Control gears for LED luminaires

In order to operate LEDs, control gear is required. There are two different types of control gear:

  • constant current sources (see figure a) and

  • constant voltage sources (see figure b).

Figure 3.194: LED connection types

Since LEDs respond to the smallest voltage fluctuations with pronounced current fluctuations due to their steep current-voltage profile, they are often operated with constant current sources. They supply the LED with ideal, stabilised direct current within the permissible operating conditions.

Figure 3.195: Schematic current/voltage profile of an LED with deviations due to material and manufacturing tolerances. The tolerances in forward voltage emerging with constant current lead to minor tolerances in power consumption.
P = (U ± ΔU) ⋅ I

Figure 3.196: Schematic current/voltage profile of an LED with deviations due to material and manufacturing tolerances. The tolerances in forward current emerging with constant voltage lead to significant tolerances in power consumption.
P = U ⋅ (I ± ΔI)

One great benefit of this type of control gear is that – to a limited extent – varying numbers of LED or LED modules can be operated in serial connection without the need for adjustments to the settings of the control gear unit in question. The maximum total power of LEDs to be operated at one control gear unit therefore depends on the maximum possible output voltage. The permissible output voltage range can be found in the control gear unit’s data sheet (see table).

Table 3.139: Example for an excerpt from an LED driver data sheet. This data sheet specifies the permissible connected load range which can be realised through serial connection of LEDs or LED modules.

Since direct voltage up to a value of 120 V is categorised as low voltage, this type of circuit facilitates operation of LEDs with a typical current consumption of 700 mA at a total power of 84 watts without exceeding the low voltage range. Furthermore, if such luminaires are also operated using a control gear unit providing safety extra-low voltage (SELV), they are categorised as safety class III and are subject to fewer restrictions for use in wet areas (see chapter ff).

Some LEDs common on the market, however, are also operated with voltage sources. In such cases, LED modules usually contain electronic components to stabilise current in the internal module circuit. Otherwise, even small tolerances in semiconductor properties would lead to pronounced differences in current/power consumption, which would make precisely defined operation impossible (see figure and figure).

Several modules of this type with internal stabilisation can be operated at one voltage source in parallel connection, going up to the number at which the control gear unit’s capacity is reached.

The LED connection has practical consequences for the integration of luminaires into an electrical installation if several luminaires are operated at one control gear unit on secondary side. To this end, the installation instructions must be observed, and the use of specific accessories for electrical connection may be required.

For luminaires with integrated control gear, surpassing safety extra-low voltage may require elevated protection against accidental contact on the inside if there is a risk of contact with live components during maintenance.