LED balancing circuits

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LED balancing circuits

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LED balancing circuit 1 (see Figure 1)

LED balancing circuit 1 (see Figure 1) contains two bipolar transistors and eight resistors.

The current passing through each load (where a 'load' is one string of LEDs in series, for

example, D1 and D2 or D3 and D4) is primarily the sum of the current through one resistor

and one parallel transistor. When the load voltages are the same, an equal amount of

current passes through the two transistors (Q1 and Q2) and an equal amount of current

passes through the two resistors (R1 and R5).

When the load voltages are different, less current passes through the resistor with the

lowest voltage (higher load voltage) and more current passes through the parallel transistor.

For either case, the total current from both loads is constant (from the power supply) and the

circuit will provide equal current through each load.

LED balancing circuit 1 uses LED loads D1 and D2, D3 and D4 and a power supply output

current through load D1and D2 is the sum of the current through R1 and Q1. The current

through the load D3 and D4 is the sum of the current through R5 and Q2. As each string

contains the same number of LEDs, any number of LEDs can be used.

The voltage at V1 is the input voltage, minus the load voltage VD1 and VD2. The voltage at

V5 is the input voltage, minus the load voltage VD3 and VD4. The current through R1 is the

voltage V1 divided by the resistor R1, and the current through R5 is the voltage V5 divided

by the resistor R5. The voltage at V3 is the voltage at V5 reduced by the divider resistors R2

and R3 (V5 x R3 / (R2 + R3)) and the current through Q1 is V3 minus the base emitter

voltage (

) divided by R4. The voltage at V7 is the voltage at V1 reduced by the divider

resistors R6 and R7 (V1 x R7 / (R6 + R7) and the current through Q2 is V7 minus the base

emitter voltage (

) divided by R8.

When the load voltages are equal, VD1 plus VD2 is equal to VD3 plus VD4, the voltage at

V1 equals the voltage at V5, the current through R1 equals the current through R5, and the

current through Q1 equals the current through Q2.

When the load voltages are not equal, VD1 plus VD2 is greater than VD3 plus VD4, V1 is

lower, V5 is higher and the current through R1 is reduced. Since V5 is higher, the current

through Q1 increases. The circuit values are selected so that the sum of the current through

R1 and Q1 remains the same. Conversely, V5 is higher and the current through R5 is higher.

Since V1 is lower, the current through Q2 decreases. The circuit values are selected so that

the sum of the current through R5 and Q2 remains constant.

If one LED (D1) is open, there is no base current to Q2 through R6 and Q2 will turn off. All of

the LED current passes through D3, D4 and R5. The maximum LED current can be limited

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