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How do I find the right transformer?

Transformers transform electric voltage from the socket outlet into a certain operating voltage (e.g 12 volt). In order to find the right transformer for your lamp, you need to satisfy the base load (minimum watts) that can be connected to the transformer. There are different types of transformers: conventional and electronic. If you want to dim your lamp, you need to use a dimmable transformer. When switching to LED, you need to consider the compatibility of all parts as well as the suitable base load.

hotel corridor lighting


The most important information at a glance:

  • Transformers transform the electric voltage
  • Conventional transformers consist of a magnetic core and copper coils
  • Electronic transformers operate with electronic circuits
  • The base load needs to be satisfied to ensure proper operation
  • For dimming, you need a dimmable transformer
  • Conventional transformers require a phase control dimmer
  • Electronic transformers require a reverse phase control dimmer
  • When switching to LED, satisfy the minimum base load


What is a transformer?

A transformer does exactly what its name implies: it transforms, specifically it transforms electrical voltage. The operating voltage of a lamp can differ from the mains voltage of the socket outlet. In order for the light to function nonetheless, the transformer transforms the mains voltage of 230 V into 12 V, for example. For lighting, this is the most common scenario. But there are also transformers for 6 and 24 V. In case a low-voltage lamp does not have an integrated transformer, it requires an external one.


How does a transformer work?

Mode of operating: conventional transformer

A conventional transformer consists of a magnet. Usually, this is a ferrite core which is wrapped with two copper coils. If both coils have the same number of windings, the voltage remains the same. If you reduce the number of windings for the second coil (secondary coil), the voltage is reduced as well.

Mode of operating: electronic transformer

Electronic transformers transform the mains voltage by means of electronic circuits. This results in a higher frequency and a more efficient mode of operating. In addition, they have an integrated overvoltage protection. Therefore, electronic transformers are not only more efficient, but also more compact and lighter. Due to these significant advantages, conventional transformers are less and less commonly used.

Dimming with a transformer


hand operating dimmer

Do you want to dim your 12 V halogen lamp? This is easily possible! You can either use a conventional or an electronic transformer. However, it is crucial that lamp, transformer and dimmer are compatible with each other.

As a rule, phase control dimmers are combined with conventional transformers. Reverse phase control dimmers are suitable for electronic transformers. In addition, there are universal dimmers that can be used with all types of transformers.

LED transformer: switching from halogen to LED

Because LEDs use so much less energy than conventional halogen lamps, you are sure to save electricity. At the same time the low energy consumption can lead to problems with the transformer. The transformer requires a so-called base load (in watts) in order to operate properly. If you connect a modern LED to the transformer, it is possible that the required base load is not met, resulting in flickering or poor light, or no light at all.

Please pay attention to the minimum (and maximum) watt the transformer requires. This information can be found on the type label of the transformer (see picture). The overall number of watts for all connected lights needs to be within the range of the base load in order to ensure proper functioning of the transformer.

Grundlast Osram HTM Halotronic auf Typenschild


Example: switching from halogen to LED with a transformer

A transformer with the base load 20-70 W is designed for two lights. In addition to the actual consumption of the LED, you have to add the so-called output reserve. In order to not impair the lifetime of the transformer, it should not run on 100 % of its maximum base load. LEDs as well as other lamps require more energy for a short time when they are switched on. If this exceeds the maximum base load, it can trip a fuse and the transformer is switched off. Our recommendation: Always factor in at least 20-30 % reserve. Accordingly, in our example, you can use two LED lights with 8 W each and still have a 4 W reserve (2x 8 W = 16 W + 20 % reserve = 20 W).




If the combination of low-voltage transformer and LEDs causes problems, switching the transformer might do the trick. There are special LED transformers that are designed for an especially low base load.

Dimming with LED transformer

Do you want to dim your LEDs? First of all, please make sure that all parts are marked as dimmable. This means that not just the (LED) transformer needs to be dimmable, but also the LED itself as well as the ballast, if required. Please pay attention if dimmer and LED are compatible with each other. Manufacturers offer relevant compatibility lists. Also note that the dimmer, as well as the transformer, requires a certain base load in order to operate correctly.


We recommend:


Osram HTM 70VA 230V Halogen/LED

Osram HTM 70VA 230V Halogen/LED

  • base load: 20-70 W
  • for two lamps
  • dimmable

Osram HTM 105VA 230V Halogen/LED

Osram HTM 105VA 230V Halogen/LED

  • base load: 35-105 W
  • for two lamps
  • dimmable

Osram HTM 150VA 230V Halogen/LED

Osram HTM 150VA 230V Halogen/LED

  • base load: 50-150 W
  • for up to six lamps
  • dimmable