Any electronic device can fail. If a stand-alone product is designed correctly, failures are mostly due to operating it outside of specifications, mains related induced failures such as lightning strikes, a grey component failing, end of life failures, etc. and it’s normally fairly easy to establish the cause of the failure. However, in the case of a complex system where multiple electronic products and often different brands are connected together, failures are much more difficult to pinpoint – very often leading to the different suppliers blaming each other for the failure in a system.
Although a dimmer connected to dimmable electronic lamps or control gear seems simple, it’s fundamentally a very complex electronic system which is further complicated due to the number of operating parameters and the sheer number of different products available. “Low voltage” dimmers, such as 0-10V, DALI, etc. which are typical “specifier” products interface often via an isolated port to the low voltage control inside the control gear. It thus does not manipulate the mains supply to the controlled device and it usually does not cause damage to the controlled device or Visa Versa.
Phase cut dimmers, leading or trailing edge, however directly influences the mains supply to the controlled device and this is where most failures are possible. Obviously dimming a fluorescent or CFL load will result in a premature burner or tube failure if the cathode heating is not optimised (premature blackening of the tube ends) – there are numerous articles and data available from leading manufacturers to substantiate this phenomenon.
But what about solid state lighting where there are no cathodes that need constant heating during dimming? The LED component relies on an internal or external electronic driver to ensure that it operates at the operating voltage and current specifications. But can the driver be damaged by a dimmer, surely the dimmer is in-line with the LED and it acts as a switch?
The simple answer is yes – there can be a fundamental incompatibility between the LED driver and the dimmer which leads to spurious and often very large current pulses into the driver. Oscillogram 1 shows the measured LED voltage (red) and current (yellow) of a GU10 LED lamp being dimmed with an incompatible dimmer. It is obvious that the wave shapes are far from ideal – there are current spikes, the voltage and current are unsymmetrical etc. and severe flickering occurs which places electrical stress on the components and can result in premature failure.
If the lamp flickers on a dimmer, one can intuitively sense that something could fail, but what if the lamp dims perfectly and flicker free? Can premature failures still occur? Unfortunately yes!
Oscillogram 2 shows the measured voltage and current of a 9W LED operating on direct mains, no dimmer. The yellow current trace appears to be smooth and there are no sharp pulses. In this case, the product data sheet states: “trailing edge dimming recommended” When the same lamp is connected to a leading edge dimmer, it dims well and does not flicker. The measurements, however, reveal a possible problem: Oscillogram 3 shows that there are 1.2A current pulses into the LED lamp, 100 times a second! This will place large electrical stress on the LED’s electronic components if it was not designed for these pulses and premature failure of the lamp is very likely. Similarly, let’s assume there are 20 of these lamps in a dimming circuit. The dimmer will be subjected to 24A current pulses 100 times a second! The dimmer’s internal power switch (Triac, transistor, MOSFET, etc.) is designed and dimensioned for a certain maximum peak and RMS operating current. If it’s operating outside of these specifications, there could be several failure mechanisms occurring due to the Electrical Over-Stress (EOS) as is shown in Photo 1 (curtesy Cypress)
Importantly, both the leading edge dimmer and LED lamp measured in the example comply to the mandatory EMI (CISPR15) requirements for any lighting related product. Should the dimmer and/or LED not comply, the damaging current pulses can be much higher.
In contrast to the leading edge dimming of the particular lamp, the recommended trailing edge dimming produces no adverse electrical conditions as is shown in Oscillogram 4.
Assuming that the correctly specified phase cut dimmer is used to dim an LED lamp and both are compliant, is optimum lifetime of both devices guaranteed? Not always – the stresses on both during start-up or turn-on must also be taken into account.
Oscillogram 5 shows that the 9W LED as well as the leading edge dimmer suffer a current pulse of 2.2A during start-up. Once again citing the example of 20 lamps per dimmer implies that the dimmer could be subjected to a 44A pulse every time the lamps are turned on – is this damaging? Most probably …
Oscillogram 6 shows a much more controlled start-up of this lamp with the recommended trailing edge dimmer.
The examples shown should however not be generalised: there are LED’s which exhibit exactly the opposite, with higher current pulses etc. using trailing edge dimmers rather than leading edge dimmers – it all depends on the design of LED driver (internal or external). It must also be emphasised that although it is always better to stress electronic components as little as possible to ensure optimum product lifetime, the high current pulses shown does not necessarily imply poor LED and dimmer compatibility. The only real way to be sure of compatibility before installation is to request a list of approved dimmers, dimmer types and maximum load per dimmer from the LED and/or dimmer manufacturers.