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:

Cutting-edge hybrid capacitors
Avnet Silica Passive Components
Panasonic Industry recently announced the launch of the ZVU Series Hybrid Capacitors, a cutting-edge solution tailored to meet the escalating demands of advanced electronic systems.

Read more...
Low-profile tantalum chip capacitors
Electrocomp Passive Components
These general-purpose tantalum capacitors from Kyocera AVX are available in multiple case sizes with low profile options.

Read more...
Compact high-performance antennas
Electrocomp Telecoms, Datacoms, Wireless, IoT
KYOCERA AVX offers a variety of extremely compact and high-performance internal, on-board, multiprotocol 2,4 GHz antennas ideal for use in SiP applications.

Read more...
Coupled inductor for high-performance applications
Passive Components
This coil with MnZn core is characterised by its high permeability and extremely low RDC values, which achieves excellent power density and very high efficiency.

Read more...
Track with precision
Electrocomp Telecoms, Datacoms, Wireless, IoT
KYOCERA AVX provides innovative antennas for cellular, LTE-M, NB-IoT, LoRa, GNSS, BLE, UWB, Wi-Fi, and future Satellite IoT.

Read more...
RGBIR sensor with I2C interface
Electrocomp Opto-Electronics
Available in a miniature opaque 2,67 x 2,45 mm package, Vishay’s VEML6046X00 includes high-sensitivity photodiodes, a low noise amplifier, and a 16-bit analogue to digital converter.

Read more...
Power inductors
iCorp Technologies Passive Components
he HTF-MP series is more suitable for complex multiphase power supply applications in design, effectively meeting the needs of ultra-thin and high-power devices.

Read more...
SMT power inductors
Future Electronics Passive Components
The Würth Elektronik WE-MXGI SMT power inductors are the latest addition to Würth Elektronik’s moulded power inductor series, engineered for high-frequency power applications.

Read more...
Large capacitance MLCCs at 100 V
RS South Africa Passive Components
TDK Corporation has expanded its CGA series for automotive multilayer ceramic capacitors to 10 µF at 100 V in 3225 size.

Read more...
Film and mica capacitors
Actum Electronics Passive Components
By utilising various polymer dielectrics plastics, Exxelia film and mica capacitors meet most technical requirements and serve all functions from standard filtering to specialised applications.

Read more...









While every effort has been made to ensure the accuracy of the information contained herein, the publisher and its agents cannot be held responsible for any errors contained, or any loss incurred as a result. Articles published do not necessarily reflect the views of the publishers. The editor reserves the right to alter or cut copy. Articles submitted are deemed to have been cleared for publication. Advertisements and company contact details are published as provided by the advertiser. Technews Publishing (Pty) Ltd cannot be held responsible for the accuracy or veracity of supplied material.




© Technews Publishing (Pty) Ltd | All Rights Reserved