APPARENT POWER
for LED light sources with 12 V alternating voltage
Optimum transformer design for LED illuminants with AC voltage
Apparent power and transformer design in a nutshell:
Apparent power plays a decisive role in the design of transformers for LED illuminants.
In addition to the rated power, the reactive power must also be taken into account in order to determine the entire apparent power. An optimum power factor close to 1 is desirable to maximize efficiency. When using 12V AC voltage, special transformer recommendations should be followed to avoid overheating and defects. With dimmable LEDs, the reactive power increases, which makes planning even more difficult.
APPARENT POWER AND TRANSFORMER DESIGN FOR LED BULBS AT 12 V AC VOLTAGE
It is crucial to deal with the apparent power of the LED bulbs in order to preserve the service life of the LED bulbs and transformers.
Undesired reacti ve power
Unlike regular light bulbs, LED bulbs are usually not ideal resistive consumers, but instead exhibit a capacitive or inductive load behaviour, depending on the design. This results in a phase shift, in addition to the nominal power (= active power) creating an undesired reactive power, which is expressed in VAr (previously blind watt bW), when in operation with AC.
Apparent power comes from reactive and active (= nominal) power
The apparent power is calculated from this reactive power and the nominal power. The apparent power is specified in VA and can therefore easily be compared with the power specifications on AC voltage transformers, which are also listed in VA.
Note:
Private or small consumers do not have to pay the additional reactive power of a bulb to the energy utility company. The incurred costs for the expansion of the distribution network required here (since more reactive power always occurs in the public mains) are shared across all consumers or only passed on to the largest consumers of reactive power (industrial companies or also solar energy suppliers).
Definition of power factor:
The ratio of nominal power to apparent power is the power factor (cos φ). The power factor specifies how much of the apparent power is converted into the desired active power. In the case of DC voltage, this is usually the factor of 1. The closer the power factor of electronic consumers with AC voltage comes to λ 1, the better and more expensive the electronics were designed. The power factor common on the market of, for example, transformers is between λ 0.6 and λ 0.95.
Transformer design is based on the apparent power
Transformer design is based on the apparent power Reactive power occurs with AC voltage, as the current and voltage do not oscillate synchronously, in contrast to DC voltage, where the current flows in one direction and does not consist of a phase shift. In capacitive or inductive loads such as LED drivers, current and voltage can get out of sync, causing the phase shift and generating reactive power. This reactive power constantly flows back and forth without doing any useful work and places a load on the transformer, any additional dimming actuator installed and the electronics of the illuminants. Therefore, not only the nominal power of the illuminants but also the apparent power must be taken into account when designing the transformer.
Definition of apparent power:
Apparent power is an operand and is made up of the actual active power (P) and the additional reactive power (Qtot). Furthermore, it is defined via the effective values of electric current (I) and voltage (U).
- S … apparent power
- U … voltage
- I … current
- P … active power
- Qtot … reactive power
Graphical representation of the apparent power
In electrical engineering, apparent power is the combination of active power (usable energy) and reactive power (non-usable energy) and is measured in volt-amperes (VA). A graphic with a glass of beer can illustrate this, with the beer representing the active power and the foam representing the reactive power - together they make up the entire apparent power.
Power factor and energy efficiency: The power factor, the ratio of nominal power to apparent power, shows how efficiently the electrical energy is used. A higher power factor, close to 1, indicates better and more efficient electronics. For AC voltage, the commercially available power factor of transformers is between 0.6 and 0.95.
Important: Never push the transformer to its power limit!
The resulting sum must NOT exceed the power of the transformer. This leads to malfunctions, failures, overheating and defects in the illuminants and, of course, in the transformer itself. The result is then the entire required power in VA. Ideally, you should add at least 10% reserve so that the transformer is never pushed to its limit.
Transformer dimensioning - table
On the following sides you will find our AR111, MR11 and MR16 LED illuminants with information on rated, nominal/effective and apparent power. Depending on the apparent power, we recommend the corresponding number of LED illuminants in the individual article boxes that can be connected to our two AC transformers. This ensures that there are no impairments, faults or failures that could be caused by incorrect transformer calculation/design.
Please note: With dimmable LED illuminants, the reactive power is sometimes elevated many times over, regardless of and disproportionately to the dimming levels! We therefore advise you to adhere to the specifications given here!
To avoid the problem of apparent power, it is also possible to use a (dimmable) DC transformer instead of an AC transformer. All our 12 V LED illuminants in the range can be operated on DC without any problems! When dimming LED illuminants such as AR111, MR11 and MR16, there may be an increase in reactive power. Here are the reasons for this:
- Non-linear loads and LED drivers:
LED illuminants use electronic ballasts (drivers) that are non-linear and produce harmonics. These harmonics distort the current flow and increase the reactive power. - Phase cut and phase cut dimmers:
Dimmers that cut off part of the sine wave of the AC voltage cause irregular current waves. These irregular waves lead to an increase in reactive power because they worsen the current and voltage ratios. - Capacitive properties of LED illuminants:
LED illuminants and their drivers have capacitive properties that cause the current to run ahead of the voltage. At different dimming levels, the phase shift between current and voltage can change, resulting in elevated reactive power. - Creating oscillating circuits and resonances
These resonances can additionally increase the reactive power, particularly with different dimming levels. - Unsuitable dimmers and transformers:
If the dimmers and transformers are not optimally matched to the capacitive properties of the LED illuminants, this can lead to instabilities and an increase in reactive power.
As reactive power occurs with alternating voltage because current and voltage do not oscillate synchronously, the use of direct voltage (DC) can solve this problem. With DC voltage, there is no phase shift between current and voltage, so there is no reactive power.
NUMBER OF LED BULBS PER AC VOLTAGE TRANSFORMER 12 VOLTS – OUR RECOMMENDATI ON!
Rated power (= light output) in watts » is the technically possible output of the LED chips Apparent power in VA » the apparent power takes the reactive power into consideration and serves as a calculation basis for the transformer design
Nominal power (= active power) in watts » is the actual power consumption in watts Transformer design in VA » is the maximum resulting apparent power of all connected LED bulbs on the square pulse AC voltage (undimmed)
AR111 G53 SPOT 11 W | 30°
Warm white: Item no. 111810
Neutral white: Item no. 111811
11,0 W Rated power LED Chip
11,0 W Nominal / active power
16,5 VA Apparent power
ONLY DD OPERATION POSSIBLE – OPERATION WITH TOROIDAL TRANSFORMER EXCEPT FOR 12 V AC
MR11 LED 2,5 W | 30°
Warm white: Item no. 111716
2,0 W Rated power LED Chip
2,0 W Nominal / active power
2,5 VAApparent power
MR11 LED 2 W | DIFFUSE
Warm white: Item no. 111718
Cold white: Item no. 111719
1,6 W Rated power LED Chip
1,6 W Nominal / active power
2,0 VAApparent power
MR11 LED 4 W | DIFFUSE | DIMMABLE
Warm white: Item no. 111973
Neutral white: Item no. 111974
4,0 W Rated power LED Chip
3,6 W Nominal / active power
4,3 VA Apparent power
MR11 LED SPOTLIGHT COB 3 W | 38° | DIMMABLE
Warm white: Art.-Nr. 111807
3,0 W Rated power LED Chip
2,6 W Nominal / active power
3,2 VA Apparent power
MR16 LED SPOTLIGHT 6 W | GLASS | DIFFUSE
Warm white: Item no. 112339
Neutral white: Item no. 112340
6,0 W Rated power LED Chip
6,1 W Nominal / active power
7,2 VA Apparent power
MR16 LED SPOTLIGHT 6 W | GLAS-COB | 70° DIMMABLE
Warm white: Item no. 112036
6,0 W Rated power LED Chip
5,3 W Nominal / active power
6,7 VA Apparent power
Questions and Answers About Apparent Power & Driver Design
Apparent power is a calculated quantity that consists of the actual active power (P) and the additional reactive power (Q). It is specified in volt-amperes (VA) and is decisive for the dimensioning of transformers.
LED lamps are not ideal resistive loads and, depending on their design, show capacitive or inductive load behavior. This leads to a phase shift between current and voltage, resulting in unwanted reactive power in addition to active power. This reactive power must be considered when selecting a transformer, as it increases the transformer load and can lead to overheating or defects.
To calculate the apparent power, add the active power (in watts) and the reactive power (in volt-amperes reactive, VAr). The resulting apparent power is specified in volt-amperes (VA) and should not exceed the maximum power of the transformer.
The power factor is the ratio of active power to apparent power. A value close to 1 means that energy is used efficiently. For LED lamps, the power factor should be as high as possible to minimize the transformer load and extend its lifespan.
The number of connectable lamps depends on their apparent power and the maximum power of the transformer. It is important to calculate the total apparent power of the connected lamps and ensure that it does not exceed the transformer’s capacity.
For dimmable LED lamps, reactive power increases with increasing dimming, making planning more difficult. Therefore, the maximum possible reactive power should be considered when selecting the transformer to avoid overloads.
Operating the transformer at its maximum power can lead to overheating, malfunctions, and premature failures. It is recommended to always plan a reserve power of at least 10 % to ensure safe and long-lasting operation.
Yes, with DC voltage, there is no phase shift between current and voltage, so no reactive power is generated. Therefore, with DC operation, the apparent power can be equal to the active power, simplifying transformer selection.
ISOLED provides tables listing the apparent power of various LED lamps and recommendations for the number of connectable lamps depending on the transformer model. These tables help in safe planning and avoiding overloads.
Incorrect dimensioning can lead to overheating, malfunctions, premature failure of LED lamps and the transformer, and increased energy consumption. Careful planning and selection of the correct transformer are therefore essential.
Yes, ISOLED provides extensive information, tables, and recommendations to assist in selecting and sizing the correct transformer for your LED lighting. For further questions, the ISOLED team is happy to help.
Do you have any questions?
If you have any questions about apparent power and transformer design, we are here for you! Fill out our form and we will get back to you as soon as we have processed your enquiry.
Register now!
Register and take advantage of the many exclusive benefits and discounts for ISOLED customers.