JPH01212364A - Load current detection circuit - Google Patents

Abstract

PURPOSE:To detect a minute current change, by inexpensive constitution such that the emitter of the first transistor is connected between a power supply and a load current detecting resistor while the other emitter thereof is connected between said resistor and load, and a current mirror circuit whose base is connected to the base of the first transistor is provided. CONSTITUTION:A resistor 2 is connected between a power supply 1 and loads 7, 8 in series. The emitters of transistors 3a, 3b are respectively connected between the power supply 1 and the resistor 2 as well as between the resistor 2 and the loads 7, 8 and the mutual bases thereof are connected to form a current mirror circuit whose characteristics don't depend on the circumferential temp. Then, a variable resistor 9 is adjusted when a switch S1 is turned OFF to equalize the emitter potentials of the transistors 3a, 3b. When a switch S is turned ON, the collector potential of the transistor 3b is largely changed by the difference between the emitter potentials of the transistors 3a, 3b and compared with the reference voltage due to a reference power supply 6 by a voltage comparing circuit 5 to detect the current of the loads.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a load current detection circuit that detects changes in load current with minute voltage drops.

Conventional technology FIG. 2 shows a conventional load current detection circuit.

In Fig. 2, 1 is a load connected in parallel (R+,
t) the power supply for 7, (R1,-)8, 2 is the resistor (value R) connected in series between the power supply 1 and the load (RL,)7, (RL*) s, 3 is the emitter It is a PNP type transistor connected to a power supply 1, a base connected to loads (Rt, , ) 7, (RL-) 8, and a collector grounded via a resistor 4.

The collector of the transistor 3 is connected to one input terminal of a voltage comparison circuit 5, and the other input terminal of the voltage comparison circuit 5 is connected to a reference voltage source 6. Sl is a switch connected between loads (RL□) 7 and (Rt, t) 8.

Next, the operation of the above conventional example will be explained.

In Figure 2, when switch S1 is turned on from off, the total current of loads (Rt, 1) 7, (RL2) s increases, and the voltage drop across resistor 2, that is, the emitter-base voltage vBE of transistor 3 increases. do. ,in this case,
The total current I of the loads (RL,) 7, (Rt, , ) s can be found as I=V, , /R.

When this voltage VBE becomes approximately 0.7V or more, the transistor 3 is turned on and the collector voltage rises. Therefore, by comparing this collector voltage with the reference voltage by the voltage comparison circuit 5, the total current ■ is determined. Can be done.

Problems to be Solved by the Invention However, in the conventional load current detection circuit described above, in order to detect the voltage drop (approximately 0.7 V) across the resistor 2 due to a change in the load current using the pace-emitter voltage VBH of the transistor 3, This operating voltage VIIE is large, and therefore there is a problem in that minute current changes cannot be detected.

Also, the pace-emitter voltage VBR of transistor 3
In order to detect the load current, the detected current changes depending on the temperature characteristics, and the voltage between the emitter and base of the transistor 3, vBE, changes due to the voltage fluctuation of the power supply 1, and therefore, the load current cannot be detected accurately. There is a problem.

Note that a method of detecting the load current using a meter or the like is also known, but in this case, minute current changes can be detected, but the operation is slow and there are problems with reliability and cost.

SUMMARY OF THE INVENTION In view of the above-mentioned conventional problems, it is an object of the present invention to provide a load current detection circuit that is inexpensive and capable of accurately detecting minute current changes.

Means for Solving the Problems In order to achieve the above object, the present invention has an emitter of a first transistor connected between a power supply and a resistor for detecting load current, and an emitter of a second transistor connected between the resistor and the load current. A current mirror circuit is provided, the pace of which is connected to the pace of the first transistor, and the current of the load is detected based on the difference between the collector voltage of the second transistor and the reference voltage. It is something.

Effect of the present invention With the above configuration, the voltage drop due to the resistor for detecting the load current is reduced to the transistor pace-emitter voltage vB.
Since it is not detected by E, it operates even with a small voltage drop.
It is possible to accurately detect minute current changes.

In addition, the fluctuation of the collector voltage of the second transistor is
, the current change can be detected accurately without being affected by fluctuations in the power supply voltage because it is obtained by the difference in emitter potential of the second transistor.

Furthermore, since the load current is detected by a current mirror circuit, it is not affected by the ambient temperature unlike the conventional example, and since the circuit elements can be integrated into ICs, it can be constructed at low cost.

EXAMPLES Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of a load current detection circuit according to the present invention, and the same constituent members as those shown in FIG. 2 are given the same reference numerals.

In Fig. 1, l is the load connected in parallel (RL-
)7, (Rtt)8, 2 is a resistor (value R) connected in series between the power supply 1 and the load (RL-)7, (Rtt)8. Sl is the load (RL, )7 and (
This is a switch connected between Rt and t)8.

9, one end is connected between power supply 1 and resistor 2, and the other end is connected to P.
It is a variable resistor connected to the emitter of an NP type transistor 3a, and the collector of the transistor 3a is a constant current source 1.
0, and the pace is connected to the constant current source 10 and to the pace of the PNP type transistor 3b.

The emitter of transistor 3b is connected to resistor 2 and load (RL-
)7 and (RL*)8, the collector is connected to one input terminal of the voltage comparison circuit 5, and the reference voltage source 6 is connected to the other input terminal of the voltage comparison circuit 5. ing.

That is, the transistors 3a and 3b constitute a current mirror circuit whose characteristics do not depend on the ambient temperature.

Next, the operation of the above embodiment will be explained.

In FIG. 1, when the switch S1 is off, when the current of the load (RL, ) 7 is I, a voltage drop of .multidot.R occurs due to the resistor 2.

Here, the value Rv of the variable resistor 9 and the current of the constant current source 10 are
When the variable resistor 9 is adjusted so that c becomes IRKRv-IC, the emitter potentials of the transistors 3a and 3b become equal, and the current flowing through the resistor 2 is still equal to the current Ic of the constant current source 10. They are almost equal.

Next, the switch S1 is turned on from off to turn on the load (R
When the total current of L, )7, and (RLri8) becomes (■10△1), a voltage drop of (■+ΔI)・R occurs due to the resistor 2.

Here, since the voltage drop Rv-IC caused by the variable resistor 9 does not change, the difference between the emitter potentials of the transistors 3a and 3b becomes ΔIR.

Therefore, the current flowing through the transistor 3b decreases due to this emitter potential difference, and the current mirror circuit (3a
, 3b), the collector potential of the transistor 3b changes greatly, and the voltage comparison circuit 5 compares this collector voltage with the reference voltage, and the total current (
+ΔI) can be obtained.

Therefore, in the above embodiment, since the voltage drop caused by the resistor 2 is not detected by the base-emitter voltage Vai of the transistor 3b, the voltage drop is small (for example, 0.IV
) and can detect minute current changes.

Further, since the variation in the collector voltage of the transistor 3b is obtained by the difference Δ■·R in the emitter potential of the transistor 3a13b, it is possible to accurately detect the current change without being affected by the variation in the power supply voltage.

Furthermore, since the transistors 3a and 3b constitute a current mirror circuit whose characteristics do not depend on the ambient temperature, they are not affected by the ambient temperature unlike the conventional example.

In addition, since the circuit elements described above can be integrated into ICs, they can be constructed at low cost, and the variable resistor 9 can detect a wide range of load current.

In the above embodiment, the variable resistor 9 and the constant current source 10 operate the current mirror circuit (3) when the switch Sl is off.
Although the structure is configured to maintain the balance between a and 3b), the variable resistor 9 may be omitted, and the constant current source 10 may be replaced with a resistor.

DETAILED DESCRIPTION OF THE INVENTION As described in the detailed description of the invention, in the present invention, the emitter of the first transistor is connected between a power supply and a resistor for detecting load current;
A current mirror circuit is provided in which the emitter of the second transistor is connected between the resistor and the load and the base is connected to the base of the first transistor, and the difference between the collector voltage of the second transistor and the reference voltage Since the load current is detected, minute current changes can be accurately detected with an inexpensive configuration.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of a load current detection circuit according to the present invention, and FIG. 2 is a circuit diagram showing a conventional load current detection circuit. DESCRIPTION OF SYMBOLS 1... Power supply, 2... Resistor, 3a, 3b... Transistor, 5... Voltage comparison circuit, 6... Reference voltage source,
7, 8... Load, 9... Variable resistance, 10... Constant current source. Name of agent Patent attorney Satoshi Nakao and 1 other person No. 1g
J Figure 2

Claims (4)

[Claims] (1) A resistor connected in series between the power supply and the load, and a first resistor connected in series between the power supply and the load.
a current mirror circuit in which an emitter of a transistor is connected between the power source and the resistor, an emitter of a second transistor is connected between the resistor and the load, and a base is connected to the base of the first transistor. and,
A load current detection circuit comprising: a circuit for detecting a current of the load based on a difference between a collector voltage of the second transistor and a reference voltage. (2) The load current detection circuit according to claim (1), wherein the first collector is connected to a constant current source, and the bases of the first and second transistors are respectively connected to the constant current source. (3) the first collector is connected to a resistor;
, and the bases of the second transistors are respectively connected to the resistor. (4) The load current detection circuit according to any one of claims (1) to (3), wherein a variable resistor is connected between the power source and the emitter of the first transistor.