When you build electronic projects, one of the first thing you need is a power supply. I have a number of pages on this site dedicated to power supply design (Ref 1). You probably have a number of power supplies in your house, like wall-warts or more sophisticated bench models. I am a ham radio operator, and like other hams, I have a number of radios and other equipment that operates from 12V, so I have a number of 12V power supplies around. It is easy to assume power supplies work, but they do not always work as well as you think, or even as well as you need. A simple way to test a power supply is to attach a load to it. In the past, I have used light bulbs to load power supplies designed for 12V and lower voltage operation. However, this is not very convenient, as there is a finite choice of readily available light bulbs, and the current drawn by a light bulb varies greatly with the applied voltage, so it is not always easy to extrapolate the measurements to other conditions. Also, light bulbs do not allow to measure the dynamic response, i.e. how the output voltage changes when the load current changes rapidly. Here I will describe a simple power supply tester, called a Dynamic Load. A Dynamic Load allows you to subject your power supply to a variety of load conditions so that you can observe its behavior. For instance, you can use it to verify the voltage regulation of a power supply, i.e. how much does the output voltage change when the load current changes from min to max and anywhere in between. You can also simulate dynamic conditions, for instance simulating the load of an audio amplifier or a ham radio transmitter operating on Single Side Band or in CW (Morse code). Here is the schematic of the complete Dynamic Load: This load can be adjusted for either constant current or constant voltage operation, and includes an oscillator to pulse the load current between 0 and a preselected value at an adjustable rate.