Passive Components


Using a PTC thermistor for degaussing

28 February 2001 Passive Components

To minimise picture distortion and beam landing error (colour impurity), the shadow mask and associated metal parts of a CRT tube must be demagnetised at switch on. This is done by passing decaying AC through the degaussing coil. An alternating magnetic field is generated which gradually decays to demagnetise the tube.

BC Components says that by implementing its dual positive temperature coefficient (PTC) thermistor in the degaussing circuit, a significant improvement of picture quality can be achieved.

Connecting a PTC thermistor (mono PTC housed or leaded) in series with the AC mains and degaussing coil; as in Figure 1, is the simplest method of producing the required decaying current. At switch on, the PTC thermistor is cold and has low resistance, so a large inrush current flows through the degaussing coil. As both the temperature and, therefore, the resistance of the PTC thermistor increase, the current and magnetic field decay. The PTC temperature stabilises after a few minutes, leaving a small alternating residual current flowing through the degaussing coil.

Figure 1. Mono PTC arrangement
Figure 1. Mono PTC arrangement

Degaussing with dual PTC thermistors

To avoid picture distortion with large-screen television and high-resolution colour monitors, it is crucial that the residual current and hence the residual magnetic field, be as low as possible. A dual PTC thermistor in the degaussing circuit can achieve this.

The degaussing PTC is connected in series with the degaussing coil; see Figure 2. The heater PTC, with a higher R25 resistance (resistance at 25°C), is in parallel with the main supply. At switch on, the inrush current through the degaussing coil is high, raising the temperature and resistance of the degaussing PTC. The temperature of the heater PTC also increases and its heat is dissipated towards the degaussing PTC. This further increases the degaussing PTC resistance, so further reducing the residual current.

Figure 2. Dual PTC arrangement
Figure 2. Dual PTC arrangement

To maximise this heating effect and thereby minimise the residual current, the two thermistors are carefully matched and clamped in close thermal contact inside a PBTP (polybutyleneteraphthalate) case. The plastic composition of the case is self-extinguishing in accordance with UL94.V.0.

Further advantages can be obtained by using double mono PTC thermistors. This consists of a parallel combination of two degaussing PTC thermistors in one standard BC Components 3-pin housing. This component offers substantial benefits compared to a single PTC degaussing thermistor. Inrush currents can be higher than with normal dual or mono degaussing PTC thermistors and by doubling the normal ceramic volume, a smoother decay is obtained.



Credit(s)



Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

Braid tail swept elbow backshells
Electrocomp Interconnection
TE Connectivity is complementing its series of banding (BT) and Tinel ring (TXR) backshells with a braid tail backshell. The new 70 Series swept elbow backshells include a pre-terminated screenbraid sock ...

Read more...
Field-installable mini industrial I/O connector
Electrocomp Interconnection
The Industrial Mini I/O connector system made by TE Connectivity is a compact, wire-to-wire and wire-to-board solution for reliable serial, bus and Ethernet connections in industrial applications. At ...

Read more...
Digital humidity sensor withstands harsh conditions
Electrocomp Analogue, Mixed Signal, LSI
Sensirion is expanding its series of fourth-generation humidity sensors to include the SHT4xI-Digital sensor platform, which has been designed specifically for challenging industrial applications. The ...

Read more...
Digital flow sensor for respiratory devices
Electrocomp Analogue, Mixed Signal, LSI
The compact SFM3119 digital flow meter is the successor to Sensirion’s SFM3100 mass flow meter. Compared to its predecessor, the SFM3119 comes with a digital I2C output as well as improved specifications.  ...

Read more...
Liquid flow sensor in small planar package
Electrocomp Analogue, Mixed Signal, LSI
With a tiny footprint of only 10 x 10 mm, Sensirion’s LPG10-1000 liquid flow sensor provides optimal performance and speed in measuring liquid flow in a planar microfluidic glass substrate, with down-mount ...

Read more...
Optional inner panels for IP68/66 plastic enclosures
Electrocomp Enclosures, Racks, Cabinets & Panel Products
Hammond Manufacturing’s 1557 family has proved to be very popular since it was launched some two years ago and is increasingly being used as a housing for electromechanical components as well as PCB-mounted ...

Read more...
Common-mode choke for 35 A commercial applications
Altron Arrow Passive Components
Vishay has introduced a new IHCM common-mode choke for high-current commercial applications up to 35 A. Available with a low-profile SMT construction, the IHCM-2321AA-10 is more robust than bulky toroid-based ...

Read more...
High-voltage semiconductor-based capacitors
RFiber Solutions Passive Components
MACOM Technology Solutions’ KV CAPS are high density, high working voltage silicon capacitors which employ a novel topology that essentially folds the parallel plates of the capacitors into three-dimensional ...

Read more...
Very compact X2 capacitors
Electrocomp Passive Components
A recent introduction by TDK is a new series of very compact EPCOS X2 capacitors for noise suppression. They are rated for 275 V a.c. and cover a capacitance spectrum from 33 nF to 1 µF. Depending on ...

Read more...
Soft ferrite cores serving many applications
Sivan Electronic Supplies Passive Components
Ferrites are polycrystalline oxides manufactured by ceramic technology and belong to a class of materials that exhibit the technically useful property of ferromagnetism. In a ferromagnetic material, magnetism ...

Read more...