LM317 Constant Current Source for Lighting LEDs
In the above links I've explored using a constant current source (CCS) and basic theory. Here I'll concentrate on the use of a constant current source for lighting light emitting diodes or LEDs. Understanding series circuits and voltage drops will help with this subject.
Briefly in a series circuit the current through all series components is the same, while the sum of the voltage drops across each series component will add back to the source voltage.
In Fig. 1 we use a LM317 in its constant current mode. The value of R3 divided into 1.25 volts will determine load current through each LED in the string. In this case current is 1.25 / 4.7 =~ 266mA. Each LED has a slightly differing voltage drop which is normal and why we want to use a CCS. The total voltage drop across the 3 LEDs equals 12.2 volts while the drop across Us and R3 is 6.8 volts. They add back to equal the 19 volt power source.
Note that total voltage drops are limited to the power supply voltage and we should maintain drop across U3 and R3 of three volts leaving a drop for whatever load of 16V in this case. Drop more than 16V and the CCS will not function.
In Fig. 2 I've removed one LED and the associated voltage drop, but the drop across the other three LEDs remains the same. That missing voltage drop adds to the voltage drop across U3 and R3.
In Fig. 3 I've changed Vin to 24V and has no effect on the LED current, which is still set by R3 at 266mA. In all three cases above the voltage drops across U3 and R3 become useless heat.
The most efficient way energy wise is to use a switching voltage regulator to reduce Vin to 3-5 volts above the load voltage required.
More CCS Circuits and Theory
- Constant Current Circuits with the LM334
- LM334 Constant Current Source with Resistive Sensors
- Experiments with TL431 Shunt Regulator
- TL431 Precision Current Regulator Circuits
- TL431 Based Current Limiter Constant Current Source Circuits
- TL431 Shunt Regulator Circuits
- LM317 Adjustable Voltage current Boost Power Supply
- LM317 High Power Constant Current Source Circuit
- LM317 Constant Current Source Circuits
- Simple Triac-SCR Test Lab for You Tube
- Basic MOSFET Transistor Test Circuits
- High Voltage MOSFET Switching Circuits
- 3 Amp LM741 Op-Amp Constant Current Source
- Current Limiter Allows Safe Testing of Zener Diodes, LEDs
- Current Limiter Circuits for Opto-Coupler Input LEDs
- LM317 Constant Current Source for Lighting LEDs
- Hall Effect Magnetic Switches and Sensors
- Transistor-Zener Diode Regulator Circuits
- Build an Adjustable 0-34 volt power supply with the LM317
- LM2575 Simple Switching Voltage Regulators
- Simple 2 Transistor LED Flasher Circuit
- Generating High Voltage with an Inductor
- Coils for Highly Selective Crystal Radio
- Neon (NE-2) Circuits You Can Build
- Understanding Xenon Flashtubes and Circuits
- Comparator Theory Circuits Tutorial
- Comparator Hysteresis and Schmitt Triggers
- Voltage Comparator Information And Circuits
- Looking at Window Comparator Circuits
- Test Power MOSFET Transistors, IGBTs Results, Observations
- High current TTL MOSFET Driver Circuit
- Insulated Gate Bipolar Transistor IGBT Circuits
- Optocouplers for TTL-CMOS Logic Level Shifting
- Light Activated SCR Based Optocouplers Circuit Examples
- Bidirectional Solid State Relay Circuits
- Build High Power MOSFET Bidirectional Switch Relay
- Digital Circuits
- Basic TTL Tri-State Buffer Circuit Examples
- Tutorial NOR Gate SR Latch Circuits
- Tutorial NAND Gate SR Latch Circuit
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