Note: Descriptions are shown in the official language in which they were submitted.
CA 02213193 1997-08-15
ELECTRONIC CIRCUIT FOR PROTECTING POWER SUPPLY TO A LOAD
This invention relates to an electronic circuit for providing power
supply protection to a load and particularly for providing limitation of
current
supplied to the load to maximum current and for halting supply of current to
the
load when the maximum current has been supplied for a predetermined time.
Various techniques are widely used for protecting power supply to a
load to prevent damage to the load in the event of a fault.
SUMMARY OF THE INVENTION
It one aspect of the present invention to provide an improved circuit
for this purpose which is relatively simple, inexpensive and can be located in
a
limited space.
According to the first aspect of the invention there is provided an
electronic circuit comprising:
a first terminal for connection to a power supply;
a second terminal for connection to a load;
a control transistor and a first resistor connected in series
between the first terminal and the second terminal so that current to the load
passes through the control transistor and the first resistor, the control
transistor
having a first connection for controlling the flow of current through the
control
transistor;
a first circuit element connected to said first connection and
arranged such that generation of a voltage across the first resistor
approaching a
predetermined voltage by a current through the first resistor and the first
transistor to the load approaching a pre-determined maximum current causes the
control transistor to limit current to the load to said pre-determined maximum
current;
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a capacitor/resistor circuit element arranged so as to be
responsive to said actuation of the control transistor to control current to
cause
charging of a capacitor at a predetermined rate;
and a threshold device responsive to charging of the capacitor
to a predetermined voltage to cause the first circuit element to actuate the
control
transistor to halt current through the control transistor to the load.
Preferably the threshold device is arranged such that discharge of the
capacitor caused by disconnection of the power supply or the load causes the
threshold device to fall below a threshold and thus allow current through the
control transistor.
Preferably the capacitor is connected in series with a second resistor
and wherein the capacitor and a second resistor are connected in parallel to
the
first resistor and the control transistor.
Preferably the threshold device is connected between the capacitor
and the second resistor.
Preferably the first circuit element comprises a transistor.
Preferably the first circuit element comprises an operational amplifier
arranged to compare the voltage across the resistor with a reference voltage.
Preferably the threshold device comprises a light emitting diode so as
to indicate actuation of the threshold device.
Preferably the control transistor is of the bipolar type and wherein
the first connection comprises a base of the control transistor.
Preferably the control transistor is of the field effect type and
wherein the first connection comprises a gate of the control transistor.
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According to the second aspect of the invention there is provided an
electronic circuit comprising:
a first terminal for connection to a power supply;
a second terminal for connection to a load;
a first transistor and a first resistor connected in series
between the first terminal and the second terminal so that current to the load
passes through the first transistor and the first resistor, the first
transistor having
a first connection for controlling the flow of current through the first
transistor;
a second transistor connected to said first connection and
arranged such that generation of a voltage across the first resistor
approaching a
predetermined voltage by a current through the first resistor and the first
transistor to the load approaching a pre-determined maximum current causes the
second transistor to control current in the first connection of the first
transistor so
as to actuate the first transistor to limit current through said first
transistor said
pre-determined maximum current;
a capacitor and a second resistor in series connected in parallel
to the first resistor and the first transistor arranged such that actuation of
the first
transistor to control current to cause charging of the capacitor at a
predetermined
rate;
a diode connected between the capacitor and the control
connection of the second transistor such that charging of the capacitor to a
voltage greater than a threshold voltage of the diode causes conduction
through
the second transistor to control current in the first connection of the first
transistor so as to actuate the first transistor to halt current through the
first
transistor to the load.
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BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described in conjunction
with the accompanying drawings in which:
Figure 1 is a first circuit schematic showing an example of a circuit
according to the present invention.
Figure 2 is a second circuit schematic showing an example of a
circuit according to the present invention.
Figure 3 is a third circuit schematic showing an example of a circuit
according to the present invention.
In the drawings like characters of reference indicate corresponding
parts in the different figures.
DETAILED DESCRIPTION
In Figure 1 is shown a power supply P and a load L together with a
protection circuit generally indicated at 10. The protection circuit comprises
a
first transistor 12 and a second transistor 14.
The circuit further includes a first resistor 16, a capacitor 18, a
second resistor 20, a third resistor 22, and a threshold device or diode 24.
The first resistor 16 and the first transistor 12 are connected in
series so that current from the power supply P passes through the first
transistor
and the first resistor.
The capacitor 18 and the second resistor 20 are arranged in series
with those elements in parallel to the first resistor and the first
transistor.
The second transistor has the gate thereof connected between the
first resistor 16 and the first transistor 12 through the third resistor 32.
The
second transistor is connected in a circuit connecting to ground and
controlling
CA 02213193 1997-08-15
the current from the base of the first transistor. Between the base of the
first
transistor and ground is connected a fourth resistor 28. The second transistor
14
is connected so that voltage across the emitter and base of the second
transistor
is responsive to the voltage across the first resistor 16.
5 In operation, normally, current to the load flows through the first
resistor and the first transistor. Provided this current is below the
predetermined
maximum current, the voltage across the first resistor is maintained below a
predetermined maximum voltage and the first transistor is maintained in
saturated
condition. The first resistor is selected so that when the voltage across the
first
resistor is less than the predetermined maximum voltage, the voltage across
the
emitter and base of the second transistor are less than the trigger voltage of
one-half volt so the second transistor is not conducting.
In the event that the current to the load approaches the
predetermined maximum current, the voltage across the first resistor also
approaches the predetermined maximum voltage which is sufficient to cause
conduction through the second transistor. This conduction causes current to
flow
to ground thus causing current to flow from the base of the first transistor
and
causing the first transistor to restrict the current to the load. The first
and second
transistors and the voltage across the first resistor 16 are therefore
maintained in
balance so the current through the first transistor is maintained at the
predetermined maximum current.
The reduction in current by the first transistor 12 causes an increase
in voltage across the capacitor 18. This voltage is dependent upon the amount
of
the overload condition which can vary from a small over voltage to a dead
short
condition. The capacitor 18 is thus charged at a predetermined rate dependent
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upon the overload condition and the selected values of the capacitance and the
resistor 20 until the voltage between the capacitor 18 and resistor 20 reaches
a
predetermined threshold voltage measured by the threshold diode 24. In the
event that the voltage exceeds the threshold voltage, the diode 24 comes into
conduction which causes the second transistor to come into conduction thus
causing the first transistor to switch off terminating the flow of current
from the
power supply to the load.
The simple circuit arrangement therefore instantaneously provides a
restriction on the current supplied to the load to a maximum predetermined
current in the event of a fault causing an over current condition at the load.
Within a predetermined time period dependent upon the overload
condition and prior to the generation of sufficient heat to cause damage, the
capacitor and threshold system cause the shut down of current flow.
Automatic resetting of the circuit occurs on discharge of the
capacitor 18 and this can be achieved by disconnecting the power supply or by
disconnecting the load in the event of a reconnection and a fresh overload
conditions, the circuit is again triggered to instantaneously limit the
maximum
current and then to halt that current after a predetermined period.
The device is particularly effective in that within a small area, an
instantaneous restriction of the current can be obtained to avoid a temporary
over
current condition causing damage to a sensitive load.
In Figure 2 is shown an arrangement which is substantially identical
to that of Figure 1 except that the control transistor 12 is replaced by a
field
effect type transistor 12A. In addition there is provided a further diode 30
which
acts to limit the forward gate voltage on the transistor 12A. A second diode
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which is LED 32 acts as an indicator in the event that the transistor 14 is
triggered by an overcurrent condition.
In Figure 3 is shown a further modified arrangement including the field
effect transistor 12A. In this arrangement the transistor 14 is replaced by
another
circuit element which is responsive to an increase in voltage across the
resistor
16 to trigger the control effect of the transistor 12A. In this arrangement
the
circuit element, provided by the transistor 14 in the previous embodiments, is
replaced by an operational amplifier 14A having an input voltage Vref for
comparison with the voltage across the resistor 16. Vref is generated by a
voltage divider 50 connected between the power supply and ground and therefore
is dependent upon any fluctuations in the power supply. This arrangement has
the
advantage that the voltage drop across the resistor 16 much smaller so that
the
value of resistor 16 can be reduced thus reducing power loss at the resistor
during normal operation.
In addition the input terminal 14B connected to Vref receives an
input from the voltage across the capacitor 18 so as to be actuated by the
charging of the capacitor 18 so as to turn off the transistor 12A as
previously
described.
Since various modifications can be made in my invention as herein
above described, and many apparently widely different embodiments of same
made within the spirit and scope of the claims without departing from such
spirit
and scope, it is intended that all matter contained in the accompanying
specification shall be interpreted as illustrative only and not in a limiting
sense.
