Creating an Adjustable Constant Current Source
Using a voltage regulator in an appropriate configuration, as demonstrated in the LM317 datasheet, is the most straightforward method to establish an adjustable constant current source. However, this conventional approach does not permit current adjustment down to zero. The design presented here overcomes this limitation by employing two distinct fixed voltage regulators with different output voltages. This ensures the op-amp always operates within its specified range.
Dual Voltage Regulator Setup
In this setup, the first voltage regulator provides a stable 15V output, serving both as a power source for the op-amp and as a voltage reference for the voltage divider composed of R3, P1, and R4. P1 is utilized to fine-tune the reference voltage appearing on the non-inverting input of the op-amp (pin 3 of IC3). The op-amp then adjusts the current at the circuit’s output (from the collector of T1 to ground) via T1. This adjustment maintains the voltage at the emitter of T1 (the instantaneous voltage) at the same level as the voltage at the wiper of P1 (the reference voltage). For this mechanism to function effectively, it is imperative to connect a load at the circuit’s output, enabling a current to flow to ground.
Adjusting Output Current Range
The range of voltages available through the adjustment of P1 is determined by the resistance values within the voltage divider circuit comprising R3-P1-R4. When the voltage on the wiper of P1 is at its minimum, the maximum achievable output current flows. This maximum current is contingent upon the value of resistor R2. The specified values are configured to provide an available current range from 0 mA to 100 mA with R2 set to 100 Ω and a range from 0 mA to 30 mA with R2 set to 330 Ω.
The calculations involve establishing a voltage range at the wiper of P1, ranging from 2 V or less (at maximum current, with 10 V or more across R2) to at least 12 V (at minimum current, with 0 V across R2). To accommodate this range while allowing for a slight tolerance in the potentiometer’s track resistance, R3 and R4 are set to 1.5 kΩ, thereby expanding the actual voltage range to 1.73 V to 13.27 V.
Voltage Variations Due to Current
Since the circuit maintains a constant output current rather than a fixed output voltage, the actual voltage at the output will naturally fluctuate. As the output current, denoted as , increases, the voltage drop across R2 () also rises. Consequently, the output voltage proportionally decreases.