I’ve been having a hard time finding oscilloscopes and adjustable power supplies in the “hobbyist” price range, but I think I have cobbled together a solution.

A few key components:

• Power supply from old PC
  • Make sure to short the correct pins to trick the power supply into turning on.  This is usually green to black.
  • yellow-black is 12 V
  • red-black is 5 V
  • orange-black is 3.3 V
  • If you want other voltages, well..  time to built a buck-boost circuit!
• Gabotronics Xprotolab mini oscilloscope
  • 2MHz sampling
  • 8 bits resolution
  • $50 (the best part)

Here is a recent test that is perfect for the components listed above: characterization of the DC buck converters for car USB chargers.  In this test, I am measuring the operating frequency and ripple of an Adreama 1A USB charger.  The charger is hooked up to the 12 V rail of the power supply, and the 5V output is applied to a power resistor as the load.

As expected, the performance of the charger is a strong function of the load.  In this case, a 5 ohm power resistor is drawing 1 A from the charger.  The buck circuit is operating at 50 kHz, but with 0.4 V of ripple.  Ouch!