Analyzing the development of circuitry protection devices heat a cathode preheating unit (CPU) cathode ray tubes (CRT), mostly television picture tubes, it is impossible not to draw attention to the lack of new technical solutions in the last few years. The main issue is the appreciation of the totality of the requirements of the protection device, since the improvement of some indicators associated with deterioration of others. This allows us to conclude the following: the possibility of circuitry based on the use of traditional hardware components, for this type of devices is practically exhausted.
Development of a device presented in this publication is based on the realization of the potential of the elemental base that have appeared in recent years. To ensure the effective protection of the heat of the CPU, increasing reliability, miniaturization and exceptions necessary adjustment device required circuit solutions:
- - reduce the number, volume and mass of the circuit elements;
- - to do without additional power sources;
- - reduce heat dissipation;
- - to carry the device on the base of high reliability.
In addition, expanded the scope of the protection device it without significant changes in the scheme can apply to any hardware displaying visual information based on the use of CRT for example in video monitors, display computers, CRT video projectors, oscilloscopes, etc [1].
Distinctive features of the proposed device are: - the use of the nonlinear element, specifically designed for smoothing the starting current to the filament of the CRT powerful thermistor direct heating with a negative temperature coefficient; - the use of contactless solid-state relay as a switching element; - locking of the CRT during warmup of the CPU.
Next, the operation of the device is illustrated with the example of the protection of the Communist party of the kinescope LK.
Schematic diagram of the device shown in the figure and consists of a thermistor R3, relay DA2, site management relay DA2 on a chip of DA1.1 and node signal blanking of the kinescope on a chip of DA1.2. The thermistor R3 type TR-16-0,8 connected in series in the heating circuit KPU of the tube and is designed to address inrush current, filament when you turn on the TV power. Cold its resistance is 16 Ohms, the resistance of the cold filament heater KPU Ro is about 3 Ohms. In this case the starting current is
Io= starting power Ro=Noa=6,3x0,33=2.1 W.
For comparison, the starting current filament unprotected CRT
I=6,3/3=2.1 A,
starting power Ro=6,3 2,1=13,23 watts.
Thus, the thermistor reduces the starting power more than 6 times. When you consider that in the filament circuit of the CPU of any modern CRT TV already has a current-limiting element is a resistor or an inductor, almost starting power is reduced to 7...8 times.
Solid state relays DA2 is designed to bypass the thermistor R3 after reaching its nominal mode. The signal of the relay is formed by a single vibrator DA1.1, the startup voltage. The duration of the current flow of heat through the thermistor R3 is set by the choice of the time constant circuit R1, C1 and is given by t[c]=1.1 R [Mω]C[µf].
The quenching voltage of the kinescope during warmup of the CPU produces a second single-shot DA1.2, the time constant of which is set by the circuit R4, NW and is calculated similarly. The voltage output from the DA1.2 is supplied to the node blanking of the kinescope, the diagram of which is determined by the model and are not included here, because its options are discussed in detail in [3]. Capacitors C2, C4 minimize the impact of noise and ripple on the supply lines to the work of single vibrators. Diodes VD1, VD2 suppress a possible surge when you turn on the TV. The power supply voltage is supplied from one of the tires of the TV and can be within 5...18 V without a significant change device settings, it is only necessary to adjust the value of the resistor R2 to provide current control relay DA2 is equal to 10 mA. The power consumed by the device in continuous mode after the warm-up of the CPU of the tube, less than 200 mW when powered 18 and 55 mW at a voltage of 5 V.
When switched on, the output of single-shot DA1.1 (pin 5) there is a low voltage, the discharge output (pin 1) is set to the low-impedance state, sunterra condenser NW and preventing it from charging. At the output of single-shot DA1.2 (pin 9) voltage high-level input at the node blanking of the kinescope, the current in the circuit of control relay DA2 (findings 10, 11) is missing. As a result, the tube is closed, the starting current of the CPU heat flows through the cold thermistor R3 and a current-limiting element provided for circuit TV that reduces the starting power 7...8 times. As the heating of the thermistor R3, the resistance decreases and the resistance of the heater increases the CPU. The output of the thermistor of this type of a nominal mode is 2...3, if necessary it can be increased if to glue the thermistor in a small radiator, the sizes of which are determined experimentally. The applied adhesive must be resistant to heat.
At the end of the time interval after turning on the TV, determined by the parameters of timing circuit R1, C1 and equal to about 10C, the output of single-shot DA1.1 goes high voltage level, and its bit output is set to high impedance. When it begins to charge the capacitor to the NW, the output of single-shot DA1.2 is still present voltage blanking of the kinescope, the controlling circuit relay DA2 begins to flow current control and power circuit is set to the low-impedance state. In the future, right up until you turn off the TV, the filament current flows through the contacts 2 and 6 relay DA2, and the thermistor R3 cools rapidly, preparing the device to the next turning on the TV. At the end of the time interval after the start of charging of the capacitor C3 is determined by the parameters of timing circuit R4, NW, and is approximately 20, the output of single-shot DA1.2 switches to the low state voltage. As a result, the tube opens, and then the TV is operating normally. Thus, the total delay time of the opening of the tube is 30 C.
The device is a dual timer ICM7556IPD firm MAXIM can be applied to any of the chipset series 556, such as specified in [4], or two chips single timer CREW (voltage - 5.-.15 In). Replace relay PA on electromagnetic impractical because of the low resource last. Next foreign counterparts: RVG612, PVAZ172N company INTERNATIONAL RECTIFIER. VD1, VD2, except indicated in the diagram, can be types KD, CKD, KD522 with any letter index. Capacitors C1, NW should take the time to failure under conditions of high ambient temperature not less than 10,000 hours and minimum leakage current. Most suitable according to the criterion of cost/efficiency are capacitors SR series Taiwanese production. Also suitable K52-16 K53-4, K53-18, K53-19. K53-29, K53-35 [2], but their cost is significantly higher. Capacitors C2, C4-type KM, K10-17.
Accurately collected from known-good elements, the device does not require networking.
Literature
Author: C. Mycin, Moscow region, Dubna; Publication: N. Bolshakov, rf.atnn.ru