We need to define the term “Digital Dimmer “ because it is often misused . There are still some manufacturers who offer as a Digital Dimmer their analog dimmer with a DMX input..
Until recently (year 2004) all major dimmer manufacturers such as Compulite , ETC, Strand were offering under this term a digital version of the
SCR
– based dimmer. This operates in FPC ( Forward Phase Control ) mode and it is suggested to call it a HYBRID Dimmer .( something between analog and digital ). The vast majority of the dimmer installed today are this type.( CompuDim , Sensor , SLD ).
The availability of IGBT as a power switch opened the way to realization of a true digital dimmer. It is the ability of IGBT (popup ch6-23 IGBT) to turn on – off anywhere within the cycle which makes it suitable for this task .
The table below summarizes the features of various types of dimmers :
A unit of four 2.4Kw digital dimmers unit, by Compulite.
FPC-Forward Phase Control
RPC-Reverse Phase Control
SIN-Sinewave
IGBT combines the features of low resistance, with simple, low-power drive which makes it ideal switch. It can be easily turned on and off in a microsecond and thus protect from
overload
by electronic circuitry which will cut off current limit in less than one microsecond,long before any electrical over-stress can occur. By electronically protecting the power switches it is not necessary to use grossly over-sized and therefore expensive devices.The ability to turn ON/OFF anywhere within the cycle led to development of a new concept called REVERSE PHASE CONTROL. Dimmers which use zero crossing (FPS) operate in the time domain and try to approximate the desired output voltage, while dimmers in RPS operate in
voltage
domain. i.e. control circuitry integrates the
AC
line
voltage
to develop an instantaneous, real-time reading of the power delivered to the
load
and turns off the
switch
when desired the output
voltage
has been reached. Any correction to the output value is made in the same half-cycle. Thus the dimmer is immune to zero-crossing noise and with always deliver the correct
voltage
to the load, inherently providing the function of
voltage
regulation. In order to reduce the
RFI
noise, the turning off can be done as slowly as desired, without being affected by the
load
value. Since no magnetic is involved, the dimmer operation is completely silent. IGBT dimmer produces a smooth dynamically controlled, exponential falling waveform whose shape is not effected by the
load
or power device.The duration of this falling transition is called “fall-time”.The RPC system still suffer from many of the same problems as any phase control dimmers.Reverse phase control dimming applies the
voltage
to the
load
at Zero volts and turns it off once the desired target
voltage
has been reached. This mode of operation is fine for tungsten (resistive) on electronic low-voltage
transformer
(capacitive), but is unusable with inductive loads. Neon, ballasted loads, fans, and small motors with generate destructive inductive sparks when dimmed by a reverse phase control dimmer. Reverse phase control dimmers must not be used with these loads and if dimming is required it must be switched to the mode forward phase control.Another problem is that both FPC and RPC dimmers generate triplen harmonics. A large amount of heat is produced in these dimmers during the transition( rise or fall time) since the IGBTs are operating in their linear region.
