LM317 used as a series pass constant current source with a single resistor.
Fig. 1

Design LM317 Constant Current Circuits

by Lewis Loflin

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.

See Constant Current Circuits with the LM334.

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.
Fig. 2

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.

LM317 operating at cutoff no longer regulates current.
Fig. 3

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.

LM317 constant current source driving 2 LEDs.
Fig. 4

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.

LM317 constant current source powering single LED.
Fig. 5

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.

LM317 constant current source trickle charging a battery.
Fig. 6

Fig. 5 shows the LM317 trickle charging a 5-volt NiCd battery pack.

This completes our introduction to the LM317 constant current source.