|NotTaR of Television Sets : Low voltage power supply fundamentals
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Low Voltage Power Supply Problems
TVs require a variety of voltages (at various power levels) to function.
The function of the low voltage power supply is to take the AC line input
of either 115 VAC 60 Hz (220 VAC 50 Hz or other AC power in Europe and
elsewhere) and produce some of these DC voltages. In all cases, the power
to the horizontal output transistor of the horizontal deflection system
is obtained directly from the low voltage power supply. In some cases,
a variety of other DC voltages are derived directly from the AC line by
rectification, filtering, and regulation. In other designs, however, most
of the low voltages are derived from secondary windings on the flyback
(LOPT) transformer of the horizontal deflection system. In still other
designs, there is a separate switchmode power supply that provides some or
all of these voltages. There are also various (and sometimes convoluted)
combinations of any or all of the above.
There will always be:
- A power switch, relay, or triac to enable main power.
- A set of rectifiers - usually in a bridge configuration - to turn the
AC into DC. Small ceramic capacitors are normally placed across the
diodes to reduce RF interference.
- One or more large filter capacitors to smooth the unregulated DC. In
the U.S., this is most often a voltage around 150-160 V DC. In countries
with 220 VAC power, it will typically be around 300-320 V DC.
- A discrete, hybrid, or IC regulator to provide stable DC to the horizontal
deflection system. Sometimes feedback from a secondary output of the
flyback or even the high voltage is used. This regulator may be either
a linear or switching type. In some cases, there is no regulator.
- Zero or more voltage dividers and/or regulators to produce additional
voltages directly from the line power. This relatively rare except for
startup circuits. These voltages will not be isolated from the line.
- A degauss control circuit usually including a thermistor or Posistor
(a combination of a heater disk and Positive Temperature Coefficient (PTC)
thermistor in a single package). When power is turned on, a relatively
high AC current is applied to the degauss coil wrapped around the periphery
of the CRT. The PTC thermister heats up, increases in resistance, and
smoothly decreases the current to nearly zero over a couple of seconds.
- A startup circuit for booting the horizontal deflection if various voltages
to run the TV are derived from the flyback. This may be an IC or discrete
multivibrator or something else running off a non-isolated voltage or
the standby power supply.
- A standby power supply for the microcontroller and remote sensor. Usually,
this is a separate low voltage power supply using a small power transformer
for line isolation. However, some sets use other (probably cheaper)
approaches. See below.
Always use an isolation transformer when working on a TV but this is
especially important - for your safety - when dealing with the non-isolated
line operated power supply. Read and follow the information in the section:
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