What You’ll Learn:
- How to measure rise time for power sources
- Understanding voltage vs. time signal measurements using an oscilloscope
- Practical steps for setting up a battery testing configuration
When testing a DC power supply, a common question often arises: “What is the rise time of a voltage source when applying a specific voltage under load?” Recently, I was tasked with determining the rise time of a power source transitioning from 0 to 350 V under a 400 mA load. Although the power source’s rise time specification was clearly stated as 0 to 600 V at 2.6 A over 250 ms, my scenario had different parameters. The challenge was to measure the performance of the voltage source under these new conditions.
Understanding Rise Time and Its Importance
The rise time of a power source is the time it takes for the voltage to transition from a lower to a higher level under a load. In my case, the task was to assess the rise time for a 0 to 350 V step with a 400 mA load. The initial specifications for the power source mentioned a 250 ms rise time for a 0 to 600 V transition at a 2.6 A load. Given that the voltage and current requirements for my test were lower (about 60% of the voltage and 15% of the current), it was logical to expect a faster rise time, but by how much?
Setting Up a Voltage Rise Time Test
Setting up this test requires more than simply connecting the power source and measuring the output. Proper configuration is essential to avoid damaging the equipment or obtaining incorrect readings. The following steps outline how to set up the test properly.
Step 1: Configuring the Test Setup
To measure the rise time of a battery tester, a test configuration is needed that includes both the power supply (device under test, or DUT) and a load capable of drawing 400 mA consistently. A measurement device, such as an oscilloscope, will capture the output voltage waveform.
Choosing the Right Load
For this test, the load must draw 400 mA while the power source transitions from 0 to 350 V. The simplest option is a resistor. Using Ohm’s Law (R = V/I), the required resistance is calculated to be 875 ohms. However, this resistor must also handle a significant amount of power, around 140 W (P = I²R).
While a resistor can achieve the desired current at 350 V, it won’t draw 400 mA consistently as the voltage rises from 0 to 350 V. For example, at 175 V (halfway to 350 V), the current would only be 200 mA. Therefore, a constant current load, such as an electronic load (eload), is a better choice for this application.
Using an Electronic Load (Eload)
An eload can ensure a constant current of 400 mA throughout the voltage rise. However, finding an eload capable of operating at such high voltages (350 V) can be challenging, as many are not rated for this range. Additionally, most eloads have a minimum operating voltage, typically around 2 V, which must be met for the eload to function correctly.
Step 2: Performing Voltage vs. Time Measurements
Once the proper load is in place, the next step is to measure the voltage rise over time. For this, an oscilloscope is the most practical tool, as it can display the voltage waveform in real-time.
Why Use an Oscilloscope?
A digital multimeter (DMM) may be tempting to use for this measurement, but it cannot capture time-varying signals. An oscilloscope, on the other hand, can accurately display the voltage waveform, allowing you to measure the rise time directly from the screen.
High-Voltage Considerations
Because the voltage being measured is quite high (350 V), a high-voltage probe is necessary. Additionally, you may want to measure the current to confirm that the 400 mA load is maintained during the rise time. This will require a current probe for the oscilloscope.
Grounding Issues
Oscilloscopes are ground-referenced, meaning they connect to the ground when measuring the DUT. In cases where the DUT floats with respect to ground, this can create a grounding path that could damage the oscilloscope or other equipment. Care must be taken to avoid unintended grounding, which could result in shorts or even catastrophic damage.
Step 3: Alternative Measurement Devices
If using an oscilloscope presents challenges due to the high voltage or grounding issues, other instruments like a power analyzer can be used. A power analyzer can measure both DC and AC waveforms and often does not require a separate current probe. Additionally, it is not ground-referenced, making it safer to use in certain setups.
Practical Setup: Using a Two-Quadrant Power Supply
For this test, we opted to use a two-quadrant power supply rated at 500 V and ± 20 A. Although this power supply offers more power than required, it easily handles the test voltage and accurately maintains the 400 mA constant current. Additionally, this supply features a built-in digitizer, eliminating the need for an oscilloscope, high-voltage probe, or current probe.
The setup was controlled through companion software, which provided a user-friendly interface for visualizing the voltage and current waveforms.
Measuring Voltage Rise Time: Results
Using the companion software, we captured the voltage rise time and analyzed the results. The voltage trace clearly showed the transition from 0 to 350 V, with the rise time measured at 77 ms—significantly faster than the original 250 ms specification at a higher load.
Interestingly, during the initial part of the test, while the DUT was at 0 V, the current was not regulated as the eload could not maintain the 400 mA until the voltage rose above 2 V. Once the voltage began to rise, the current briefly overshot 400 mA before stabilizing.
Conclusion: Key Takeaways for Battery Testing
Measuring the rise time of a power source is an essential step in understanding its performance, especially when testing battery modules. By carefully selecting the right equipment, such as a constant current eload and an oscilloscope (or power analyzer), accurate and safe measurements can be made.
In this test, the rise time was successfully measured at 77 ms, much faster than the original specification under higher voltage and current conditions. This demonstrates that even with less stringent conditions, accurate measurements are crucial for reliable testing outcomes.
