A constant current source source can supply a fixed current to a load regardless of input voltage or load change. LM317 constant current source is one of the simplest design.
The LM317 IC is quite useful as a constant current source, works on a wide input voltage range, from 3 V up to 40 V, and cheap too, here’s the datasheet.
So, here’s the LM317 based constant current source, it’s design and a little about it’s working principle.
LM317 constant current source circuit diagram
First, have a look at the circuit diagram, it’s pretty basic.
Only three components, excluding the power source and connecting wires. It’s really simple, you can build the circuit on a bread board or just by soldering components to each other.
LM317 is said to be sensitive to reverse polarity, most designs uses a protection diode before the input. But that will add another voltage drop of around 0.7 V to 1 V, so I excluded it.
LM317 constant current calculator
The value of R sets the amount of constant current, use the formula below.
Where I is the amount of constant current and R is series resistor.
So R has to be precise enough and it should be capable of dissipating the generated heat.
Metal film resistors are good choice for their 1% tolerance. But you can’t find a 12.5 Ohm, 6.25 Ohm or 1.25 Ohm resistor for 100 mA, 200 mA and 1 Amp current respectively.
So the the trick is to use some higher value resistors in parallel combination. Like for a 12.5 Ohm resistor, you can use eight 100 Ohm resistor in parallel. Or for a 1.25 Ohm resistor, use eight 10 Ohm half watt resistor in parallel.
Using resistors in parallel has many advantage over a single resistor.
- A resistor bank can be much more precise than a single high power resistor.
- They can dissipate the heat more effectively than a single resistor.
- It’s easier to get small resistors than a single high power resistor, cheaper too.
What about the heat dissipation on the LM317 regulator IC ? LM317 is available in various packages, the commonly available TO-220 package can dissipate up to 500mW without heatsink for few minutes.
Prototype and testing
I built a prototype 100 mA LM317 constant current source on a breadboard.
The resistor bank is eight 100 Ohm carbon film resistors in parallel, around 12.8 Ohm resistance, measured with a cheap multimeter.
The output constant current is around 99.4 mA, quite close to the estimated value. The input voltage was around 4.10 V, a single 18650 Li-Ion cell.
I’ve used this to measure few low value resistors, 1 Ohm, 0.33 Ohm, and 0.05 Ohm. It performed surprisingly well, around 1% to 2% difference compared to the marked values.
You can use this current source in many ways, few of them below.
- Current regulated power supply.
- LED forward voltage measurement in constant current mode.
- Low resistance measurement.
- Battery capacity measurement.
- Constant current battery charger.
- Reliable LED driver, though not much efficient.
So, that’s all about this constant current source. The next constant current source will be a variable type, 4 selectable ranges, 1 A, 500 mA, 100 mA, 10 mA, stay tuned ! If you have any suggestion, question, just drop a comment.