Before trusting that bargain-bin power adapter, make sure it’s not packing surprises. This DIY AC power adapter tester quickly identifies voltage, polarity, and load behavior for nearly any plug-in supply.

The Project

A while back, Charles Hansen found himself staring at a box full of mysterious AC power adapters — the kind you collect from yard sales and long-forgotten gadget drawers. Some were clearly labeled, others had worn or missing tags, and all of them begged the same question: What exactly are these things outputting? To bring order to the chaos, Chuck built a compact test fixture that accepts common 2.5 mm and 2.1 mm barrel plugs as well as 3.5 mm tip-ring connectors, making it easy to identify voltage, polarity, and more at a glance. Plug in, say, a 14-V DC adapter, and the tester’s red and green LEDs instantly reveal whether it’s DC and which way the polarity runs. It is a simple, elegant solution to the classic “mystery wall wart” problem.
 
Power adapter tester 1
The AC power adapter tester

The Circuit

"The power adapter is plugged into the appropriate matching jack K1, K2 or K3," Hansen noted. "The center parts of K1/K2 and the tip connection of K3 are connected to the anode of the red LED and the cathode of the green LED, both contained in bicolor LED LED1. Likewise the outer sleeves of K1 and K2, and the ring of K3 are connected to the second bicolor LED, LED2. LED1 and LED2 are interconnected through R1, which limits the LED currents. If AC is applied, both LEDs light bright orange/yellowish."
 
Power adapter tester 2
Two bicolor LEDs should end all worries and confusion about cheap AC power adaptors.
The tester includes banana sockets for attaching a resistive load so you can check how a power adapter performs under load—typically about 95% of its no-load voltage, though results can vary. A BNC connector lets you safely measure output voltage, with a built-in series resistor to protect your meter if you forget to set the proper range. While careful users may not need this safeguard, it’s always smart to start with a high voltage range and step down. If you mount the banana jacks externally, space them 0.25 inches apart to support standard dual-banana test leads.
 
Power adapter tester 3
Example of lettering and signage on the front panel.

The AC Power Adapter Tester Project

The original article, “AC-AC & AC-DC Power Adapter Tester,” appeared in Elektor March 2014. You can read the article for free during the two-week period following the publication of this post. Enjoy!
Editor's Note: This article first appeared in a 2014 edition of Elektor. Given the project’s age, some components might not be readily available. Still, we think the design will inspire you to start a project of your own.

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