A Comparison of Light-Emitting Diode 

Power Supply Circuits 

by Arthur Harrison 

ARL-TN-284 July 2007 

Approved for public release; distribution unlimited. 
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Army Research Laboratory 

Adelphi, MD 20783-1197 

ARL-TN-284 July 2007 

A Comparison of Light-Emitting Diode 

Power Supply Circuits 

Arthur Harrison 

Sensors and Electron Devices Directorate, ARL 

Approved for public release; distribution unlimited. 
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A Comparison of Light-Emitting Diode Power Supply Circuits 

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Arthur Harrison 

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13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

In applications where LEDs use a large proportion of power and cell life is an important consideration, a power-conserving 

technique utilizing a switched circuit offers a considerable improvement in cell life. Two circuits, one utilizing a conventional 

method of resistive current limiting, and the other utilizing a switching method, were constructed and evaluated. The switching

method yielded a 39% improvement in the duration of LED illumination. 

15. SUBJECT TERMS

LED 

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Arthur Harrison 

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Contents 

List of Figures iv

Overview 1

Synopsis 1

Conclusion 3

References 5

Distribution List 7 

iii
List of Figures 

Figure 1. Simple LED circuit..........................................................................................................1 

Figure 2. Switched LED circuit. .....................................................................................................2 

Figure 3. Schematic of circuit used for testing. ..............................................................................3 

iv
Overview 

The Prognostics and Diagnostics program at the Army Research Laboratory uses various circuits 

that have light-emitting diodes (LEDs) as visible status indicators, powered by a 3-volt lithium 

cell. In these applications, where cell life is an important consideration, LEDs use a large 

proportion of power. This technical note provides the results of an experiment conducted to 

examine the merits of a power-conserving technique used in a LED power supply circuit. 

Synopsis 

A visible LED may be illuminated with a direct current provided by a 3v lithium cell, where the 

current delivered to the LED is determined by a series resistor, as shown in figure 1. 

Figure 1. Simple LED circuit. 

The LED current is defined by the equation: 

ILED = (VB1-VfLED) / R1 = 990 microamperes (uA) 

where: 

B1 = 3 Volts (V) lithium cell 

VB1 = 3.0V 

Vf(LED) = Voltage across LED = 2.1V 

 R1 = Resistance = 909ohms 

The voltage across the resistor, VR1, 0.9V, results in a power loss defined by the equation: 

Ploss = (VR1)^2 / R1 = 0.891 milliwatts (mW) 

1
Another method for illuminating the same LED is to dispense with the resistor, and replace it 

with a semiconductor switch, metal oxide semiconductor field effect transistor (MOSFET), Q1, 

controlled with a rectangular waveform, as shown in figure 2. 

Figure 2. Switched LED circuit. 

When 3V is applied to the gate of Q1, its channel resistance becomes less than 1ohm, providing 

nearly the full available cell current to the LED, limited only by the internal resistance of the 

cell, the on resistance of the MOSFET, and the resistance of the circuit's conductors. Since the 

MOSFET exhibits a very low on resistance of less than 1ohm, the associated power loss in the 

MOSFET is nearly zero. 

If the MOSET were to remain on, the resulting steady-state current may exceed the LED's 

maximum power rating and damage it. However, by regulating the on time and frequency of the 

MOSFET gate waveform, the average brightness of the LED may be set to equal the same 

steady-state value obtained in figure 1, while still operating the LED within its allowable pulsedmode ratings. 

Figure 3 shows the practical circuit employed for implementing the switching scheme. The type 

CD40106BE complementary metal oxide semiconductor (CMOS) six-section inverting Schmitt 

trigger integrated circuit (IC) was used for its low voltage operation capability. A relaxation 

oscillator is comprised of inverting Schmitt trigger U1A in conjunction with frequencydetermining components resistor, R1, and capacitor, C1. The oscillator provides a frequency of 

approximately 800Hz. The oscillator's output is followed by a differentiator, comprised of 

capacitor, C2, and resistor, R2, with inverting Schmitt trigger, U1B, providing waveform 

restoration. The values of C2 and R2 are selected to provide an appropriate on-off ratio (duty 

cycle) for the LED. The four remaining inverting Schmitt triggers serve as a gate driver for the 

MOSFET. Capacitors C3 and C4 provide bypassing for the cell, lowering the power supply 

impedance to prevent the LED current impulses from affecting the oscillator. The figure 3 

circuit, exclusive of the LED, uses approximately 10uA of battery current, which amounts to a 

power loss of 30 microwatts (uW), which is approximately one-thirtieth the power loss apparent 

in the circuit of figure 1. 

2
Figure 3. Schematic of circuit used for testing. 

For the circuit shown in figure 3, the initial conditions were as follows: 

MOSFET on time = 22uS 

MOSFET off time = 1200uS 

Oscillator frequency = 818Hertz (Hz) 

LED duty cycle = 1.80% 

Cell voltage = 2.25V with the LED on 

After 500 hours, the conditions were: 

MOSFET on time = 6uS 

MOSFET off time = 2700uS 

Oscillator frequency = 370Hz 

LED duty cycle = 0.22% 

Cell voltage = 2.40V with the LED on 

The large variation in conditions results from the cell’s voltage decrease during the test. More 

consistency of the timing parameters would be achieved in a system-based circuit employing a 

crystal-controlled timing generator, however, the autonomous circuit of figure 3 satisfied the 

requirement to evaluate the power-saving merits for a single LED, using a circuit with a 

minimum number of components. 

Conclusion 

The circuits of figures 1 and 3 were constructed with LEDs and power cells from the same lot, 

and were run concurrently. The brightness of the LEDs was evaluated on a subjective basis by 

several individuals, with the initial brightnesses being equal. After approximately 360 hours of 

3
continuous operation, the LED in figure 1 exhibited approximately half of its initial brightness, 

while the LED in figure 2 exhibited half of its initial brightness after approximately 500 hours, 

yielding a 39% improvement in duration. At 500 hours, the cell voltage in the figure 1 circuit 

was 1.50V, and the LED was almost completely extinguished. 

4
References 

 Data Sheet, Toshiba TLRA120 LED. 

 Data Sheet, International Rectifier IRLD024 MOSFET. 

 Data Sheet, Panasonic BR2320-1HE Lithium Cell. 

5
INTENTIONALLY LEFT BLANK

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