All simple constant-current sources generally operate on the same principle: a current is allowed to flow through a resistance and some sort of regulator is used to try to hold the voltage across this resistance constant. If this is done using a transistor, there must be a voltage drop of approximately 0.6 V over the resistor in order to forward bias the base. However, in some cases, this yields an excessive loss, so an opamp with a reference source is used instead. The type LM334 adjustable current source has all of this ‘on board’ and regulates with a voltage drop of 64 mV. The associated schematic diagram shows a practical example of a current source using this indestructible IC.
Here R1 is the sense resistor that determines the current level. Its value is calculated using the formula
R1 = 0.064 ÷ current
For example, for a current of 20 mA, the value of R1 must be 3.2 Ω.
The illustrated circuit is exclusively intended to be used for small voltage spans and small currents since T1 can dissipate 100 mW at most. However, you are free to experiment with other components and component values. With the values shown on the schematic, the circuit is eminently suitable for powering a white LED with an operating voltage of 3.6 V from a 4-V lead-acid battery or a 4.5-V battery.
The LM134/LM234/LM334 are 3-terminal adjustable current sources characterized by:
- an operating current range of 10000: 1
- an excellent current regulation
- a wide dynamic voltage range of 1V t 10V
The current is determined by an external resistor without requiring other external components. Reverse voltages of up to 20V will only draw a current of several microamperes. This enables the circuit to operate as a rectifier and as a source of current in a.c. applications.
- Operates From 1V to 40V
- 0.02%/V Current Regulation
- Programmable From 1μA to 10mA
- True 2-Terminal Operation
- Available as a Fully Specified Temperature Sensor
- ±3% Initial Accuracy
- Current Mode Temperature Sensing
- Constant Current Source for Shunt References
- Cold Junction Compensation
- Constant-Gain Bias for Bipolar Differential Stage
- Micropower Bias Networks
- Buffer for Photoconductive Cell
- Current Limiter