DETECTEUR DE SURINTENSITES

CURRENT OVERLOAD DETECTOR

Abrégé anglais

ABSTRACT OF THE DISCLOSURE
A voltage drop develops across a thermistor
connected between a power source and a load in pro-
portion to the current draw. A zoner diode becomes
conductive in the reverse direction when the voltage
drop increases to a threshold, thereby connecting
the voltage drop to a light emitting diode (LED).
For detecting excessive current draw, the LED is
normally off.

Revendications

The embodiments of the invention in which an
exclusive property or privilege is claimed are defined
as follows:-
1. A circuit for providing an indication of an
excessive current drain on a DC source by a load and
for limiting the current drain on the source by the
load comprising:
a thermistor connected between the source and
the load for providing a voltage drop indicative of the
current drain and for limiting the current drain, where-
in the resistance of the thermistor increases in pro-
portion to the current there through,
a light emitting diode (LED) connected in
forward bias across the thermistor for providing the
indication of excessive current drain when energized,
and
a zener diode connected in reverse bias in
series with the LED across the thermistor so that the
LED is energized when the voltage drop is at least
the reverse breakdown voltage of the zoner diode.

Description

I
-- 1
This invention relates to the detection of
current overloads by solid state means.
historically, power systems utilized fuses
or circuit breakers for protection against faults in
wiring or loads. Both means isolate faults by mock-
animally breaking contact with the power source.
Both means are dependent on a thermal reaction to
circuit overload currents, thus are slow and tempt
erasure dependent. The thermal element of a fuse
is destroyed in the isolation process, while circuit
breakers are large and heavy as compared to circuit
i components.
Therefore, it is an objection of this
invention to protect power sources and distribution
equipment from faults in wiring or loads using solid
state means that are space and weight efficient. It
is a further object to provide for protection with-
out necessarily disconnecting the faulty circuit or
load. It is another object to detect current over-
load conditions.
In accordance with a particular embodiment circuit for providing an indication of an excessive
current drain on a DC source by a load and for limit-
in the current drain on the source by the load
includes a thermistor connected between the source
and the load for providing a voltage drop indicative
of the current drain and for limiting the current
drain, wherein the resistance of the thermistor in-
creases in proportion to the current there through.
A light emitting diode (LED) is connected in forward
bias across the thermistor for providing the indict-
lion of excessive current drain when energized. A
zoner diode is connected in reverse bias in series
with the LED across the thermistor so that the LED
is energized when the voltage drop is at least the
reverse breakdown voltage of the zoner diode.
I

~66;~Z
-- 2 --
According to the invention, a power source
is connected to a load by a thermistor. Under normal
operating conditions the voltage drop across the
thermistor is small. When the load draws excess
current (fault), such as in a short circuit, the
voltage drop across the thermistor increases, non-
linearly. In response to a threshold voltage drop
across the thermistor, a zoner diode becomes conduct-
ivy in the reverse direction (reverse breakdown) and
flows current to an indicator, such as an LED.
The foregoing and other objects, features
and advantages of the present invention, will become
more apparent in the light of the following detailed
description of the invention.
Figure 1 is a schematic of a prior art current
overload detector circuit.
Figure 2 is a schematic of the current over-
load detector circuit of this invention.
Figure 1 shows a prior art current overload
detector circuit. A resistor 10 is connected between
a source 12 and a load 14. As load current increases,
the voltage drop across the resistor 10 increases,
linearly, according to Ohm's law (E = IT). A-t a
threshold voltage drop, a zoner diode 16 connected in
reverse bias across the resistor 10 becomes conductive
and flows current through a series-connected light
emitting diode 18. Illumination of the LED is indict-
live of a current overload. This basic concept is
disclosed in I. S. Patent No. 4,418,342 (Aschoff,
1983). In Figure 2 therein, the elements are a nests-
ion 1, a zoner diode 8', and an LED 6'.
Consider the following example. The load 14
draws 1.0 - 2.0 milliamps under normal operating condo-
lions. It is desirable to indicate a current overload
at currents above 2.0 milliamps. The combination of a
12 volt zoner diode and a 6000 ohm resistor will

:~266Z2
-- 3 --
illuminate the LED at exactly 2.0 milliamps (discount-
in forward thresholds in the LED). However, the
voltage drop will fluctuate from 6.0 to 12.0 volts
under normal operating conditions. Such a wide range
of voltages may be highly undesirable. Furthermore,
the 6000 ohm resistor will consume as much as 0.024
watts of power under normal operating conditions, and
more under fault conditions.
Suppose, however, that a 4000 ohm resistor
were used. Power consumption would be reduced by one-
third and there would be less voltage fluctuation
(4 to 8 volts) under normal operating conditions.
However, the zoner diode would not reach reverse
breakdown until 3.0 milliamps of current were drawn
by the load. Thus, there would be a 'dead band" of
non detection between 2.0 and 3.0 milliamps.
With a 2000 ohm resistor, the voltage flue-
tuition at normal operating current would be 2.0 to
4.0 volts, but the dead band would be between 2.0 and
6.0 milliamps.
As is readily concluded from the above
examples, there is a tradeoff involved in the use of
a resistor as the voltage drop means in the current
overload detector circuit. The present invention
overcomes these limitations.
Figure 2 shows the current overload detector
circuit of this invention. A nonlinear resistive
element, such as a thermistor 20 is connected between
a source 22 and a load 24. The resistance of the
thermistor 20 increases in proportion to the current
there through. As in the circuit of Figure 1, at a
threshold voltage, a threshold conductivity means,
such as a zoner diode 26 connected in reverse bias
across the thermistor 20, becomes conductive and
flows current through a series connected indicator
means, such as a forward-biased LED 28. Illumine-
lion of the LED 28 is indicative of a current overload.

~Z2~;~22
Consider the following example. The thermistor
has a resistance related linearly to current, as follows:
1000 ohms at 1.0 milliamps, 2000 ohms at 2.0 milliamps,
and 3500 ohms at 3.5 milliamps. In the normal operate
in range of 1.0 to 2.0 milliamps, the voltage drop would be 1 volt and 4 volts, respectively. A fault
would be indicated at about 3.5 milliamps, which is
substantially lower than either a 1000 ohm or a 2000
ohm resistor would result in. Thus, using a thermistor
produces an acceptable dead band (2.0 to 3.5 milliamps),
while normal operating current voltage drop fluctuations
are greatly reduced. Power loss is commensurately mini-
mixed.
The operating characteristics of the therms-
ion of this example are simply meant to illustrate the concept of this invention. It is well within the
scope of one skilled in the art to select an approp-
rite nonlinear resistive element depending on the
particular load, acceptable voltage fluctuations,
and acceptable dead band.
Although the invention has been shown and
described with respect to an exemplary embodiment
thereof, it should be understood by those skilled
in the art that the foregoing and various other
changes, omissions and additions may be made therein
and thereto, without departing from the spirit and
scope of the invention.