JPH0668522B2 - Current detection circuit - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a current detection circuit for detecting a current that is extremely smaller than the allowable current of the circuit.

"Prior art and its problems" Generally, when measuring the current flowing through a circuit in an electronic circuit, a low resistance is inserted in the circuit, the voltage across the resistance is measured, and the current value is estimated by this. This method is often used.

However, if the current in the circuit is much smaller than the permissible current of this circuit, for example, 1/100 to 1/100 of the permissible current.
It is extremely difficult to accurately detect this value when a current of 1/000 is flowing. For example, in FIG. 6, the current detection resistor R1 is connected to the emitter of the transistor Q1, and the connection point d is connected to one input terminal of the comparator 6. In this circuit, the detected current flowing in the transistor Q1 is detected by the current detection resistor. When the voltage across the resistor is detected by R1 and the voltage across the resistor is compared with the reference voltage VR by the comparator 6, the value of the resistor R1 may be increased to increase the detection sensitivity. If the value is large, when a large current flows through the transistor Q1, the voltage drop of the current detection resistor R1 becomes too large, and in the switching circuit, the voltage when the switch is on becomes high, which is not preferable. Also, the reference voltage V
If R is simply reduced, the operation may become unstable due to noise or the like because the detection voltage is low.

[Object of the Invention] In view of such conventional problems, the present invention can stably detect a current that is extremely smaller than the allowable current of the circuit, and can detect the voltage even when a large current is applied. An object of the present invention is to provide a current detection circuit with a small drop.

"Structure of the Invention" Connecting the second transistor to the first transistor,
The second transistor is driven by the voltage across the current detection resistor connected to the first transistor, and this voltage is compared with the reference voltage by the comparator. By selecting the value of the current detection resistor appropriately,
When the current flowing through the second transistor Q2 is small, the second transistor is almost off, and most of the current flows through the first transistor. Conversely, when the current is large, the second transistor Q2 receives most of the current in addition to the first transistor. Allow the current to flow.

[Example] An example in which the current detection circuit of the present invention is applied to a two-wire type non-contact switch will be described below. That is, in FIG. 1, the detection circuit 1 is connected between the terminals a and b via the constant voltage circuit 2. The detection circuit 1 is specifically a proximity switch or a photoelectric switch. The load 3 is connected in series to the power source 4, and both terminals of the series circuit of the load 3 and the power source 4 are a and b.

Further, a bandgap reference circuit 10 that operates according to the output of the detection circuit 1 is connected between the terminals a and b. The bandgap reference circuit transistor _{Q B, Q C, Q D} , Q E, is composed of _{Q F} resistors _{R A} ~ resistor _{R D,} it is generally broadly applied to this type of switch.

A switching circuit 5 is connected between the terminals a and b at the output end of the detection circuit 1, and the output end of this switching circuit is connected to the comparator 6, and the output end of this comparator is connected to the drive circuit 7. This drive circuit is connected between the power supply terminals a and b in a state of being connected in series with the light emitting diode 8 driven by this drive circuit. In the switching circuit 5, the emitter of the first transistor Q1 is connected to the terminal a, and the collector of the first transistor Q1 is connected to the current detection resistor R1. This resistor and the first transistor Q1 form a series circuit between the terminals a and b. Is forming. That is, this current detection resistor R1
Is adapted to detect the current of the transistor Q1. The second transistor Q2 is the first transistor Q1.
And has the opposite polarity and is connected in parallel to the series circuit. That is, the collector of the second transistor Q2 is connected to the terminal a, and the emitter thereof is connected to the terminal b. Further, its base is connected to the connection point between the collector of the first transistor Q1 and the current detection resistor R1. The on / off of the transistor Q2 is controlled by the voltage across the current detection resistor R1, that is, the emitter current of the first transistor Q1. The connection point is the comparator 6
Is connected to one input terminal, that is, to the base of the transistor Q3, the emitter of which is connected to the terminal a via the current source I1 and the collector to the terminal b. The transistor Q4 has its emitter connected to the current source I1, that is, the emitter of the transistor Q3 and its collector transistor Q5, whose emitter is connected to the terminal b. And the collector base of this transistor is connected to each other. The base of the transistor Q5 is connected to the base of the transistor Q6, the emitter of this transistor is connected to the terminal b, and the collector is connected to the drive circuit 7. A reference voltage source 9 composed of a series circuit of a current source I2 and a resistor R2 is connected between the terminals a and b, and a base of a transistor Q4 is connected to a connection point between the current source I2 and the resistor R2.

The voltage V1 across the current detection resistor R1 is the reference voltage source 9
Is compared with the reference voltage V2 of
It is output from the collector of 6. Transistor _{Q F} is a switching transistor, to the transistor Q1, are Darlington connected. Or base of the transistor Q1 is connected to the emitter of the transistor Q _F, the collector of the transistor Q _F is connected to the collector of the transistor Q1. The base of transistor Q _F is connected to the collectors of transistor Q _E and transistor Q _C.

"Operation" The bandgap reference circuit 10 adjusts the currents of the transistors Q _F , Q1 and Q2 when turned on.
The voltage between V _{+ and} GND is kept at a predetermined voltage V _ON . This voltage V _ON is roughly shown as follows.

V _ON = 1.23 * (R _A + R _B ) / R _A transistor Q _A acts as a switch, and the base current I
_{When BA} flows, it conducts, the above operation is performed, and when I _BA is cut off, the above circuit current stops flowing at all.

Then, in the bandgap reference circuit 10, when the transistor Q _A is turned on, the transistor Q _D and the transistor Q _E pass through the transistor Q _A and the resistor R _A.
A base current flows to the collector of the transistor Q _D. This current causes the base current of the transistor Q _F to flow through the transistor Q _B and the transistor Q _C forming the current mirror circuit. Since the transistor Q _F and the transistor Q1 are Darlington connection,
When a current flows through the base of the transistor Q _F, an amplified current also flows through the collector, whereby the transistor Q _F
The current I _B1 flows in the base of 1, the further amplified current flows in the collector, and a part of it becomes the base current I _B2 of the transistor Q2. When the further amplified current increases, the voltage between the terminals a and b decreases. However, since the base current of the transistor Q _F decreases, the collector current of the transistor Q2 also decreases, and eventually the voltage between the terminals a and b settles at a certain constant voltage.

Since the transistor Q _E has an area N times as large as that of the transistor Q _D , the transistor Q _B and the transistor Q _{E of the} current mirror circuit have different base-emitter voltages V _BE . It represented the difference in V _BE as follows When [Delta] V _BE.

ΔV _BE = (kT / q) * 1 nN where k; Boltzmann's constant T; absolute temperature K q; electron charge 1.6 * 10 ^-19 C. This ΔV _BE is 36 mV at room temperature when N = 4, for example, It shows a positive temperature coefficient of +0.125 mV.

On the other hand, the V _{BE of the} transistor Q _D is normally 650 mV at room temperature, which shows a negative temperature characteristic of −2 mV / ° C. If the temperature characteristic of the voltage V _RD appearing across the resistor R _D is +2 mV,
The voltage V _REF between V ₊ and the base of the transistor Q _D can be kept constant regardless of temperature. Δ of transistor Q _D
V _BE is generated at both ends of the resistor R _C , and this is V _RC . Temperature coefficient TC (V _RD) and V _RC temperature coefficient TC of the resistor R _D V _RD Considering that 2 times the current in the resistor R _C flows through the
The relation with (V _RC ) can be expressed as follows.

_{TC (V RD) = TC (} V RC) * (2R D / R C) TC (V RD) = 2mV / ℃ and To TC (V _RC) = 0.125mV / order ° C. at which _R D / R
_{If C is set} to 8, the temperature characteristic of V _REF can be eliminated.
The value of this time V _RD is for approximately 580mV at room temperature, V _REF becomes _{V REF = V RD + V BED} = 580 + 650 = 1230 That 1230MV.

Thus the voltage V _ON between the _V + to GND resistor _R A, can be determined by _{R B} in the following manner. (Above).

V _ON = 1.23 * (R _A + R _B ) / R _{A In} FIG. 1 and FIG. 2, the second transistor Q2 is supplied while the output current of the switching circuit 5 is small, that is, while the current flowing through the current detection resistor R1 is small. Is off and a current flows only through the first transistor Q1. When the output current of the switching circuit 5 increases, the voltage across the current detection resistor R1 increases, the second transistor Q2 is biased, and most of the current flows through the second transistor Q2 in addition to the first transistor Q1. Like

Here, if the output current of the switching circuit 5 is IT, the emitter current of the first transistor Q1 is IE1, and the collector current of the second transistor Q2 is IC2, then IT = IE1 + IC2, and between O and IT1 in FIG. A relatively large voltage V1 almost proportional to IT is obtained across the detection resistor R1. Then, this voltage becomes the input voltage of the comparator 6, and this comparator detects the voltage V1.

In FIG. 1, when the current of the transistor Q1 is now small, the voltage V1 across the current detection resistor R1 and the reference voltage V2
Since V1 <V2, the transistor Q3 is turned on and the transistors Q4, Q5, Q6 are turned off.

When the current of the transistor Q1 increases and V1 <V2, the transistor Q3 turns off and the transistor Q3
4, Q5 and Q6 are turned on.

The point at which a current flows into the second transistor Q2, that is, the value of IT1 in FIG. 3 depends on the base-emitter voltage VBE2 of the second transistor Q2. Since this base-emitter voltage has a relatively large temperature coefficient, that is, normally −2 mV / ° C., if the reference voltage V2 is set so that the comparator 6 inverts near this IT1, the current detection level fluctuates due to temperature changes. In order to prevent this, as shown in FIG. 4, a diode D1 is inserted so that the reference voltage V2 of the comparator 6 has the same temperature coefficient as the detection voltage V1, and the forward voltage thereof is set to the resistance R3,
It may be divided by R4 and added to the input of the comparator 6, that is, the base of the transistor Q4.

Now, when the transistor Q6 is turned on, the drive circuit 7 is driven, whereby the light emitting diode 8 is turned on, and it is displayed that the switching circuit 5 is in the operating state. Of course, at this time, a sufficient current is flowing through the load 3 to operate it.

Needless to say, the same operation is performed even if the polarities of the respective transistors and the polarities of the power supplies are all inverted as shown in the circuit diagrams of FIGS. 1, 2, and 6.

"Effect" The present invention connects the first transistor and the current detection resistor in series as described above, and also connects the second transistor in parallel with the series circuit, and the second transistor is the first transistor. Is driven by
Moreover, because the voltage generated by the current detection resistor is applied to the input of the comparator, even a very small current can be detected stably compared to the rated current of the circuit, and the voltage can be detected even when a large current is applied. There is an advantage that the effect is extremely small. In particular, since the voltage across the first transistor is controlled and the voltage between the power supply terminals is also controlled, the light emitting diode is turned on when the current detection circuit of the present invention is applied to a two-wire type contactless switch. In this case, even if an inductance load is connected as a load, the light emitting diode can be reliably turned on.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a current detection circuit according to the present invention, FIG. 2 is a circuit diagram of a main portion, FIG. 3 is a characteristic diagram of voltage and current in FIG. 2, and FIG. 5 is a circuit diagram showing another embodiment of the present invention, FIG. 5 is a circuit diagram showing another embodiment of the present invention, and FIG. 6 is a conventional circuit diagram. 1 ... Detection circuit 2 ... Constant voltage circuit 3 ... Load 4 ... Power supply 5 ... Switching circuit 6 ... Comparator 7 ... Drive circuit 8 ... Light emitting diode 9 ... Reference voltage source 10 ... Bandgap reference Circuit Q1-Q6 ... Transistors I1, I2 ... Current source R1 ... Current detection resistance R2-R4 ... Resistance D1 ... Diode

Claims (1)

[Claims] 1. Terminals a and b of a series circuit of a power source 4 and a load 3.
The first connected between the first and the second detection circuit 1 operates according to the output of the detection circuit 1.
A series circuit of the transistor Q1 and the current detection resistor R1 for detecting the current flowing through the first transistor Q1,
A collector is connected to the terminal side of the first transistor Q1, a base is connected to a connection point between the first transistor Q1 and one end of the current detection resistor R1, and an emitter is connected to the other end side of the current detection resistor R1. And a reference voltage for generating a reference voltage corresponding to the potential of the connection point between the first transistor Q1 and the current detection resistor R1 to a certain extent when the second transistor Q2 is almost off. The generation circuit 9 and the reference voltage generated by the reference voltage generation circuit 9 are compared with the voltage at the connection point of the first transistor Q1 and the current detection resistor R1 and the voltage at the connection point is higher. In this case, a current detection circuit including a comparator 6 that outputs a current detection signal.