CRT filament protection device. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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Analyzing the development of the circuitry of the filament protection devices of the cathode-heating unit (CPU) of cathode ray tubes (CRT), mainly television kinescopes, one cannot help but pay attention to the lack of new technical solutions over the past few years. The main problem remains the satisfaction of the entire set of requirements for the protection device, since the improvement of some indicators is associated with the deterioration of others. This allows us to draw the following conclusion: the possibilities of circuitry based on the use of a traditional element base for this type of device are practically exhausted.

The development of the device presented in this publication is based on the implementation of the capabilities of the element base that has appeared in recent years. To ensure effective protection of the KPU glow, increase reliability, miniaturization and eliminate the need for device adjustment, circuitry solutions were required that allowed:

• reduce the number, volume and weight of circuit elements;
• do without additional power sources;
• reduce heat generation;
• build the device on an element base of increased reliability.

In addition, the scope of the protection device has been expanded - it can be used without significant changes in the circuit in any equipment for displaying visual information based on the use of a CRT, for example, in video monitors, displays: computers, CRT video projectors, oscilloscopes, etc. [1].

Distinctive features of the proposed device are: - the use of a non-linear element, specially designed to smooth the inrush current of CRT heating - a powerful direct-heated thermistor with a negative temperature coefficient; - application of a solid-state non-contact relay as a switching element; - locking the CRT during the CPU warm-up.

Further, the operation of the device is considered using the example of protecting the KPU of the kinescope 61LK5Ts.

(click to enlarge)

The schematic diagram of the device is shown in the figure and consists of a thermistor R3, a relay DA2, a relay control unit DA2 on a DA1.1 chip and a kinescope blanking signal generation unit on a DA1.2 chip. The thermistor R3 type TR15-16-0,8 is connected in series to the KPU filament circuit of the kinescope and is designed to eliminate the inrush current of the filament when the TV is turned on. In a cold state, its resistance is 16 ohms, the resistance of the cold filament of the KPU heater Ro is about 3 ohms. In this case, the starting current is

Io= starting power Po=HoIo=6,3x0,33=2,1 W.

For comparison: the inrush current of an unprotected kinescope

I=6,3/3=2,1A,

starting power Po=6,3 2,1=13,23 W.

Thus, the thermistor reduces the starting power by more than 6 times. If we take into account that in the filament circuit of the KPU of the kinescope of any modern TV there is already a current-limiting element - a resistor or inductance, then in practice the starting power is reduced by 7 ... 8 times.

Solid state relay DA2 is designed to bypass the thermistor R3 after it reaches the nominal mode. The relay activation signal is generated by a DA1.1 single vibrator, which is triggered when the supply voltage is turned on. The duration of the filament current flow through the thermistor R3 is set by the choice of the time constant of the circuit R1, C1 and is calculated by the formula t[c]=1.1R [MΩ]C[μF].

The kinescope blanking voltage during the KPU warm-up is generated by the second DA1.2 single vibrator, the time constant of which is set by the R4, C3 circuit and is calculated similarly. The voltage from the DA1.2 output is supplied to the kinescope blanking unit, the circuit of which is determined by the TV model and is not given here, since its options are discussed in detail in [3]. Capacitors C2, C4 minimize the effect of interference and ripple in the power circuits on the operation of single vibrators. Diodes VD1, VD2 suppress possible voltage surges when the TV is turned on. The supply voltage is supplied from one of the TV buses and can be in the range of 5 ... 18 V without a significant change in the parameters of the device, it is only necessary to adjust the value of the resistor R2 from the condition that the control current value of the DA2 relay is equal to 10 mA. The power consumed by the device in a long-term mode after the end of the KPU kinescope warm-up does not exceed 200 mW when powered by 18 V and 55 mW when powered by 5 V.

When the TV is turned on, a low-level voltage appears at the output of the DA1.1 single vibrator (pin 5), the bit output (pin 1) is set to a low-resistance state, bypassing the capacitor C3 and preventing it from charging. At the same time, at the output of the DA1.2 single vibrator (pin 9), there is a high-level voltage supplied to the kinescope blanking unit, there is no current in the control circuit of the DA2 relay (pins 10, 11). As a result of this, the kinescope is closed, the starting current of the KPU filament flows through the cold thermistor R3 and the current-limiting element provided by the TV circuit, which reduces the starting power by 7...8 times. As the thermistor R3 warms up, its resistance decreases, and the resistance of the KPU heater increases. The time it takes for a thermistor of this type to reach the nominal mode is 2 ... 3 s, if necessary, it can be increased by gluing the thermistor to a small radiator, the dimensions of which are determined experimentally. The glue used must be heat-resistant.

At the end of the time interval after turning on the TV, determined by the parameters of the timing circuit R1, C1 and equal to approximately 10 s, the output of the DA1.1 single vibrator switches to a high voltage level, and its discharge output is set to a high-resistance state. At the same time, the capacitor C3 begins to charge, the kinescope blanking voltage is still present at the output of the single vibrator DA1.2, the control current begins to flow in the control circuit of the relay DA2, and its power circuit is set to a low-resistance state. In the future, until the TV is turned off, the filament current flows through contacts 2 and 6 of the DA2 relay, and the thermistor R3 cools down quickly, preparing the device for the next turn on of the TV. At the end of the time interval after the start of charging the capacitor C3, determined by the parameters of the timing circuit R4, C3 and equal to approximately 20 s, the output of the single vibrator DA1.2 switches to a low voltage state. As a result of this, the kinescope opens, and then the TV operates in normal mode. Thus, the total delay time for opening the kinescope is 30 s.

In the device, instead of the dual timer ICM7556IPD from MAXIM, you can use any of the microcircuits of the 556 series, for example, those indicated in [4], or two microcircuits of a single timer KR1006VI1 (supply voltage - 5.-.15 V). It is not advisable to replace the 5P19A1 relay with an electromagnetic one due to the low resource of the latter. The closest foreign analogues: RVG612, PVAZ172N from INTERNATIONAL RECTIFIER. VD1, VD2, in addition to those indicated in the diagram, can be types KD509, KD510, KD522 with any letter index. Capacitors C1, C3 must have a time between failures in conditions of elevated ambient temperature of at least 10000 hours and a minimum leakage current. The most suitable in terms of cost / efficiency are Taiwan-made SR series capacitors. Also suitable are 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 assembled from known good elements, the device does not require adjustment.

Literature

1. Vukolov N.I., Gerbin A.I., Kotovshchikov G.S. Receiving cathode-ray tubes: Handbook / Ed. Ulasyuka V.N. - M.: Radio and communication, 1993.
2. Capacitors: Handbook / I.I. Chetvertkov, M.N. Dyakonov, V.I. Presnyakov and others: Ed. I.I. Chetvertkova. - M.: Radio and communication, 1993.
3. Vetoshkin P. The device for "soft" turning on the kinescope. - Radio, 1994, N9, p.7.
4. Dudnik Yu. ICs of analog timers AS555N, AS556N. - Radio amateur, 1998, N1.C.40.

Author: S. Mitsin, Moscow region, Dubna; Publication: N. Bolshakov, rf.atnn.ru

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