Control

1. Voltage mode

Voltage mode, e.g. with a battery or a power supply unit, is a simple and cost-effective way of controlling LEDs. All that is needed is an additional series resistor R_V. Due to the linear behavior of an ohmic resistance, R_V makes the overall circuit much less sensitive to voltage changes, resulting in robust LED control. Due to the ohmic resistance, however, the power loss of the control increases and is given off in the form of heat (P_V = R_V ⋅ I_LED²).

The current I_LED through the LED results from the ratio I_LED = U/R_V.

The series resistor R_V is calculated as follows:

R_V = U_R /I_LED

The current through the LED is known. The voltage U_R, which is dropped across the series resistor to be calculated, is missing. This voltage is formed from the operating voltage minus the voltage dropped across the LED. The voltage U_LED that is dropped across the LED at I_LED can be read from the U/I characteristic curve for the LED in the data sheet.

U_R = U - U_LED

If the brightness of an LED with a series resistor is to be adjusted, the applied voltage must be reduced (darker) or increased (brighter).

However, the disadvantage of this type of control is that the luminous flux cannot be controlled precisely. As described at the beginning, a small change in voltage leads to a big change in current and thus to a big change in the luminous flux. In the case of voltage control, therefore, fluctuations in the supply voltage may have a direct influence on the luminous flux of the LED. It should also be borne in mind that the electrical characteristics of the resistor and the LED are temperature dependent.

• Advantages: simple design, simple control, the brightness of the LED can be set directly via the voltage
• Disadvantages: additional resistance, resulting in waste heat

2. Current mode

An LED can be operated directly if a power source (electronic circuit) is used instead of a voltage source (e.g. battery). With current control, the luminous flux of the LED can be adjusted directly via the specified current value, without resistance. Fluctuations in the supply voltage thus have no influence on the luminous flux of the LED. The luminous flux is constant and reproducible with current control. Current control is thus recommended, for example, in machine vision applications.

• Advantages: no additional components; the brightness of the LED can be adjusted directly via the current
• Disadvantages: complex power source may be required