Troubleshooting An HP DC Power Supply: 18.6V DC Output Showing 40V AC Signal

Have you ever encountered an issue where your HP DC power supply, rated at 18.6V DC, is showing a 40V AC signal on your multimeter? This can be quite perplexing, but understanding the possible causes and troubleshooting steps can help resolve the problem effectively.

Understanding the Issue

When your HP DC power supply is displaying an unexpected 40V AC signal on your multimeter, several factors could be at play. This article will explore the most common causes and provide step-by-step guidance on how to troubleshoot this issue.

Steps to Troubleshoot the Issue

1. Multimeter Settings

Always start by double-checking the multimeter settings. Ensure that the multimeter is set to the correct mode to measure DC voltage and not AC voltage. Misconfigured multimeter settings can lead to misleading and inaccurate readings.

2. Ripple Voltage

If your power supply is a switching type, it may have some ripple or noise on its output. This ripple can sometimes be observed as an AC voltage when measured by a multimeter. If the power supply is functioning correctly, the DC output should be stable despite any small AC ripple.

3. Faulty Power Supply

The power supply itself could be malfunctioning. Circuitry issues such as a damaged output filter can allow AC signals to pass to the output. This can result in high levels of AC noise superimposed on the DC output. Carefully inspect the power supply for any visible signs of damage.

4. Grounding Issues

Ensure that your power supply is properly grounded. A lack of proper grounding can cause the power supply to pick up stray AC voltage from the environment. Check if the outlet is functioning correctly and that the power supply is correctly grounded.

5. Measurement Technique

Verify that you are measuring the voltage correctly. Poor contact with the multimeter probes or measuring in a noisy environment can lead to inaccurate readings. Make sure that your probes are making good contact with the power supply terminals.

Troubleshooting Techniques

1. Verify Multimeter Settings

Before proceeding with any other checks, verify the multimeter settings. Ensure that the multimeter is set to measure DC voltage and not AC voltage.

2. Measure DC Voltage

Measure the DC voltage again using the correct settings on the multimeter. This simple step can often resolve the issue.

3. Inspect Power Supply

Inspect the power supply for any visible signs of damage. Look for bulging capacitors, loose connections, or other visible issues that may indicate a problem.

4. Use Oscilloscope (Optional)

If possible, use an oscilloscope to obtain a better view of the output waveform. This can help in diagnosing any issues with the power supply’s internal circuitry.

5. Additional Tricks for Tricky Cases

In some cases, a simple workaround can help. For example, try connecting a small night-light (4 or 7 watts) in parallel with the meter leads. This can help absorb some of the noise and may eliminate the 40V AC signal.

For more precision, consider using a specialized tool like the Fluke SV225 Stray Voltage Adapter. This adapter provides a 3000-ohm load and can be helpful in accurately measuring the true DC output of the power supply. However, it is a bit more expensive at around $80.

Conclusion

By following the steps and optional tricks mentioned in this article, you should be able to diagnose and potentially resolve the issue with your HP DC power supply. If the problem persists, it might be best to replace the power supply or consult a professional technician.

Remember, a thorough inspection and correct measurement technique are key to accurately diagnosing and resolving power supply issues. Happy troubleshooting!