Current detector

When a circuit is activated, bases of transistors Q1 and Q2 making up a first current mirror circuit 7 are grounded through switch means 10, whereby a current is forcibly made to flow into the bases of the transistors Q1 and Q2. Thus, the circuit operation can be made reliable at the activation time. A part of load current is shunted with good accuracy via a resistor R1 having a first resistance value of a predetermined magnification of a resistance value of a shunt resistor Rs, the first current mirror circuit 7, and a second current mirror circuit 8, and the output end of the shunted current is grounded via a resistor R2 having a second resistance value, so that an output voltage VOUT proportional to a load current IL can be obtained with good accuracy from the resistor R2 and the load current IL can be found with good accuracy from the output voltage VOUT.

BACKGROUND OF THE INVENTION
 This invention relates to a current detector for detecting a supply current
 supplied to a load with good accuracy.
 Hitherto, a high-accuracy current detector has been demanded for the
 purposes of finding the charge amount of a secondary battery used with a
 personal computer, etc., with good accuracy, monitoring a current
 distribution circuit in an automobile, etc. A method of placing a shunt
 resistor of a high-accuracy low resistance element in series with wiring
 through which the current to be detected flows and detecting a voltage
 drop occurring in the shunt resistor, thereby detecting a current value is
 known.
 For example, in FIG. 10, a current I12 proportional to a load current IL
 flowing into a shunt resistor Rs is formed using pnp transistors Q11 and
 Q12 and a resistor R11 and is converted into an output current VOUT using
 a resistor R12, thereby detecting the load current IL.
 Hitherto, a method of using an operational amplifier IC to differentially
 amplify the potential difference occurring across a shunt resistor of a
 high-accuracy low resistance element and output the voltage proportional
 to a load current has been known. In this case, however, the offset
 voltage of the operational amplifier IC is superposed on the output
 voltage, thus a problem is involved in the accuracy particularly when the
 load current is small. In contrast, there is also an operational amplifier
 IC whose output voltage can be adjusted by connecting a variable resistor
 to an external terminal, but the operational amplifier ICs are not
 suitable for mass production. A method of canceling the effect of offset
 voltage by a circuit shown in FIG. 11 is known.
 In FIG. 11, if an operational amplifier OP1 has a sufficiently large
 amplification factor, the potential difference between an inversion input
 terminal and a noninversion input terminal of the operational amplifier
 OP1 can be assumed to be zero. Therefore,
EQU I14.multidot.R14=(I13+IL).multidot.RS.apprxeq.IL.multidot.RS
 where R14 is the resistance value of a resistor R14, RS is the resistance
 value of a shunt resistor Rs, I13 is a current flowing into the inversion
 input terminal of the operational amplifier OP1, and I14 is a current
 flowing into the noninversion input terminal of the operational amplifier
 OP1.
 Thus, the collector current I14 of a pnp transistor Q13 becomes
EQU I14.apprxeq.I1.multidot.RS/R14
 and a current proportional to load current IL can be output.
 However, in the circuit previously described with reference to FIG. 10, the
 range in which the voltage proportional to the load current IL flowing
 into the shunt resistor Rs of a high-accuracy low resistance element is
 provided is narrow, thus to monitor a small current area and to monitor a
 large current area, different circuits need to be used, for example, in
 such a manner that the resistance value of the resistor R12 is changed;
 this is a problem. In [DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS]
 covered later, using FIG. 3, comparison with the circuit forming the
 invention is made for the relationship between the load current and the
 output voltage in the circuit in FIG. 10.
 Use of the circuit previously described with reference to FIG. 11 using the
 operational amplifier IC requiring a large number of circuit elements is
 not preferred considering implementing of the current detection circuit in
 IC form, the manufacturing costs, etc.
 SUMMARY OF THE INVENTION
 It is therefore an object of the invention to provide a current detector
 having a simple configuration capable of detecting a current with good
 accuracy.
 According to a the invention, there is provided a current detector using a
 current detection resistor placed between a power supply section and a
 load to detect a supply current value supplied from the power supply
 section to the load, the current detector comprising a first resistance
 element having a first resistance value of a predetermined magnification
 of a resistance value of the current detection resistor, a first current
 mirror circuit made up of a plurality of negative-polarity-type
 transistors and having one end connected to an end part of the current
 detection resistor on the power supply section side thereof via the first
 resistance element and an opposite end connected to an end part of the
 current detection resistor on the load side thereof, a second current
 mirror circuit made up of a plurality of positive-polarity-type
 transistors and having both input ends connected to both output ends of
 the first current mirror circuit, a second resistance element placed
 between at least either of both output ends of the second current mirror
 circuit and ground and having a second resistance value, and current
 detection means for detecting a voltage of the second resistance element
 on the second current mirror circuit 8 side thereof, characterized by
 switch means being placed between control terminals of the transistors
 making up the first current mirror circuit and ground. Preferably, the
 current detector of the invention further includes current value detection
 means using the detected voltage to find the supply current value. Here,
 the negative-polarity-type transistors are transistors (current control
 elements) such as pnp transistors and p-channel FETs (field-effect
 transistors) of the type wherein when a current is drawn out from a
 control terminal, current terminals are brought into conduction, and the
 positive-polarity-type transistors are transistors (current control
 elements) such as npn transistors and n-channel FETs of the type wherein
 when a current is input to a control terminal, current terminals are
 brought into conduction. The control terminals of the transistors making
 up the second current mirror circuit are connected to one input end of the
 second current mirror circuit and the control terminals of the transistors
 making up the first current mirror circuit are connected to an opposite
 input end of the second current mirror circuit.
 According to the configuration, when the circuit is activated, the control
 terminals of the transistors making up the first current mirror circuit
 are grounded through the switch means, whereby a current is forcibly made
 to flow into the control terminals, so that it is made possible to make
 the circuit operation at the activation time reliable. A part of load
 current is shunted with good accuracy via the first resistance element
 having the first resistance value of the predetermined magnification of
 the resistance value of the current detection resistor, the first current
 mirror circuit, and the second current mirror circuit, and the output end
 of the shunted current is grounded via the second resistance element
 having the second resistance value, so that an output voltage proportional
 to the load current can be obtained with good accuracy from the second
 resistance element in a simple configuration and the load current can be
 found with good accuracy from the output voltage.
 Preferably, in the current detector of the invention, the switch means
 comprises a transistor circuit capable of grounding the control terminal
 of the transistor.
 According to the configuration, the switch means can be easily implemented
 as transistor circuit.
 Preferably, in the current detector of the invention, the switch means
 comprises a one-shot pulse generation circuit for activating the
 transistor circuit.
 According to the configuration, a one-shot pulse from the one-shot pulse
 generation circuit can be used easily to turn off the transistor circuit
 after turning on the transistor circuit only for a predetermined time.
 Preferably, the current detector of the invention further includes control
 means for controlling so that the voltage detection means detects the
 voltage after on/off control of the switch means is performed (in the
 embodiment, control means 11 or microcomputer 11a). Preferably, the
 current detector of the invention further includes control means for
 controlling so as to output a current detection start signal to the
 voltage detection means after on/off control of the switch means is
 performed (in the embodiment, control means 11).
 According to the configuration, it is made possible to take the voltage
 detection timing reliably and well.
 Preferably, in the current detector of the invention, the control means
 controls so as to detect the voltage every constant period and performs
 on/off control of the switch means every constant period while the voltage
 is not detected.
 According to the configuration, on/off control of the switch means is
 performed every constant period. Thus, if the current voltage conversion
 circuit made up of the first and second current mirror circuits
 malfunctions and stops due to noise, etc., for example, it is again
 activated in the next period and the detected voltage can always be
 monitored. Therefore, noise resistance of the first and second current
 mirror circuits is enhanced.
 Preferably, in the current detector of the invention, the first current
 mirror circuit comprises a first transistor having an emitter connected to
 the power supply section side of the current detection resistor via the
 first resistance element and a second transistor having an emitter
 connected to the load side of the current detection resistor and a base
 and a collector connected to a base of the first transistor, the first and
 second transistors being implemented as pnp transistors, the second
 current mirror circuit comprises a third transistor having a collector
 connected to a collector of the first transistor and a fourth transistor
 having a collector connected to the collector of the second transistor and
 a base connected to a base and the collector of the third transistor, the
 third and fourth transistors being implemented as npn transistors, and an
 emitter of at least either of the third and fourth transistors is grounded
 via the second resistance element.
 According to the configuration, the first and second current mirror
 circuits can be configured more easily.
 Preferably, the current detector of the invention further includes a third
 resistance element having the second resistance value, a fourth resistance
 element having the same resistance value as the first resistance element,
 and a fifth resistance element having a resistance value different from
 the second resistance value, wherein the first current mirror circuit
 further includes a fifth transistor of a pnp transistor having an emitter
 connected to the power supply section side of the current detection
 resistor via the fourth resistance element and a base connected to the
 base of the first transistor, wherein the third transistor has the emitter
 grounded via the second resistance element, the fourth transistor has the
 emitter grounded via the third resistance element, and the fifth
 transistor has a collector grounded via the fifth resistance element, and
 wherein the voltage detection means further detects a voltage value of the
 collector of the fifth transistor.
 According to the configuration, if the voltage of the second resistance
 element on the second current mirror circuit side thereof and the voltage
 of the fifth resistance element on the fifth transistor side thereof are
 changed in response to the change range of the detected voltage caused by
 change in the load current and the voltage is detected, the load current
 can be found suitable with good accuracy if the voltage range that can be
 detected by the voltage detection means is narrow as compared with the
 change range of the detected voltage caused by change in the load current.
 Also in this case, it is made possible to make the circuit operation at
 the activation time reliable by the switch means.
 Preferably, the current detector of the invention further includes a
 predetermined number of sixth resistance elements each having the second
 resistance value, wherein the first current mirror circuit has the
 predetermined number of sixth transistors of pnp transistors having
 emitters connected to the emitter of the first transistor and bases
 connected to the base of the first transistor, and wherein collectors of
 the sixth transistors are grounded via the sixth resistance elements.
 According to the configuration, the currents flowing into the second
 resistance element and the sixth resistance elements become equal levels
 to each other, thus the detected voltage becomes low and the voltage
 detection means can be made up of circuit parts consuming low power and
 the circuit can be miniaturized and the costs can be reduced. Also in this
 case, it is made possible to make the circuit operation at the activation
 time reliable by the switch means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 is a diagram to show the configuration of a current supply unit
 containing a current detector of one embodiment of the invention. In FIG.
 1, a current supply unit 1 comprises an FET 3 for current supply and a
 load 4 connected in series between a power supply section 2 and ground,
 wherein the FET 3 is controlled based on a drive current from a drive
 circuit 5 for supplying a load current IL from the power supply section 2
 to the load 4. A current detector 6 is placed between the FET 3 and the
 load 4 for detecting the load current IL supplied from the power supply
 section 2 to the load 4.
 The current detector 6 comprises a shunt resistor Rs (current detection
 resistor) placed between the FET 3 and the load 4, a resistor R1 (first
 resistance element) connected to the FET 3 side of the shunt resistor Rs,
 a first current mirror circuit 7, a second current mirror circuit 8, a
 resistor R2 (second resistance element) placed between both output ends of
 the second current mirror circuit 8 and ground, and a voltage detection
 circuit 9 for detecting an output voltage VOUT of the resistor R2 on the
 second current mirror circuit 8 side thereof and using the output voltage
 VOUT to determine whether or not the load current is within the normal
 range, and converts a current proportional to the load current IL into a
 voltage with the resistor R2 from the shunt resistor Rs through the first
 current mirror circuit 7 and the second current mirror circuit 8, and the
 voltage detection circuit 9 detects the voltage. The current detector 6
 further comprises switch means 10 placed between control terminals (bases)
 of transistors making up the first current mirror circuit 7 and ground and
 control means 11 for controlling so as to cause the voltage detection
 circuit 9 to output a voltage detection start signal after the switch
 means 10 is turned on/off.
 The shunt resistor R2 is a high-accuracy low resistance element having a
 known resistance value RS and provides a current detection resistor. The
 resistor R1 has a first resistance value of a predetermined magnification
 of the resistance value of the shunt resistor Rs.
 The first current mirror circuit 7 has one input end connected to one end
 of the shunt resistor Rs on the power supply section 2 side thereof via
 the first resistance element and an opposite input end connected to one
 end of the shunt resistor Rs on the load 4 side thereof. Specifically, the
 first current mirror circuit 7 is made up of a transistor Q1 (first
 transistor) and a transistor Q2 (second transistor) which are pnp
 transistors having the same transistor characteristic. The transistor Q1
 has an emitter connected to the power supply section 2 side of the shunt
 resistor Rs via the resistor R1 and a base connected to a base and a
 collector of the transistor Q2 and the transistor Q2 has an emitter
 connected to the load 4 side of the shunt resistor Rs. The emitter sides
 of the transistors Q1 and Q2 form both input ends of the first current
 mirror circuit 7 and the collector sides of the transistors Q1 and Q2 form
 both output ends of the first current mirror circuit 7.
 The second current mirror circuit 8 is made up of a transistor Q3 (third
 transistor) and a transistor Q4 (fourth transistor) which are npn
 transistors having the same transistor characteristic. The transistor Q3
 has a collector connected to the collector of the first transistor Q1 and
 a base connected to the collector of the transistor Q3 and a base of the
 transistor Q4 and the transistor Q4 has a collector connected to the
 collector of the transistor Q2. The collector sides of the transistors Q3
 and Q4 form both input ends of the second current mirror circuit 8 and the
 emitter sides of the transistors Q3 and Q4 form both output ends of the
 second current mirror circuit 8.
 In the circuitry in FIG. 1, the transistor group T1 consisting of the pnp
 transistors Q1 and Q2 and the transistor group T2 consisting of the npn
 transistors Q3 and Q4 have the same transistor characteristics such that
 the base-emitter voltage Vbe difference can be almost ignored by using
 adjacent transistors on the same semiconductor wafer, etc.
 For example, to make up a circuit of discrete parts, one package housing
 two adjacent transistors is commercially available and thus may be used.
 To use an IC, transistors Q1 and Q2 are placed adjacent to each other and
 transistors Q3 and Q4 are placed adjacent to each other on semiconductor
 wafer, whereby the base-emitter voltage Vbe difference can be placed at a
 level to such an extent that it can be almost ignored.
 The resistor R2 has a second resistance value and is placed between the
 emitters of the transistors Q3 and Q4 and ground.
 The voltage detection circuit 9 forms voltage detection means for detecting
 the output voltage VOUT of the emitters of the transistors Q3 and Q4. It
 also forms load current determination means (current value detection
 means) for using the detected voltage value to determine whether or not
 the load current IL is within the normal range. Specifically, for example,
 if the voltage detection circuit 9 is implemented as one comparator, the
 comparator can compare the output voltage VOUT with the reference voltage
 (lower or upper limit reference voltage) and whether or not the load
 current IL is within the normal range can be determined by output of the
 comparator. If the voltage detection circuit 9 is made up of two
 comparators, the first comparator can compare the output voltage VOUT with
 the lower limit reference voltage and whether or not the load current IL
 is within the lower limit range can be determined by output of the first
 comparator. The second comparator can compare the output voltage VOUT with
 the upper limit reference voltage and whether or not the load current IL
 is within the upper limit range can be determined by output of the second
 comparator. Outputs of the first and second comparators can be input to an
 AND gate and whether or not the load current IL is within the normal range
 can be determined by output of the AND gate.
 The switch means 10 has a transistor circuit 101 and a one-shot pulse
 generation circuit 102 for activating the transistor circuit 101, as shown
 in FIG. 2.
 The transistor circuit 101 comprises a transistor Q8, which has a collector
 connected via a resistor R6 to the bases of the transistors Q1 and Q2 and
 an emitter grounded. The one-shot pulse generation circuit 102 has an
 output end connected to a base of the transistor Q8 and outputs a one-shot
 pulse of a predetermined duration to the base of the transistor Q8 as a
 trigger signal is input to an input end of the one-shot pulse generation
 circuit 102. Power supply Vcc is supplied to the one-shot pulse generation
 circuit 102 from the power supply section 2.
 The control means 11 controls so as to output a trigger signal as a circuit
 drive signal to the one-shot pulse generation circuit 102 only once when
 the power is turned on and after the expiration of a predetermined time,
 the control means 11 controls so as to output a drive start signal of the
 voltage detection circuit 9.
 The reason why the switch means 10 is provided is as follows: If a route
 where a base current flows into the base of the transistor Q1 or Q2 does
 not exist, the circuit does not operate. If the switch means 10 is not
 provided, the first current mirror circuit 7 and the second current mirror
 circuit 8 can operate, but the stability of the operation is not
 sufficient. That is, in the actual circuit operation, current IL0 shown in
 FIG. 1 flowing because of a voltage drop occurring in the shunt resistor
 Rs causes the transistor Q1 to allow a collector current to flow, which
 then becomes a base current of the transistors Q3 and Q4 at the following
 stage, and currents I1 and I2 flow into the first current mirror circuit 7
 and the second current mirror circuit 8. Since the currents also flow via
 the base-emitter parasitic capacitance of the transistor Q2, the first
 current mirror circuit 7 and the second current mirror circuit 8 operate.
 Alternatively, the circuit may operate due to the collector leakage
 current of each transistor. Thus, the above-described circuit operation is
 realized based on the internal structure (parasitic capacitance) of each
 of the actual transistors and the stable operation is not reliably
 performed depending on the combination of the transistors and the circuit
 constants. A malfunction caused by noise or temperature change (circuit
 stop caused by parasitic capacitance change, etc.,) can also occur.
 Therefore, to perform the stable circuit operation reliably, the circuit
 is grounded via the switch means 10 when the circuit is started, whereby
 the route where the base current flows into the bases of the transistors
 Q1 and Q2 is forcibly provided.
 Next, the reason why driving the voltage detection circuit 9 is started
 after the expiration of the predetermined time since driving the
 transistor circuit 101 is as follows:
 As the transistor Q8 is turned on, the base current is made to flow into
 the bases of the transistors Q1 and Q2 and the first current mirror
 circuit 7 and the second current mirror circuit 8 are forcibly operated.
 When the transistor Q8 is on, the bases of the transistors Q1 and Q2 are
 grounded, thus voltage not related to the load current IL (voltage not
 proportional to the load current IL) is output as the output voltage VOUT.
 Thus, while the transistor Q8 is on, the output voltage VOUT is not
 monitored by the voltage detection circuit 9. Then, if the transistor Q8
 is turned off, the currents I1 and I2 continue to flow because of the
 sol-called self-hold circuit function, so that the first current mirror
 circuit 7 and the second current mirror circuit 8 continue to operate and
 the output voltage VOUT proportional to change in the load current IL is
 provided.
 Next, the principle of finding the load current IL from the output voltage
 VOUT in the current value detection means will be discussed. First, the
 resistor R1 having resistance value R1 N times the resistance value RS of
 the shunt resistor Rs (predetermined magnification) as shown in the
 following expression 1 is adopted:
 (Expression 1)
EQU R1=N.multidot.RS
 The following expression 2 is obtained for the potential difference between
 the bases of the transistors Q1 and Q2 and the source of the FET 3:
 (Expression 2)
EQU I2.multidot.R1+Vbe(Q1)=(I1+IL).multidot.RS+Vbe(Q2)
 where I1 is the collector current flowing into the transistor Q2, I2 is the
 collector current flowing into the transistor Q1, Vbe (Q1) is the
 base-emitter voltage of the transistor Q1, and Vbe (Q2) is the
 base-emitter voltage of the transistor Q2, as shown in FIG. 1.
 Since the transistors Q1 and Q2 adopt transistors of almost the same
 characteristic, as described above, the following expression 3 is
 obtained:
 (Expression 3)
EQU Vbe(Q1)=Vbe(Q2)
 When the expressions 1 and 3 are assigned to the expression 2, the
 following expression 4 is obtained:
 (Expression 4)
EQU I2=(I1+IL)/N
 In the circuitry in FIG. 1, the transistors Q3 and Q4 make up so-called
 current mirror circuit. In the current mirror circuit, the common emitter
 resistor R2 is connected and the transistors of almost the same
 characteristic are adopted as described above, so that the base-emitter
 voltages equal and therefore the following expression 5 is obtained:
 (Expression 5)
EQU IC(Q3)=IC(Q4)
 where IC(Q3) is the collector current flowing into the transistor Q3 and
 IC(Q4) is the collector current flowing into the transistor Q4.
 Generally, hFE=(collector current)/(base current) of transistor is
 sufficiently large, thus the emitter current and the collector current of
 each transistor can be assumed to be equal, and the following expressions
 6, 7, and 8 are obtained:
 (Expression 6)
EQU IC(Q3)=I2
 (Expression 7)
 IC(Q4)=I1
 (Expression 8)
EQU IC(Q3)=IE(Q3)
EQU IC(Q4)=IE(Q4)
 Therefore, from the expressions 5, 6, and 7,
 (Expression 9)
EQU I2=I1
 Therefore, from the expressions 4 and 9,
 (Expression 10)
EQU I2=I1=IL/(N-1)
 From the expressions 6 and 8,
 (Expression 11)
EQU I2=IE(Q3)
 Therefore, from the expressions 10 and 11, the output voltage VOUT to the
 voltage detection circuit 9 is
 (Expression 12)
 ##EQU1##
 Since the resistance value R2 of the resistor R2 and the numeric value N
 are known, in the voltage detection circuit 9
 (Expression 13)
EQU IL=VOUT.multidot.(N-1)/(2.multidot.R2)
 Thus, the load current IL can be found.
 For example, assuming that RS=10 (m.OMEGA.), R1=1 (k.OMEGA.), and R2=33
 (k.OMEGA.), VOUT=2 (V) when IL=3 (A).
 The operation of the circuitry having the configuration described above is
 as follows: First, when the power is turned on, the power supply section 2
 is driven, driving the drive circuit 5 is also started, and driving the
 control means 11 is also started. When the drive circuit 5 is driven, the
 FET 3 is turned on and the load current IL is supplied to the load 4 via
 the shunt resistor Rs. When the control means 11 is driven, it outputs a
 trigger signal to the one-shot pulse generation circuit 102 and upon
 reception of the trigger signal, the one-shot pulse generation circuit 102
 outputs a one-shot pulse to the base of the transistor Q8, whereby the
 transistor Q8 is turned on only for a predetermined time, the bases of the
 transistors Q1 and Q2 are grounded, a base current flows into the bases of
 the transistors Q1 and Q2, and the first current mirror circuit 7 and the
 second current mirror circuit 8 are forcibly operated.
 Next, although the transistor Q8 is turned off, currents I1 and I2 continue
 to flow into the first current mirror circuit 7 and the second current
 mirror circuit 8, so that the first current mirror circuit 7 and the
 second current mirror circuit 8 continue to operate and the output voltage
 VOUT as current voltage conversion output proportional to change in the
 load current IL is provided.
 At this point in time, the control means 11 controls so as to output a
 drive start signal of the voltage detection circuit 9, and the voltage
 detection circuit 9 monitors and detects the output voltage VOUT.
 Further, the voltage detection circuit 9 finds the load current IL from the
 detected output voltage VOUT by the load current determination means and
 determines whether or not the load current IL is within the normal range.
 Determination display can be produced and the value of the load current IL
 can be displayed in response to the determination result.
 FIG. 3A is a drawing to show change in the load current IL from 5 (A) to 50
 (A) and there are no units on the horizontal axis. FIG. 3B is a comparison
 drawing of change in (load current IL)(output voltage VOUT) between the
 circuits in FIGS. 1 and 10 when the load current IL changes as in FIG. 3A.
 In FIG. 3B, the minimum value and the maximum value of (IL)/(VOUT) in the
 circuit in FIG. 1 are 23.4 an 24.8 respectively, and the minimum value and
 the maximum value of (IL)/(VOUT) in the circuit in FIG. 10 are 19.8 an
 23.75 respectively.
 That is, variation of (IL)/(VOUT) is placed within .+-.3% in the circuit in
 FIG. 1, while variation of .+-.9% occurs in the circuit in FIG. 10.
 Therefore, to use the circuit in FIG. 10 to detect an area with a small
 load current with good accuracy, the resistance value of the resistor R12
 in FIG. 10 needs to be changed, but need not be changed in the circuit in
 FIG. 1.
 As described above, according to the embodiment, when the circuit is
 started, the bases of the transistors Q1 and Q2 are grounded through the
 switch means 10, whereby the route where the base current flows is
 forcibly provided, so that the circuit operation at the starting time can
 be made reliable. That is, according to the embodiment, in the structure
 wherein the first current mirror circuit 7 and the second current mirror
 circuit 8 are simply connected, the circuit operation is realized based on
 the internal structure (parasitic capacitance) of each of the actual
 transistors and the stable operation is not reliably performed depending
 on the combination of the transistors and the circuit constants and a
 malfunction caused by noise, temperature change, etc., (circuit stop
 caused by parasitic capacitance change, etc., also occurs; however,
 according to the embodiment, the problems can be resolved.
 The transistor Q1 is connected to the power supply section 1 side of the
 shunt resistor Rs via the resistor R1, the transistor Q2 is connected to
 the load 4 side of the shunt resistor Rs, and the transistors Q3 and Q4
 making up the second current mirror circuit 8 are connected to the
 transistors Q1 and Q2, whereby a part of the load current IL is shunted
 with good accuracy and the output end of the shunted current is grounded
 via the resistor R2, so that the voltage proportional to the load current
 IL can be provided with good accuracy and the load current IL can be found
 with good accuracy accordingly.
 The invention is not limited to the above-described embodiment and the
 following modifications can be adopted:
 (1) In the above-described embodiment, when the power is turned on, the
 control means 11 outputs a trigger signal to the one-shot pulse generation
 circuit 102 for turning on the transistor circuit 101, but the control
 means 11 may output a circuit drive pulse signal for turning on the
 transistor circuit 101 only for a predetermined time when the power is
 turned on directly to the transistor circuit 101, as shown in FIG. 4,
 without providing the one-shot pulse generation circuit 102. In this case,
 only the transistor circuit 101 forms switch means 10a.
 (2) In the above-described embodiment, the one-shot pulse generation
 circuit 102 and the control means 11 for turning on the transistor circuit
 101 only for the predetermined time and further the voltage detection
 circuit 9 are provided, but these functions can also be provided by a
 microcomputer 11a as shown in FIG. 5. In this case, the transistor Q8 is
 once operated by a circuit drive signal from an I/O port and after the
 transistor Q8 is turned off in a given time, the output voltage VOUT can
 also be read through an A/D port under the control of the microcomputer
 11a. In the circuit configuration, the A/D port does not continuously read
 the voltage value and reads the voltage value every constant period, so
 that the transistor Q8 can also be turned on/off every period while the
 voltage value is not read. If the current voltage conversion circuit made
 up of the current mirror circuits malfunctions and stops due to noise,
 etc., as in the embodiment wherein the transistor Q8 is turned on and off
 only once when the circuit is started, the transistor Q8 is turned on and
 off every constant time and the current mirror circuit is forcibly
 started, so that the modification has the advantage that the current
 mirror circuit has resistance to noise. In this case, the microcomputer
 11a contains the functions of the above-described voltage detection means,
 the current value detection means for calculating and finding the load
 current IL from the detected voltage value, and the load current
 determination means for determining whether or not the found load current
 IL is within the normal range.
 In current value calculation means, calculation is executed based on the
 above-described principle of finding the load current IL from the output
 voltage VOUT.
 (3) FIG. 6 is a circuit diagram to show one modification of the current
 detector previously described with reference to FIG. 1. Parts identical
 with those previously described with reference to FIG. 1 are denoted by
 the same reference numerals in FIG. 6. In FIG. 6, a current detector 12
 comprises a resistor R3 having a resistance value R2 (third resistance
 element), a resistor R4 having a resistance value R1 (fourth resistance
 element), a resistor R5 having a resistance value R5 (R5.noteq.R2) (fifth
 resistance element), and a transistor Q5 of a pnp transistor with the
 base-emitter voltage almost equal to that of a transistor Q1 (fifth
 transistor) in addition to the components of the current detector in FIG.
 1.
 An emitter of a transistor Q4 is not connected to an emitter of a
 transistor Q3 and is grounded via a resistor R3. Therefore, in the circuit
 in FIG. 6, an output voltage VOUT1 from the emitter of the transistor Q3
 becomes
 (Expression 14)
EQU VOUT1=R2.multidot.IL/(N-1)
 The transistor Q5 has an emitter connected to the power supply section 1
 side of a shunt resistor Rs via the resistor R4, a base connected to a
 base of a transistor Q1, and a collector grounded via the resistor R5. A
 voltage detection circuit 9 further detects a voltage value VOUT2 of the
 collector of the transistor Q5 and finds a load current IL from the
 detected voltage value.
 In the circuit in FIG. 6, the resistors R1 and R4 have equal resistance
 values R1 and the transistor Q5 has the same characteristic as the
 transistor Q1, so that a current I2 equal to that of the transistor Q1
 flows into the transistor Q5 as shown in FIG. 6.
 Therefore, the output voltage VOUT2 from the collector of the transistor Q5
 becomes
 (Expression 15)
 VOUT2=R5.multidot.IL/(N-1)
 According to the modification, connection is made to the FET 3 side of the
 shunt resistor Rs via the resistor R4 of the same resistance value as the
 resistor R1, the transistor Q5 having the same characteristic as the
 transistor Q1 is provided, and the resistance value of the collector
 resistor R5 is set to a proper value different from the resistance value
 of the resistor R2, whereby the voltages VOUT1 and VOUT2 at different
 levels can be output at the same time to the load current IL at the same
 level.
 Therefore, if the voltage range that can be detected by the voltage
 detection circuit 9 is narrow as compared with the change range of the
 output voltage VOUT caused by change in the load current IL, the load
 current IL can be detected suitably with good accuracy.
 (4) FIG. 7 is a circuit diagram to show another modification of the current
 detector previously described with reference to FIG. 1. Parts identical
 with those previously described with reference to FIG. 1 are denoted by
 the same reference numerals in FIG. 7. In FIG. 7, a current detector 13
 comprises a resistor R3 having a resistance value R2 (third resistance
 element), resistors R61 to R64 each having a resistance value R2 (sixth
 resistance elements), and pnp transistors Q61 to Q64 each with the
 base-emitter voltage almost equal to that of a transistor Q1 in addition
 to the components of the current detector in FIG. 1.
 An emitter of a transistor Q4 is not connected to an emitter of a
 transistor Q3 and is grounded via a resistor R3. The pnp transistors Q61
 to Q64 have emitters connected to an emitter of the transistor Q1, bases
 connected to a base of the transistor Q1, and collectors grounded via the
 resistors R61 to R64.
 According to the modification, the currents flowing into the resistor R2
 and the resistors R61 to R64 become equal levels to each other and thus
 become each I1/5, so that a voltage detection circuit 9 can be made up of
 circuit parts consuming low power and the circuit can be miniaturized and
 the costs can be reduced.
 The modification in FIG. 7 comprises the four resistors R61 to R64 and the
 four pnp transistors Q61 to Q64, but the number of parts is not limited to
 four and may be a predetermined number M. Thus, the currents flowing into
 the M resistors and the resistor R2 become I1/(M+1) and the power
 consumption can be reduced.
 In the modification, the characteristics of the transistors Q1, Q2, and Q61
 to Q64 need to be matched with each other to detect a load current IL with
 good accuracy, thus preferably the transistor circuit is implemented as an
 IC uniformly.
 Here, variations in characteristics of elements in an integrated circuit
 (IC) formed on a semiconductor wafer will be discussed.
 To manufacture ICs, a large number of identical circuits are formed by
 executing a known circuit formation process on one of wafers cut out from
 a semiconductor (generally, silicon) ingot, then the wafer is diced and
 molded for each circuit (chip).
 Therefore, the variations in characteristics of the elements in IC can be
 classified into those occurring between the chips in one wafer, those
 between the wafers, and those between ingots from which wafers are cut
 out.
 The variations in characteristics of the elements in IC are caused by
 variations in the circuit formation process, namely, variations in etching
 process, variations in exposure process, variations in diffusion degree in
 impurity diffusion process, variations in temperature in each process,
 etc.
 The etching process, the exposure process, and the impurity diffusion
 process causing the variations are executed for each wafer and the
 temperature of each process is also the same with the same wafer, thus the
 variations in characteristics are hard to occur between the chips in one
 wafer. Particularly, the variations between the elements formed close to
 each other in the same chip can be almost ignored.
 Therefore, relative variations in characteristics of the transistors Q1,
 Q2, and Q61 to Q64, relative variations in characteristics of the
 transistors Q3 and Q4, and relative variations in resistance values of the
 resistors R2, R3, and R61 to R64 can be placed at very low levels.
 (5) FIG. 8 is a circuit diagram to show still another modification of the
 current detector previously described with reference to FIG. 1. Parts
 identical with those previously described with reference to FIG. 1 are
 denoted by the same reference numerals in FIG. 8. In FIG. 8, a voltage
 detection circuit 9 is grounded via a diode D1 of a rectification element,
 whereby when a power supply section 2 made of a battery is connected to
 the circuit, for example, if the positive and negative poles are connected
 oppositely in error, the internal circuit of the voltage detection circuit
 9 can be protected. However, in the circuit configuration, the voltage
 detected by the voltage detection circuit 9 is raised as much as the
 forward voltage of the diode D1.
 Then, in the modification in FIG. 8, the connection position of a resistor
 R2 is changed from that in FIG. 1 and the voltage detection point of the
 voltage detection circuit 9 is also changed. That is, the resistor R2 is
 placed between collectors of transistors Q1 and Q3 and the collector of
 the transistor Q1 is connected to the voltage detection circuit 9.
 Thus, an output voltage VOUT is raised by the base-emitter voltage of the
 transistor Q3 equal to the forward voltage of the diode D1, so that a load
 current IL can be detected suitably with good accuracy, as in the first
 embodiment.
 Only the voltage detection point of the voltage detection circuit 9 may be
 changed from that in FIG. 1 and the connection position of the resistor R2
 may remain at the same position as that in FIG. 1, namely, the position of
 block A indicated by the dashed line in FIG. 8. Also in this case,
 likewise, the output voltage VOUT is raised by the base-emitter voltage of
 the transistor Q3. Since the resistor R2 is connected between emitters of
 transistors Q3 and Q4 and ground, a current mirror circuit made up of the
 transistors Q3 and Q4 can be operated suitably as compared with the
 modification in FIG. 8.
 (6) In the embodiment previously described with reference to FIG. 1, the
 collector and the base of the transistor Q3 are directly connected;
 instead, an npn transistor Q7 may be provided as shown in FIG. 9. That is,
 the transistor Q7 has a base connected to the collector of the transistor
 Q3, an emitter connected to the base of the transistor Q3, and a collector
 connected to the FET 3 side of a shunt resistor Rs.
 According to the modification, the decrease drawn out from the collector
 current of the transistor Q3 becomes 1/hFE, so that the characteristics of
 the current mirror circuit made up of the transistors Q3 and Q4 can be
 improved.
 (7) In the embodiment and the modifications described above, the loads 4
 are lamps as shown in FIGS. 1 and 6 to 9, but not limited to lamps. For
 example, a secondary battery is adopted, whereby the charge current as
 supply current supplied from a power supply section to the secondary
 battery can be detected with good accuracy and the charge amount of the
 secondary battery can be found with good accuracy.
 As described above, according to the invention as claimed in claim 1, 2,
 when the circuit is activated, the control terminals of the transistors
 making up the first current mirror circuit are grounded through the switch
 means, whereby a current is forcibly made to flow into the control
 terminals, so that the circuit operation can be made reliable at the
 activation time. A part of load current is shunted with good accuracy via
 the first resistance element having the first resistance value of the
 predetermined magnification of the resistance value of the current
 detection resistor, the first current mirror circuit, and the second
 current mirror circuit, and the output end of the shunted current is
 grounded via the second resistance element having the second resistance
 value, so that an output voltage proportional to the load current can be
 obtained with good accuracy from the second resistance element and the
 load current can be found with good accuracy from the output voltage.
 According to the invention as claimed in claim 3, the switch means for
 grounding the control terminals of the transistors of the first current
 mirror circuit can be easily implemented as transistor circuit.
 Further, according to the invention as claimed in claim 4, a one-shot pulse
 from the one-shot pulse generation circuit can be used easily to turn off
 the transistor circuit after turning on the transistor circuit only for a
 predetermined time.
 Further, according to the invention as claimed in claim 5, the control
 means controls so as to output a current detection start signal to the
 voltage detection means after on/off control of the switch means is
 performed, so that the voltage detection timing can be taken reliably and
 well.
 Further, according to the invention as claimed in claim 6, on/off control
 of the switch means is performed every constant period. Thus, if the
 current voltage conversion circuit made up of the first and second current
 mirror circuits malfunctions and stops due to noise, etc., for example, it
 is again activated in the next period and the detected voltage can always
 be monitored. Therefore, noise resistance of the first and second current
 mirror circuits can be enhanced.
 Further, according to the invention as claimed in claim 7, the first and
 second current mirror circuits can be configured easily.
 Further, according to the invention as claimed in claim 8, if the voltage
 range that can be detected by the voltage detection means is narrow as
 compared with the change range of the detected voltage caused by change in
 the load current, the load current can be found suitable with good
 accuracy. Also in this case, the circuit operation can be made reliable at
 the activation time by the switch means.
 Further, according to the invention as claimed in claim 9, the currents
 flowing into the second resistance element and the sixth resistance
 elements become equal levels to each other, so that the voltage detection
 means can be made up of circuit parts consuming low power and the circuit
 can be miniaturized and the costs can be reduced. Also in this case, the
 circuit operation can be made reliable at the activation time by the
 switch means.