Application Note

V I S H A Y I N T E R T E C H N O L O G Y, I N C .
Resistive Products
Application Note
PTC Application Lighting
Ceramic PTC thermistors are widely used in CFL lamp
ballast designs to regulate the preheating and ignition timing
of the lamp. To properly design-in a PTC element, one
needs to know specific design parameters of the lamp to
make the right selection.
Some parameters have a direct influence on the pre-heating
time of the lamps filament. Some of them are related to the
lamp circuit, some of them are related to the PTC
parameters. To align them, a good understanding of how
they work together is needed.
Here is a description of the main parameters that will
determine pre-heat time:
• PTC related items:
- Cold resistance of the PTC (R25): lower values will
increase the trip or pre-heat time.
- PTC ceramic volume: smaller parts will decrease the
- Curie or switching temperature: the point where the PTC
resistance value will sharply increase will determine the
initial ignition time. The higher this temperature, the
longer it will take.
- PTC materials used: good thermal conducting leadwires like copper will slightly increase trip-times. Also
coating material on the PTC will lengthen somewhat the
ignition time. High thermal conductive leadwires,
seating plane distance to the ceramic body and the use
of coating will also determine partly the power
consumption of the PTC when in the operating “hot”
• Ambient and environment related items:
- Ambient temperature: lower ambient temperature inside
the lamp ballast will increase lamp ignition time.
• Lamp circuit related items:
- The amplitude of pre-heat current (effective value) that
is generated and is dependent on the load (PTC +
filament resistance + eventual parallel or serial capacitor
impedance) and max. output current that can be
produced: the higher this current, the faster the PTC will
heat-up, although the PTC itself will partly determine the
current flow. Only the portion of the current that runs
through the PTC will determine the ignition time.
- The frequency of the initial pre-heat current: as this
frequency is usually lower than the lamp operating
voltage frequency, it can have an influence on the
pre-heat time, taken into account that ceramic PTC’s
have a certain parallel capacitance value that will
determine which part of the current will generate the
necessary Joule effect to heat-up the PTC.
Document Number: 29020
Revision: 03-May-10
For technical questions, contact: [email protected]
As many parameters influence pre-heat or ignition time, it is
advisable to take some reference when choosing a specific
PTC for such an application. All published PTC’s are
specified at 25 °C and with trip-times measured at the
moment of sharp resistance increase or lamp-ignition,
currents being standardized at 50 Hz/60 Hz. Typically lamp
design engineers are looking for standard pre-heat times
between 0.5 s and 1.0 s. Longer ignition times have the
negative effect of giving rise to subjective impression that
the lamp has a starting problem. At extremely low ambient
temperatures this can result in a lamp that will not ignite. Too
short ignition times will lead to shortened life time of the
Application Note
PTC Application Lighting
A suggested design sequence first selects PTC (R25)
resistance to obtain desired pre-heat filament current. Next,
select PTC mass (diameter) and/or Curie switch
temperature for proper cathode pre-heat (pre-ignition)
timing. A wide variety of standard Vishay PTC Lighting
Thermistors are available for laboratory experimentation.
Since many parameters are influencing each other, trip or
pre-heat times can only be estimated. As a reference, typical
trip-times are specified at a certain current which needs to
be checked if it is a valid value in the application. When the
pre-heat current for a certain load resistance (PTC + lamp
filament + other impedances) is known, a trip-time can be
found in the relevant specifications of the Vishay PTC
For proper design-in of Ceramic PTC’s into HF lamp
ballasts, other factors will determine lamp and PTC life.
Among these factors are the wearing of solder-joints due to
repetitive cold-heat cycles. For the mostly used PTC’s with
the relatively low Curie or switching temperatures, cycling
time or number of switch-on cycles can easily go up to 50k
cycles or more. In some lamp ballast designs it is important
to know what will happen when the lamp itself is at end of
life and will not ignite anymore. In that case a relatively high
voltage can become present on the PTC for a prolonged
time. When selecting a device, this should be taken into
account as well as maximum peak ignition voltage.
Rectified Mains
(310 VDC for
230 VAC Mains)
Cathode Cathode
36 kHz
PTC Thermistor
Fig. 1 - Typical Electronic Ballast Circuit
Ignition Time Lamp (s)
PTC Current (mA)
R25: 100 Ω
Diameter: 5.5 mm maximum
- 20 °C
Tamb: 25 °C
Fig. 2 - Typical Current-Time Characteristic
More information on product selection can be found on:
For technical questions, contact: [email protected]
Document Number: 29020
Revision: 03-May-10