Video signal splitter with DC short circuit protection

A system for supplying control signals, DC power and video signals to a plurality of video system components includes a DC signal splitter and an RF signal splitter. The RF signal splitter carries video signal to a multiplicity of components. The DC signal splitter carries DC power and low frequency control signals to the multiplicity of components. Each of the signal paths for the DC power and the control signals includes DC short circuit protection, which prevents the occurrence of a short circuit in one output from causing the entire system to fail.

BACKGROUND OF THE INVENTION 
This invention relates generally to signal splitters that carry RF signals, 
DC power and low frequency AC control signals. This invention relates 
particularly to a signal splitter for connection to a coaxial cable that 
carries modulated video signals, DC power and low frequency control 
signals. Still more particularly, this invention relates to apparatus and 
methods for providing DC short circuit protection in a signal splitter for 
connection to a coaxial cable that carries RF video signals, DC power and 
low frequency control signals. 
Many homes and businesses have video signal distribution systems for 
distributing video signals to two or more devices such as television 
receivers and video cassette recorders. The source of the video signal is 
typically an antenna or cable television (CATV) system. The video signals 
are typically distributed between various video devices via coaxial 
cables. 
In a typical home where CATV is the video signal source, a decoder box is 
connected directly to the CATV outlet, and other devices receive video 
signals directly or indirectly from the decoder box. Signal splitters are 
typically used to interconnect the components of a video system. 
Television sets and video cassette recorders may often be operated by 
remote control systems for functions such as channel selection and volume 
control. Such control systems typically use infrared signals to transmit 
control signals between a hand-held transmitter and a receiver included in 
the video device. Ordinarily a direct line of sight is required between 
the transmitter and the receiver. 
It is possible to use the coaxial cables that carry the video signals for 
carrying control signals and DC power. This permits a viewer in a location 
not having a direct line of sight between the receiver and the hand-held 
transmitter to have control of a decoder box, a VCR or a television set. 
The transmitter sends infrared (IR) control signals to an auxiliary 
receiver having a direct line of sight to the transmitter. The auxiliary 
receiver is connected to the coaxial video distribution cables and 
converts the IR control signals into electrical signals that are carried 
on the coaxial cables to a signal converter. The signal converter that 
converts the electrical signals back into IR control signals that are 
retransmitted via IR to the video device whose operation is to be 
controlled. 
In modern video distribution systems it is possible to supply power to the 
signal converter by means of the coaxial cable. The signal converter may 
typically require 12 volts DC for its operation. Therefore, the coaxial 
cables carry video signals, control signals and DC power. A video system 
may include several components that draw DC power from the coaxial cable. 
When all the components of a video system are properly connected, there is 
ordinarily no problem with using the coaxial cables to carry DC power and 
more than one type of AC signal. However, it is possible for components to 
be improperly connected during installation so that a DC short circuit 
exists. In present video systems a short circuit on one outlet of a signal 
splitter stops operation of the entire system and may damage the DC power 
supply. Determining the cause of the malfunction or locating an improperly 
connected device is similar to the problem of determining which light bulb 
is faulty in a series string of Christmas tree lights. 
SUMMARY OF THE INVENTION 
The present invention provides short circuit protection in a signal 
splitter that carries video signals, control signals and DC power to a 
multiplicity of components in a video system. The present invention thus 
prevents the occurrence of a short circuit in one output from causing the 
entire system to fail. If a short circuit should occur in one component, 
it is a simple matter to identify which component is inoperable. 
A system according to the present invention for supplying DC power, control 
signals and video signals to a plurality of video system components 
comprises a coaxial cable that guides the video signals, the control 
signals and the DC power. The invention includes means for separating the 
control signals and the DC power from the video signals so that the 
control signals and the DC power follow a plurality of paths separate from 
the video signals. Each DC path includes means for limiting the electrical 
current that may be conducted to provide low frequency short circuit 
protection in the plurality of paths. 
The means for limiting the electrical current may comprise a thermistor 
connected in series in each of the paths that carry the control signals 
and the DC power. The means for limiting the electrical current may 
comprise a transistor circuit in each path designed to limit the current 
to a predetermined value. 
The transistor circuit in each path may comprise a sensing transistor, a 
sensing resistor and a series pass transistor. If there is no short 
circuit at the output of the series pass transistor, the DC power and the 
control signals pass through the sensing resistor into the emitter of the 
series pass transistor and out of the collector of the series pass 
transistor. The sensing resistor is connected across the base-emitter 
junction of the sensing transistor. The collector of the sensing 
transistor provides the base drive to the series pass transistor to 
control whether the series pass transistor is in a conducting or 
non-conducting state. When the electrical current through the sensing 
resistor exceeds a predetermined value, then the base drive of series pass 
transistor is reduced to prevent excessive current flow out of the 
collector of the series pass transistor. 
The system according to the present invention may further include a 
capacitor connected across the emitter-base junction of the sensing 
resistor to provide a resistance-capacitance circuit having an RC delay 
that prevents shut down of the system in the presence of transient 
overloads having a duration less than a predetermined time.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention is described with reference to a four-way signal splitter 
merely for purposes of illustrating the structure and advantages of the 
invention. The invention is not in any way limited to a four-way signal 
splitter. The invention may be included in a signal splitter that splits 
an input electrical signal into any number of electrical signal outputs. 
Referring to FIG. 1, a video signal distribution system 10 includes a power 
supply 12 connected to supply DC electrical power to a coaxial cable 14. 
The coaxial cable 14 is connected to the input port of a four-way signal 
splitter 16. The signal splitter 16 divides electrical signals input 
thereto and provides electrical signal outputs to coaxial cables 20-23. 
The coaxial cables 20-23 are connected to provide signals to video system 
components 24-27. 
Thermistors 30-33 are connected between the input and the four outputs, 
respectively of the signal splitter 16. The thermistor 30, for example, 
provides DC short circuit protection if the video system component 24 
should for any reason become a DC short circuit. The upper curve of FIG. 3 
illustrates current-voltage characteristics of a typical positive 
temperature coefficient thermistor. For a range of voltages, the ratio of 
the current to voltage is a constant, which for a typical thermistor may 
be about one ohm. After the voltage reaches a predetermined value, the 
current remains constant. 
The knee in the thermistor current-voltage curve represents a current that 
is less than what a 75 .OMEGA. load would draw without causing damage to 
the device connected to the output terminal. In the video signal splitter 
according to the present invention the constant current output of the 
thermistor is set to a value that does not damage components connected to 
the thermistor. Because each leg of the signal splitter has its own 
thermistor, a short circuit in one leg cannot cause other legs to cease 
proper operation. 
The current I.sub.IN that is shown being input to the video signal 
distribution system 10 includes the RF video signals, low frequency 
control signals and the DC power. A suitable inductor 34 connected to the 
splitter 16 prevents the RF video signals from passing through the 
thermistors 30-33. The thermistors 30-33 have no effect on the 
transmission of the RF video signals to the video system components 24-27. 
An RF signal splitter 35 divides the RF video signals and directs them to 
each of the video system components 24-27. DC blocking capacitors 36-39 
connected to the outputs of the RF signal splitter 35 prevent low 
frequency signals from following the same path as the RF video signals to 
the video system components 24-27. 
Therefore, it may be seen that the RF video signals are separated from the 
DC power and the low frequency control signals. The low frequency short 
circuit protection has no effect on the transmission of the RF video 
signals to the video system components 24-27. However, if any of the video 
system components includes a DC short circuit, the thermistors 30-33 
prevent the DC power and the low frequency control signals from exceeding 
predetermined values. 
The invention also may be embodied in a transistor circuit that is 
essentially the equivalent of a thermistor. Referring to FIG. 2, a four 
way video signal splitter 48 includes a transistor circuit in each of its 
four legs. The signal splitter 48 may include the RF signal splitter 35 
connected to the blocking capacitors 36-39 of FIG. 1. 
A signal input I.sub.IN that includes RF video signals, low frequency 
control signals and DC power is input to the four way video signal 
splitter 48. The RF components of the signal I.sub.IN are directed to the 
RF signal splitter 35. The low frequency control signals and the DC 
component of the signal I.sub.IN are directed to a low frequency signal 
splitter 60 having legs 60A, 60B, etc. 
The DC signal splitter 60 includes four legs 61A, 61B, 61C and 61D, which 
may be essentially identical in structure. Therefore, only the structure 
and method of operation of the leg 61A is described in detail. The legs 
61A-61D are arranged to provide control signals and DC power to terminals 
indicated by the labels OUT 1-OUT 4, respectively in FIG. 2. The 
components of the leg 61A are referenced by a numeral followed by the 
letter "A." Components of the legs 61B, 61C and 61D are referenced by the 
same numeral is in the leg 61A followed by the letters "B," "C," and "D" 
as appropriate. The DC component of the signal I.sub.IN first passes 
through an inductor 62 that functions as an RF choke to prevent the RF 
portion of the signal I.sub.IN from being affected by the DC signal 
splitter 60. 
The control signals and the DC power are applied to the emitter of a 
sensing transistor 64A and to a sensing resistor 66A that has one terminal 
connected to the emitter of the sensing transistor 64A. The sensing 
transistor 64A may be a 2N2907, and the sensing resistor 66A may have a 
resistance of approximately 5.1 .OMEGA.. The other terminal of the sensing 
resistor 66A is connected to the emitter of a series pass transistor 70A, 
which may also be a 2N2907. 
The sensing resistor 66A and the emitter of the series pass transistor 70A 
are connected to a resistor 72A, which is connected to the base of the 
sensing transistor 64A. The resistor 72A may have a resistance of about 
100 .OMEGA.. 
The collector of the sensing transistor 64A is connected to a first 
terminal of a resistor 74A. The second terminal of the resistor 74A is 
grounded. The resistor 74A may have a resistance of about 5.1 K.OMEGA.. 
A resistor 76A has a terminal connected between the base of the sensing 
transistor 64A and the collector of the series pass transistor 70A. The 
resistor 76A may have a resistance of about 2 K.OMEGA.. The base of the 
transistor 70A is connected to the collector of the sensing transistor 
64A. The other terminal of the resistor 76A is connected to the collector 
of the series pass transistor. 
A capacitor 78A has its terminals connected between the emitter and base of 
the sensing transistor 64A. From the circuit diagram of FIG. 2, it may be 
seen that the capacitor 78A is also connected between the emitter of the 
sensing resistor 66A and the resistors 72A and 76A. The capacitor 78A may 
have a capacitance of about 100 .mu.F. The capacitance of the capacitor 
78A is preferably sufficiently large that transients will be smoothed and 
squashed. When a 75 .OMEGA. load is connected to the corresponding output 
of the signal splitter 48, the capacitor 78A begins to charge, and the leg 
61A begins to cut off. 
The lower current-voltage curve of FIG. 3 illustrates the characteristics 
of the legs of the signal splitter 60. The resistors in the legs 61A-61D 
preferably are adjusted so that the current throughput of each leg is 
limited to about 50 ma and then reduced to about 20 ma when the capacitor 
78A is fully charged. 
The resistors 72A and 76A provide current feedback to the transistor 64A. 
These resistors 72A and 76A act to sense the voltage across the transistor 
70A to maintain an essentially constant voltage across its 
emitter-collector junction. The resistor 74A is a DC biasing resistor that 
turns on the transistor 70A. The transistor 64A works against the resistor 
74A to shut off the transistor 70 if a short circuit should occur in the 
video system component connected thereto. The voltage divided effect of 
the resistors 72A and 76A causes the transistor 64A to reduce the base 
drive current to the transistor 70 as the voltage across the transistor 70 
increases. 
An inductor 80A has one terminal connected to the resistor 76A and the 
collector of the series pass transistor 70A. The inductor 80A may have an 
inductance of about 3.3 .mu.H, which allows the inductor 80A to act 
essentially as a low frequency short circuit. The other terminal of the 
inductor 80A is connected to an output terminal of the signal splitter 48. 
The output of the leg 61A of the signal splitter 60 must therefore pass 
through the inductor 80A. 
The series pass transistor 70A normally is saturated so that current input 
to the sensing transistor 66 flows through the sensing resistor 66A to the 
emitter of the series pass transistor 70A. The current then flows out of 
the collector of the series pass transistor 70A, through the inductance 
80A to the load that is connected to the terminal OUT 1 of the signal 
splitter 48. If the load should become a short circuit and draw a current 
greater than a predetermined value, then the voltage across the sensing 
resistor 66 rises to a value sufficient to cause conduction between the 
emitter and collector of the sensing transistor 64A. As the sensing 
transistor 64A becomes conductive, it removes the base drive from the 
series pass transistor 70A. The series pass transistor 70A therefore shuts 
off when the sensing transistor 64A becomes sufficiently conductive and no 
current is delivered to the terminal OUT 1. 
The structures and methods disclosed herein illustrate the principles of 
the present invention. The invention may be embodied in other specific 
forms without departing from its spirit or essential characteristics. The 
described embodiments are to be considered in all respects as exemplary 
and illustrative rather than restrictive. Therefore, the appended claims 
rather than the foregoing description define the scope of the invention. 
All modifications to the embodiments described herein that come within the 
meaning and range of equivalence of the claims are embraced within the 
scope of the invention.