is stable. Occasionally, a large capacitive load can cause ringing in the transient response of a power supply. When the output voltage is reprogrammed using a large capacitive load, the power supply may temporarily move into constant current mode or unregulated mode.

The slew rate is the maximum rate of change of the output voltage as a function of time. The slew rate is limited to the output current limit divided by the total load capacitance, as expressed in the following equation:

 (ΔV/Δt) = (I/C) 

where 

• ΔV is the change in the output voltage
• Δt is the change in time
• I is the current limit, and
• C is the total capacitance of the load

Series resistance or lead inductance from cabling can affect the stability of the power supply. In some situations, it might be necessary to increase the capacitive load or locally bypass the circuit or system being powered to stabilize the power supply.

Inductive Loads

In constant voltage mode, inductive loads remain stable. However, in constant current mode, inductive loads form parallel resonance with the output capacitor of the power supply. Parallel resonance can cause ringing of the current in the load, but it should not affect power supply stability.

Pulse Loads

The current of a load can vary between a minimum and a maximum value in some applications. In the case of a varying load, or pulse load, the constant current circuit of the power supply limits the output current. Occasionally, a peak current may exceed the current limit and cause the power supply to temporarily move into constant current mode or unregulated mode.

Reverse Current Loads

Caution: Reverse currents can cause the power supply to move into an unregulated mode and can damage the power supply.

Occasionally, an active load may pass a reverse current to the power supply. To avoid reverse current loads, use a dummy load resistor to preload the output of the power supply. Ideally, a dummy load resistor should draw the same amount of current from the power supply that an active load may pass to the power supply.

Note: The sum of the current of the dummy load and the current supplied to the load must be less than the maximum current of the power supply.

Relevant NI Products

Customers interested in this topic were also interested in the following NI products:

• Programmable DC Power Supplies and Precision Sources
• Modular Instruments (digital multimeters, digitizers, switching, etc...)
• LabVIEW Graphical Programming Environment
• SignalExpress Interactive Software Environment

For the complete list of tutorials, return to the NI Measurement Fundamentals Main page

Reader Comments | Submit a comment »