Fig. 1

# LM334, LM317, TL431 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. Fig. 1 illustrates a simple constant current source (CCS) circuit using a PNP bipolar transistor.

The values of the Ic = Ib × Hfe (Beta) of the transistor. A constant current circuit can also be used as a current limiter.

Ic is collector-emitter current, Ib is base-emitter current, and Hfe is the DC gain of the transistor.

The above circuit suffers one weakness, current will drift with transistor temperature changes.

Maxim Semiconductor notes the following on why we need to use a constant current source:

When applying white LEDs for display back lighting or other illumination applications, there are two reasons to drive them with constant current: To avoid violating the Absolute Maximum Current Rating and compromising the reliability.

To obtain predictable and matched luminous intensity and chromaticity from each LED...The forward current vs. forward voltage of six random white LEDs (three from each of two manufacturers) ... driving these six LEDs with 3.4V, for instance, will cause their forward current to vary from 10mA to 44mA, depending upon the LED."

Besides LEDs constant current sources are used with resistive sensors such as photocells and thermistors for greater stability and for current limited power supplies. Also useful for testing and prototyping.

Fig. 2

Fig 2 illustrates a constant current source with a 741 op amp. See 3 Amp LM741 Op-Amp Constant Current Source.

In Fig. 1 Ib is controlled by a 1K resistor and a 5K potentiometer. With a VCC of 12-volts, we drop 0.6 volts across the base-emitter junction of Q1. We adjust the potentiometer for a base current of 3mA (.003 Amps.) If Q1 has a hfe of 50: Ic = 0.003 × 50 = 150mA or 0.15A.

These circuits are a must for operating high-power light emitting diode (LED) arrays. The circuit above is simple, can be a little unstable due to temperature drift with Q1 causing the current drift. That problem is minor compared to power supply drift can cause far greater instability.

Other constant current sources use the LM317 a popular variable voltage regulator.

The TL431A is another popular part in a small TO-92 package. In simple terms the TL431A acts a temperature compensated variable/adjustable Zener diode.

It can also act as a voltage reference or constant current source.

Fig. 3

Fig 3 uses a LM334 a three-terminal current source designed to operate at current levels from 1uA to 10mA as set by an external resistor Rset. The device operates as "a true two-terminal current source, requiring no extra power connections." It can also operate as a temperature sensor.

In this example I'm using the LM334 to control Ib on Q3. Rset is the R1 and R2 combination adjusted for 100 ohms. Iset = Ib = 67.7mV / Rset = 677uA. Ic = Ib * hfe; Ic = 677uA * 180 = 120mA. Q3 was a 2N2907. See LM334 Spec sheet.

This is far superior to the two earlier circuits because power supply swings produced little measurable change in Ic. But the LM334 suffers from a maximum drive current of only 10mA and there are many applications where far higher currents are need.

In our next section we explore using the LM317 variable voltage regulator in its constant current source mode.

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