JPH0152976B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention mainly relates to a burnout detection circuit for lamps in automobiles, etc., in which a voltage drop across a burnout detection resistor connected in series to a plurality of lamps is detected when one or more lamps are burnt out. This relates to a disconnection detection circuit that utilizes the fact that the wire becomes smaller when

FIG. 1 is a circuit diagram showing the basic principle of this type of disconnection detection mechanism, in which 101 is a power source, 102 is a disconnection detection resistor R ₁ , and 103 is a lamp.

FIG. 2 is a diagram showing how the voltage drop across the disconnection detection resistor 102 changes when the power supply voltage Vcc is changed in FIG. b is the characteristic when one wire is disconnected.

In this way, when the power supply voltage is changed, the voltage drop across the disconnection detection resistor 102 becomes saturated at a high voltage.

The reason for this is thought to be that when the power supply voltage is increased, the lamp filament temperature becomes higher and the lamp resistance becomes higher.

Furthermore, when the ambient temperature changes, the above change is small. The reason for this is that the energy generated from _the lamp is almost radiant energy, and the radiant energy obeys Steff-Amboltzmann's law, σ(T _F ⁴ − Ta ⁴ ), T _F
>>Ta, this is because the filament temperature becomes the dominant factor.

The voltage range required for normal operation in automobiles is 8~
16V, and in this power supply voltage range, the characteristics shown in FIG. 2 can be approximated by the following equation as shown by c.

E=A・Vcc+B……(1) In equation (1), A and B are constants.

Therefore, by generating the voltage shown by equation (1) and comparing it with the voltage drop across the disconnection detection resistor, the disconnection state can be detected with high accuracy.

FIG. 3 shows a conventional circuit for generating a voltage according to equation (1), in which Z is a Zener diode and R ₂ to _{R 5} are resistors.

In this circuit, if the zener voltage is V _Z , E ₁ is expressed by the following equation.

E ₁ = R ₂ R ₄ /R ₂ R ₃ +R ₂ R ₄ +R ₃ R ₄・V _CC +R ₂ R ₃ /R ₂ R ₄ +R ₂ R ₃ +R ₃ R ₄・V ……(2) Figure 4 1 is a conventional disconnection detection circuit using this circuit, and 104 is a comparator.

In the disconnection detection circuit, in order to eliminate unnecessary power loss, the voltage drop across the disconnection detection resistor _R1 shown in FIG. 1 is usually set to a small value of 50 to 100 mV. In the circuit shown in Figure 3, normally V _Z is about 5V and V _CC is about 13V, so E ₁ is 50 to 100 m.
When trying to obtain the value of V, R ₂ <<R ₄ and R ₂ << _{R 3} .

Inserting this condition, the above equation (2) becomes as follows.

E ₁ = R ₂ /R ₃・V _CC +R ₂ /R ₄・V _ZIn the above equation, R ₂ /R ₃・V _CC is the voltage drop that occurs in R ₂ due to the current flowing through R ₃ , and R ₂ / R ₄ ·V _Z represents the voltage drop that occurs across R ₂ due to the current flowing through R ₄ .

Generally, in order to compensate well for the characteristics of the lamp, it is necessary that R ₂ /R ₃ ·V _CC to _{R 2} /R ₄ ·V _Z be satisfied. That is, it is necessary that the current flowing through R ₃ and the current flowing through R ₄ be equal at the rated power supply voltage. The rated power supply voltage is V _CC = 13V, and V _Z =
If the current flowing through R ₃ and R ₄ at 5V is 200 μA, then R ₃ = 65KΩ and R ₄ = 25KΩ. If we were to achieve this circuit using the most standard bipolar integrated circuit with a sheet resistance of 200Ω/□, we would need a resistance of 325□ for R ₃ and 125□ for R ₄ to achieve the above resistance. , this requires a large chip area.

In this way, the circuits shown in FIGS. 3 and 4 are
Current consumption cannot be reduced unless relatively large resistors are used for R ₃ and R ₄ . (Especially in integrated circuits, where various functions are incorporated into one package, it is necessary to reduce the number of individual circuits as much as possible.) Furthermore, in order to keep the current flowing through the Zener diode Z, the current flowing through the resistor _R5 must be compared,
The current flowing through resistor _R4 must be kept small, but to do so, resistor _R4 must be set to a large resistance value.

The present invention has been made to solve this problem, and FIG. 5 shows an embodiment of a disconnection detection circuit according to the present invention.

In the figure, 101 is a power supply, 102 is a resistor R ₁ ,
103 is a lamp, 105 is a resistor R ₂ , 106 is a resistor R ₃ , and 107 is a comparator with offset, which is a feature of the present invention.

In Figure 5, the voltage drop across resistor _R2 is Vcc×
R ₂ /R ₂ +R ₃ . Also, since the offset comparator 107 has a constant offset voltage Voff, it is inverted when the voltage drop E ₁ across the disconnection detection resistor R ₁ reaches the following value.

E ₁ =Vcc×R ₂ /R ₂ +R ₃ +Voff...(3) Equation (3) has the same form as equation (1).

FIG. 6 shows a specific circuit example of the comparator with offset 104.

In the figure, the circuit consisting of collector pitch resistor Q ₁ , transistors Q ₂ , Q ₃ , and resistor Rd is a circuit that generates a bias voltage, and the base voltage of Q ₃ is
Set to 2V _BE . Therefore, currents of V _BE /Re and V _BE /Rf flow through the collectors of Q ₉ and Q ₁₀ , respectively.
Rb and Rc are circuits for distributing the emitter currents of Q ₄ and Q ₈ into currents proportional to the reciprocal of the resistance at the inversion level of the comparator. Therefore, at the inversion level of the comparator, a current proportional to V _BE flows through Ra, and Q ₄
The current ratio between Q ₈ and Q 8 is determined by the resistance values of Rb and Rc, and Q ₄
And the V _BE of _Q8 will have a difference.

Here, the threshold value of the comparator composed of Q ₆ , Q ₇ , Q ₅ , and Q ₁₁ (the point where Q ₁₁ switches from on to off) is Q ₆
This is when the base potentials of Q7 and _Q7 are equal. The reason will be explained below.

The size of the collector connected to the collector of Q ₆ and the collector connected to the collector of Q ₇ of multi-collector PNP transistor Q ₅ are equal, and
If h _FE of Q ₅ is sufficiently large and its base current can be ignored, an outflow current equal to the collector current of Q ₆ flows through the collector of Q ₅ connected to the collector of Q ₇ . Therefore, when the base potentials of Q ₆ and Q ₇ are equal and the collector currents of Q ₆ and Q ₇ are equal, Q ₅ 's
The current flowing through the collector connected to the collector of Q ₇ will be equal to the collector current of Q ₇ .

Therefore, when the base potential of Q ₇ becomes higher than the base potential of Q ₆ and the collector current of Q ₇ becomes larger than the collector current of Q ₆ , the base current of Q ₁₁ flows, and Q ₁₁ is turned on.

On the other hand, when the base potential of _Q7 becomes lower than the base potential of _Q6 and the collector current of _Q7 becomes smaller than the collector current of _Q6 , _Q11 turns off.

When the base potentials of Q ₆ and Q ₇ are equal, the voltage applied to resistor Rb and the voltage applied to resistor Rc are equal. Therefore, the current flowing through the resistor Rb at this time is Ib, and the resistor Rc
If the current flowing through is Ic, the following equation holds true.

Rb・Ib=Rc・Ic...(4) The voltage applied to the resistor Re is the base-emitter voltage _VBE of _Q2 . Therefore the emitter current of Q ₉
I _9E becomes as follows.

I _9E = V _BE /Re... (5) If h _FE of Q ₉ is sufficiently large, the emitter current of Q ₉ will be equal to the collector current of Q ₉ .

Since the sum of Ib and Ic is equal to the collector current of _Q9 , the following equation holds true.

Ib + Ic = V _BE /Re...(6) Calculating Ib and Ic from equations (4) and (6) gives the following.

Ib＝Rc／Rb＋Rc・_VBE ／Re……(7) Ic＝Rb／Rb＋Rc・_VBE ／Re……(8) Therefore, at this time, the voltage drop across the resistor Ra
Va, base-emitter voltage of transistor _Q4
V _BE4 , base-emitter voltage of transistor _Q8
V _BE8 is such that h _FE of transistors Q ₆ and Q ₇ is high enough
If the base current can be ignored, the following equation is obtained.

Va＝Ra・Ib＝Ra・Rc／Rb＋Rc・V _BE /Re……(9) V _BE4 ＝kT／qlnIb／Is＝kT／qlnRc・V _BE /Is・(Ra・R
c)・Re...(10) V _BE8 =kT/qlnIc/Is=kT/qlnRb・V _BE /Is・(Ra・R
c)・Re ……(11) Therefore, the voltage difference Voff between the non-inverting input terminal T ₁ and the inverting input terminal T ₂ in the threshold state, that is, the state where the base potentials of Q ₆ and Q ₇ are equal, is as follows. Become.

Voff＝Va＋V _BE4 −V _BE8 ＝Ra・Rc／Re(Rb＋Rc)・V _BE ＋kT／qlnRc／Rb……(
12) Generally, it is known that when V _BE and a voltage proportional to absolute temperature T are added together at an appropriate ratio and α is selected so that V _BE + αT = 1.25V, the temperature coefficient becomes 0.

In equation (12), the first term is proportional to V _BE and the second term is proportional to T, so by appropriately selecting Ra, Rb, Rc, and Re, a voltage with an arbitrary value of temperature coefficient 0 can be created. It also shows that it is possible to create voltages with arbitrary temperature coefficients.

Note that if V _CC is smaller than the intersection point of a and c shown in Figure 2, a disconnection detection output will be output even though there is no disconnection, but this intersection point is less than the practical power supply voltage range. Since the power supply voltage is sufficiently low (an area in which the lamp is not seen to be lit) and disconnection in this range is not a problem, no inconvenience will occur.

As described above, according to the disconnection detection circuit of the present invention,
By using a comparator with an offset, there is no need to use a high resistance to generate a comparison voltage as in the conventional case, and a disconnection detection circuit that can be easily constructed using an integrated circuit can be obtained.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the principle of lamp burnout detection, Fig. 2 is a diagram showing the relationship between the power supply voltage and the voltage drop of the burnout detection resistor, Fig. 3 is a diagram for compensating for conventional lamp characteristics, and Fig. 4 Figure 5 is a circuit diagram showing a conventional disconnection detection circuit, Figure 5 is a circuit diagram showing an embodiment of the disconnection detection circuit of the present invention, and Figure 6 is a specific example of a comparator with offset used in the present invention. FIG. In the figure, 101 is a power supply, 102 is a disconnection detection resistor,
103 is a lamp, and 107 is a comparator with offset. In the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1 a power source and a first resistor connected to the power source;
a lamp connected between the first resistor and ground; second and third resistors connected between the power source and ground;
A comparator with an offset having an input terminal connected to a connection point between the first resistor and the lamp, and a connection point between the second and third resistors, and having a constant offset voltage independent of the power supply voltage. A disconnection detection circuit characterized by: