Design LM317 Constant Current Circuits
YouTube videos for this project:
- Constant Current Source Tutorial YouTube
- LM334 CCS Circuits with Thermistors, Photocells YouTube
- LM317 Constant Current Source Circuits YouTube
- Build H-Bridge Motor Control without Fireworks
A constant current source (CCS) in electronics is a device/circuit that produces a constant value of current regardless of source voltage or load resistance. A constant current circuit can also be used as a current limiter.
Earlier we discussed various constant current circuits using a PNP transistor, a LM741 OP-AMP and settled for a LM344 constant current source as the most stable. The downside is the LM334 can only source 10mA.
The next series of circuits will use the LM317. While designed as a variable voltage regulator, it will double as a stable constant current source. With only three pins as shown above, one can produce a current of about 1-amp max. See LM317 Spec sheet.
Fig. 1 shows the LM317 used as a series pass constant current source with a single resistor. Resistor range is: 0.8 ohms < R1 < 120 ohms. The formula is Iout = 1.25V / R1. The 10 ohm resistor produces and current output of 125mA. For the TO-220 package I max = 1A while the TO-92 package is 500mA.
LM317 driving three high-power white LEDs at 120mA. One can vary Vin from 15 to 35 volts and Iout will stay a constant 120mA.
In Fig 3 the current has dropped below 120mA due to Vin of 12-volts to two close to the output voltage for the load. In general Vin > Vload + 5V. This makes LM317 useless as a constant current source below 5-volts.
In Fig 4 we removed a LED for a voltage drop on the load at 6.4V restoring current regulation. Once we have the proper Vin vs. Vout the load has no effect on the current.
In Fig. 5 we have only a single LED yet the current remains 120mA. The load doesn't change the current from a constant current source in fact I could short out the output and maintain 120mA. Note the LM317 will get hot - and voltage not dropped by the load will be dropped by the pass transistor, LM317 etc. Don't go wild with a large Vcc powering a small voltage load.
Fig. 5 shows the LM317 trickle charging a 5-volt NiCd battery pack.
This completes our introduction to the LM317 constant current source.
Above we boost the current out of a LM317. See LM317 Adjustable Voltage current Boost Power Supply
- Experiments with TL431 Shunt Regulator
- TL431 Precision Current Regulator Circuits
- TL431 Based Current Limiter Constant Current Source Circuits
- TL431 Shunt Regulator Circuits
- Comparing ATmega328 Based Transistor Checkers
- Basic MOSFET Transistor Test Circuits
- High Voltage MOSFET Switching Circuits
- LM317 Adjustable Voltage current Boost Power Supply
- LM317 High Power Constant Current Source Circuit
- Constant Current Circuits with the LM334
- LM334 Constant Current Source with Resistive Sensors
- LM317 Constant Current Source Circuits
- Introduction Hall Effect Switches, Sensors, and Circuits
- Using Ratiometric Hall Effect Sensors
- Pulse Width Modulation Power Control for Microcontrollers
- Introduction to PIC12F683 Programming
- Basic Transistor Driver Circuits for Micro-Controllers
- Opto-Isolated Transistor Drivers for Micro-Controllers
- Introduction to Python Bitwise Operations
- Python Bitwise Operations by Example
- ULN2003A Darlington Transistor Array with Circuit Examples
- Tutorial Using TIP120 and TIP125 Power Darlington Transistors
- Driving 2N3055-MJ2955 Power Transistors with Darlington Transistors
- Understanding Bipolar Transistor Switches
- N-Channel Power MOSFET Switching Tutorial
- P-Channel Power MOSFET Switch Tutorial
- H-Bridge Motor Control with Power MOSFETS
- More Power MOSFET H-Bridge Circuit Examples
- Build a High Power Transistor H-Bridge Motor Control
New January 2018:
- Arduino Nano Test Template
- Arduino Solid State Relay Motor Enable Control
- Arduino Blink LED Tutorial
- Arduino SSR Power Enable Program
- SSR Based High Voltage H-Bridge
- Arduino H-Bridge Motor Control Program with LCD Display
- Arduino XOR Blinks LED
- Arduino IF Statement Code Examples
- Arduino Motor Control Program Using IF
- LM317 Adjustable Voltage Source Current Boost
- Geiger Counter Adventures in Radioactivity
- Introduction to Geiger-Mueller Counters and Electronics
- Astable CD4047 Geiger Counter Power Supply
- CD4047 Monostable Multivibrator Circuit
- Getting Real About Radiation Myths and Hazards
- Uranium Hype-Facts and Virginia Uranium
- Uranium Basics and Isotopes
- Climate Change, Climate Scams, and Volcanoes
- Technology is Why the Jobs are Not Coming Back
- Off-the-Shelf Technology for Space Exploration