Most occasions in electronics call for a voltage source set to a specific value. However, some situations are more concerned about how many electrons are flowing as opposed to the voltage, and this is what a constant current source does. Imagine your load needs a fixed current like an LED we cannot power it over 20mA, it may damage the LED. Hence some sensitive electronic components and Integrated Circuits require a constant current source power supply.
A constant current source in electronics is a device/circuit that produces a constant value of current regardless of source voltage or load resistance. Here we design a simple and easy constant current DC power supply circuit using Three terminal adjustable regulator IC LM317. This circuit will provide different voltage output according to the input voltage and constant current output depends on the value of reference resistor R1.
Hardware Components
The following components are required to make DC Power Supply Circuit
| S.no | Component | Value | Qty |
|---|---|---|---|
| 1. | Transformer | 0-9V DC Output | 1 |
| 2. | Regulator IC | LM317 | 1 |
| 3. | Diode | 1N4007 | 4 |
| 4. | Electrolyte Capacitor | 100μF/16V | 1 |
| 5. | Ceramic Capacitor | 0.1μF | 1 |
| 6. | Resistor(Depend on Load) | – | 1 |
LM317 Pinout
For a detailed description of pinout, dimension features, and specifications download the datasheet of LM317
DC Power Supply Circuit
Working Explanation
As we can see in the circuit, first we need to build a rectifier circuit by implementing a step-down transformer and bridge rectifier to make a constant current DC power supply. Here we used 230V input primary, 0-9V AC secondary step-down transformer, and 1N4007 diode to form a bridge rectifier. Capacitors C1 and C2 are placed as a filter to remove AC ripples from the DC supply, then to regulate the current output we used an IC LM317.
IC LM317 from Texas Instruments is a three-terminal adjustable regulator and it can handle output current up to 1.5A it has internal short circuit current limiting options and a thermal overload protection circuit. Now the adjusted terminal of LM317 is connected with the output load after the R1 reference resistor and it functions as current feedback. In the prototype circuit, we used a 10Ω / 5W resistor and obtained a 125mA constant current output DC power supply.
Iout = Vref/R1
Vref = 1.25V for LM317 we can calculate the R1 resistor value with this formula, the R1 resistor value depends on your current output requirements.
Applications
These are useful for laser and electroplating applications.
