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CIRCUIT DESCRIPTION
Commonly Used Shutdown Detection Circuits
Excessive Current Detection
One very common circuit used in Zenith television products is the B+ Excessive Current Sensing circuit. In
this circuit there is a low ohm resistor in series with the particular power supply, (labeled B+ in the drawing).
The maximum current that is allowable within a particular power supply determines the value of this resistor.
In the case of
Figure 1
, the value is shown as a 0.47 ohm, however it could be any low ohm value. When
the current demand increases, the voltage drop across the resistor increases. If the voltage drop is sufficient
to reduce the voltage on the base of the transistor, the transistor will conduct, producing a Shutdown signal
that is directed to the appropriate circuit.
Voltage Loss or Excessive Load Detection
The second most common circuit used is the Voltage Loss Detection circuit. This is a very simple circuit that
detects a loss of a particular power supply and supplies a Pull-Down path for the base of a PNP transistor.
This circuit consists of a diode connected by its cathode to a positive B+ power supply. Under normal
conditions, the diode is reversed biases, which keeps the base of Q1 pulled up, forcing it OFF. However, if
there is a short or excessive load on the B+ line, the diode in effect will have a LOW on its cathode, turning
it ON. This will allow a current path for the base bias of Q1, which will turn it ON and generates a Shut-
down Signal.
B+ Voltage Too High Detection.
In this circuit, a Zener diode is connected to a voltage divider or in some cases, directly to a B+ power
supply. If the B+ voltage increases, the voltage at the voltage divider or the cathode of the zener diode will
rise. If it gets to a predetermined level, the zener will fire. This action creates a Shutdown Signal.
Negative Voltage Loss Detection.
The purpose of the Negative Voltage Loss detection circuit is to compare the negative voltage with its
counter part positive voltage. If at any time, the negative voltage drops or disappears, the circuit will pro-
duce a Shutdown signal. In Figure 5, there are two resistors of equal value. One to the positive voltage,
(shown here as +12V) and one to the negative voltage, shown here as -12V. At their tie point, (neutral
point), the voltage is virtually zero (0) volts. If however, the negative voltage is lost due to an excessive load
or defective negative voltage regulator, the neutral point will go positive. This in turn will cause the zener
diode to fire, creating a Shutdown Signal.
ZP94/95 Shutdown Circuit
There are a total of 16 individual Shutdown inputs. In addition, there are also two Shutdown inputs that are
specifically detected by the main power driver IC, I901 that protect it from excessive current or over
voltage. The four previously described circuits can categorize all of the Shutdown detection circuits.
Voltage Loss Detection
· Shorted SW+2.5V on Signal PWB through Protect 1 to (D957) on Sub Power Supply PWB
· Shorted SW+9V (D015) on Signal PWB through Protect 1 to (D959) on Sub Power Supply PWB
· Shorted SW+5V (D014) on Signal PWB through Protect 1 to (D959) on Sub Power Supply PWB
· Shorted SW+3.3V (D016) on Signal PWB through Protect to (D959) on Sub Power Supply PWB
· Shorted Stby+3.3V on Signal PWB through Protect 2 to (D959) on Sub Power Supply PWB
· Shorted Stby+5V (D032) on Signal PWB through Protect 2 to (D959) on Sub Power Supply PWB
· Shorted Stby+9V (D007) on Signal PWB through Protect 2 to (D959) on Sub Power Supply PWB
· Shorted Stby+3.3V (D016) on Signal PWB through Protect 2 to (D959) on Sub Power Supply PWB
