An adjustable load – design and build @technoblogy
Technoblogy presents an adjustable load that provides a constant-current load for testing power supplies and batteries. It allows setting the load current to up to 1.05A, using a potentiometer, and displays the current on a three-digit LED display controlled by an ATtiny84.
As well as testing power supplies, the adjustable load can be used for testing rechargeable NiMH and Li-ion batteries under different load conditions. It measures the current over time, and displays the battery’s capacity in mAh or Ah. This is useful for comparing different makes of battery, or for deciding whether a battery has reached the end of its life.
The constant-current circuit is based on an LT3080 regulator. This uses a current reference of 10µA, rather than a voltage reference like regulators such as the LM317, and will operate down to 1.5V and a few mA.
The current is sensed by a 1Ω resistor, and this is fed to the sense input of the regulator, via a 100kΩ potentiometer and 5.1kΩ resistor. Since the current through the potentiometer and resistor is 10µA, at the potentiometer’s maximum setting the voltage across them will be 1.05V, so the output of the regulator will be maintained at 1.05V across the 1Ω resistor, producing a constant current of 1.05A.
The current is displayed on a three-digit 7-segment display. I used a 0.28″ three-digit common anode seven-segment LED display.
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