Here’s a design circuit for active load that can be used in Power supply with base in 5V 2A. This circuit is based on a scrap of perf-board using a medium power NPN transistor. I used a TIP41B because I had some, but those are not recommended for new designs. Almost any decent NPN power transistor or N-channel power FET will work in this application. This is the figure of the circuit.
The '555 is set up as a free-running
square wave generator (well, within a few percent of being a 50 percent duty
cycle square wave). Frequency is just below 100Hz. The actual frequency doesn't
matter as much as the rapid rise and fall of the waveform. This is the way the
circuit was used: I connected the 2.49Ω resistor (LOAD SINK) to my power supply
being tested. I also had an additional 27Ω resistor connected to the power
supply. When the power transistor is on, load current was about 2A; when off,
around 200mA. I monitored the voltage at the LOAD SINK terminal with my scope.
The SYNC terminal is used if I needed a separate sync input to the scope for
more accurate triggering. More on that in a minute. The tester for the
circuit is excessive ringing in the output voltage as the high-current draw
snapped on and off. If the voltage just overshot slightly and then quickly
settled to a final value, then my loop compensation was probably just right. If
the phase margin was insufficient, the ringing would last a while. That SYNC
input comes in handy if the output is ringing excessively. It's tough to set a
good, consistent trigger point on such a waveform. The SYNC output provides a
clean, square waveform with no ringing.
