GenVI Dimmer
PTFD Explained
Operator Manual V1.0
Page 27
7
PTFD Explained
7.1
OVERVIEW
PTFD stands for “Pulse Tranformer Fired Dimmer”. It is a tried and proven piece of dimmer
technology that is used in top quality dimmers including LSC’s GenVI range. However, many
dimmer manufacturers now use low cost opto-isolator circuitry in their dimmers which can
have some short comings as described below.
7.2
WHAT IS PTFD?
PTDF dimmers use a small “pulse transformer” to control their internal power control device,
usually a triac or an SCR. We will use the Generic term “triac”. The triac is the part of the
dimmer that actually controls the voltage coming out of the dimmer to the load. A 12
channel dimmer needed 12 transformers, one per channel. The transformer is used to
provide isolation between the high voltage (100-240v) mains output and the low voltage
(~12v) of the dimmer electronics. A pulsed signal is fed into the input of the transformer,
and the output is connected to the input of the triac. The result is a dimmer that can control
virtually any type of connected load. This includes inductive loads such as anything with a
transformer in it. Examples are pin spots, 12v EVL dichroic lamp systems, reactive loads,
discharge lamps and fluorescent lamps.
In the 1980’s a new integrated circuit device became available known as an “opto-isolator”
(also known as an optocoupler or a MOC). This device uses a LED and a phototransistor to
provide the same level of high-low voltage isolation as a pulse transformer but at a far
lower cost. Due to the lower cost and the fact that an IC is easier to work with than a
transformer in manufacturing, the vast majority of dimmer manufacturers switched to this
great new device.
7.3
OPTO-ISOLATED PROBLEMS
The opto-isolators used in low cost dimmers have one major disadvantage. They do not
provide the power to drive the triac! Instead the dimmer circuitry relies on the connected
load to power the triac. Therefore the opto-isolated dimmer’s performance is dependent on
the load connected to it. In the case of a simple high power load such as a 1000 watt lamp
this is not usually a problem. The problems occur when you try to control any of the
following loads:
Very low power circuits such as a 15w festoon lamp.
Inductive loads, such as Pin spots or ELV 12v dichroic lamp systems.
Transformers such as gobo rotator power supplies.
Motors, such as mirror ball rotators.
Motorised Disco Effects. These often contain a motor and a transformer.
Reactive loads such as Intelligent lights, Discharge lamps and Fluorescent lamps
If the load is very small there is just not enough power to fire the triac. The result is that
most opto-isolated dimmers require a minimum load of 100 watts in order to work. If the
load is inductive or reactive then there is a problem with the voltage and current getting out
of phase with each other which can cause a false trigger of the triac, which causes the lights
to flicker or flash. If the load is both of the above such as a small transformer driving a
gobo rotator or a 35 watt dichroic lamp then some opto-isolated dimmers will turn on but
never turn off, so your light or gobo rotator keeps working at full power, even with the
dimmer control at zero!
A PTFD dimmer rack does not have any of these problems. It simply dims the light as the
operation of the triac is completely independent of the load.
7.4
HARD FIRING
Some manufacturers use opto-isolators but employ a technique called “Hard Firing” to try
and overcome the problems listed above. Hard firing involves sending a string of small
pulses to the opto-isolator (just like we do with our PTFD’s), so that if/when the triac
misfires or turns off from a lack of power, it will automatically re-trigger. Whilst this can
help the situation the dimmer still cannot control certain loads. Some sales jargon will try to
convince you that Hard Firing solves all the problems with opto-isolation. It does not!
Summary of Contents for Gen VI
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