JP4432503B2 - Deuterium lamp drive device - Google Patents

Description

  The present invention relates to a deuterium lamp driving device for driving and driving a deuterium lamp used as a light source for an ultraviolet-visible spectrophotometer or the like.

  A deuterium lamp is widely used as an ultraviolet light source of, for example, an ultraviolet-visible spectrophotometer. As a driving circuit for this deuterium lamp, for example, the one described in Patent Document 1 has been known. FIG. 2 is a block diagram schematically showing the deuterium lamp driving circuit described in this document.

  In FIG. 2, the main power supply unit 3 includes a commercial AC power source and a transformer, and steps down a commercial 100 V (or 200 V) AC voltage to a predetermined AC voltage, and converts the AC voltage into a DC voltage by the rectifier circuit 4. Give to the drive circuit. The driving circuit includes a constant current source 2 that supplies a constant current to the deuterium lamp 1, a heater power supply unit 6 that preheats the deuterium lamp 1, and a trigger voltage that applies a pulsed high voltage to the deuterium lamp 1 at the start of lighting. A generator 5 is provided.

  At the start of lighting of the deuterium lamp 1, the heater power supply unit 6 applies a preheating voltage to the heater of the deuterium lamp 1 to emit thermoelectrons from the heater, and then the trigger voltage generating unit 5 triggers a voltage of about several hundred volts. Apply. As a result, a discharge is generated between the electrodes of the deuterium lamp 1. After lighting, the discharge is maintained by supplying a substantially constant drive current from the constant current source 2 to the deuterium lamp 1. Since the amount of light emitted during the discharge of the deuterium lamp 1 is proportional to the drive current supplied from the constant current source 2, it is necessary to stabilize the drive current in order to stabilize the amount of light emitted.

  In the ultraviolet-visible spectrophotometer using the deuterium lamp as described above, there are a double beam method and a single beam method depending on the configuration of the optical path. In the case of the double beam method, it is possible in principle to cancel the influence of the light quantity fluctuation of the light source in the process of calculating the absorbance, but in the case of the single beam system, the influence of the light quantity fluctuation of the light source cannot be corrected. Variations in the amount of emitted light almost directly affect variations in absorbance. Therefore, in order to improve the accuracy of absorbance, it is necessary to increase the stability of the drive current of the deuterium lamp 1.

  Although various forms can be considered as the constant current source 2 in the deuterium lamp driving circuit, in order to improve the stability of the driving current as described above, a feedback loop is configured by an error amplifier using an operational amplifier (op amp). It is common. FIG. 3 is a circuit diagram illustrating the circuit configuration of the constant current source 2 in detail. In this figure, the trigger voltage generator 5 is omitted, and the input DC voltage is simply shown as a voltage source V1.

In the constant current source 2 in FIG. 3, the lamp current I _L flowing through the deuterium lamp 1 flows through the current detection resistor 21 having a resistance value Rd, and a voltage drop of I _L · Rd = V _L occurs. The error amplifier 22 compares the voltage V _L with the reference voltage V _ref applied by the reference voltage source 23, and outputs a voltage corresponding to the difference. For example, when the voltage V _L is higher than the reference voltage V _ref , the output of the error amplifier 22 increases. Then, the base current flowing from the base terminal of the second transistor 27 via the diode 24 decreases, and the current flowing from the collector terminal of the second transistor 27 to the base terminal of the first transistor 26 also decreases. As a result, the collector current of the first transistor 26, that is, the lamp current _IL is also reduced. Conversely, when the voltage V _L is smaller than the reference voltage V _ref , the output of the error amplifier 22 decreases and the base current of the second transistor 27 increases. As a result, the base current of the first transistor 26 also increases, and the lamp current _IL increases. In this way, as a result, the lamp current I _L is always feedback-controlled so as to be kept at V _ref / Rd, and the lamp current I _L is kept constant.

However, in the configuration of the conventional constant current source as described above, there are error factors as described below. That is, considering the constant current source 2 shown in FIG. 3 in more detail, the base current I _b2 of the second transistor 27 flows from the output terminal of the error amplifier 22 to the ground potential (GND) via the internal power supply circuit. Now, assuming that the current amplification factor of the first transistor 26 is β1 and the current amplification factor of the second transistor 27 is β2, the collector current I _c2 of the second transistor 27 is
I _c2 = β2 ・ I _b2
The emitter current I _e2 is
I _e2 = (1 + β2) · I _b2
It is. The collector current I _c1 of the first transistor 26 is
I _c1 = β1 · I _b1 = β2 · I _c2 = β1 · β2 · I _b2
It is. Therefore, the lamp current I _L is
I _L = I _e2 + I _c1 = (1 + β2) · I _b2 + β1 · β2 · I _b2 = (1 + β2 + β1 · β2) · I _b2
And
I _b2 = I _L / (1 + β2 + β1 · β2)
Because
I _L = (V _ref / Rd) −I _L / (1 + β 2 + β 1 · β 2)
It becomes. That is, it is considered that there is an error in the constant current source 2 by the amount of the base current I _b2 of the second transistor 27. As shown in FIG. 4, a current corresponding to the base current is parallel to the _ideal constant current source i _ideal. It can be considered that the source i _b2 is connected in the reverse direction, and this is an error component of the constant current source 2.

As described above, in the deuterium lamp 1, the error in the lamp current I _L becomes the error in the amount of light. Normally, in a spectrophotometer, the absorbance is obtained from the fluctuation rate of the amount of received light, so even if the absolute value of the light amount includes a steady error, this error does not appear in the fluctuation rate of the light amount and does not cause a big problem. However, when the lamp current I _L fluctuates in a short time in the span than measured, whereby the light emission amount varies, it may appear as a change in absorbance. Now, assuming that the light emission quantity of the deuterium lamp is I ₀ and the fluctuation amount of the light emission quantity is ΔI, the absorbance change ΔA of the single beam spectrophotometer is
ΔA = −log {(I ₀ + ΔI) / I ₀ } = − log {1+ (ΔI / I ₀ )} = − log (1 + Ni)
It becomes. Here, Ni is the variation rate of the amount of emitted light, and if the amount of emitted light varies by the variation rate Ni during one absorbance measurement, the absorbance varies by ΔA shown in the above equation.

As an example, if I _L = 300 mA and β 1 = β 2 = 50, I _b2 is 118 μA. Here, for example, if both β1 and β2 increase by 10% due to temperature change, I _b2 decreases to 97 μA. The light quantity change rate with this temperature change is
ΔI / I ₀ = (118 μA−97 μA) / 300 mA = 7 × 10 ⁻⁵
When this change in light quantity is converted into a change in absorbance, 3 × 10 ⁻⁵ (Abs) is obtained.

JP-A-9-210780

There are various factors that affect the stability of the absorbance, such as the light emission stability of the light source itself. Conventionally, the influence of such other factors has been relatively large. It wasn't too much of a problem. However, in recent years, as the performance of spectrophotometers has improved, the required accuracy has become very high. For example, a high accuracy of the order of 10 ⁻⁶ (Abs) has been required for absorbance stability. ing. In such a situation, the change in the amount of light due to the error of the constant current source 2 as described above cannot be ignored. That is, such a change in the amount of light of the deuterium lamp is one of the factors that influence the measurement accuracy of the absorbance, and it is necessary to solve the above-described problem in order to increase the measurement accuracy to a certain extent.

  The present invention has been made to solve such problems, and the object of the present invention is to provide a lamp current supplied to a deuterium lamp even when there is a temperature change of a transistor constituting a constant current source. The deuterium lamp driving device can maintain the stability of the deuterium lamp and thereby maintain the high stability of the light emission quantity of the deuterium lamp.

As described above, the cause of the error in the constant current source 2 is that the base current I _b2 of the second transistor 27 flows into the error amplifier in the first place, and flows into the GND from the power circuit of the error amplifier. Therefore, this base current I _b2 does not flow through the current detection resistor 21 that originally detects the voltage reflecting the lamp current I _L, and is therefore excluded from the current correction by the feedback loop via the error amplifier 22. Will end up. Therefore, in order to solve such a problem, the current flowing through the control signal input terminal of the current control transistor may be returned to the current detection resistor without flowing into the error amplifier.

That is, the present invention, which has been made to solve the above-described problems, provides a deuterium lamp that is controlled so that the current flowing in the deuterium lamp is constant under the condition that a constant DC voltage is applied to the deuterium lamp. A current detection resistor for detecting a lamp current flowing in the hydrogen lamp; a reference setting unit for setting a reference voltage corresponding to a target value of the lamp current; and the reference voltage and a voltage detected by the current detection resistor. In a deuterium lamp driving device comprising a constant current source that includes an error amplifier that compares and outputs a signal corresponding to the difference, and a current control transistor that controls the lamp current according to an output signal from the error amplifier.
An optical isolation means for interrupting current while transmitting information on the signal is inserted on a signal transmission path between the output terminal of the error amplifier and the control signal input terminal of the current control transistor. It is said.

Here, for example, a photocoupler can be used as the optical isolation means. In the deuterium lamp driving device according to the present invention, when the voltage value by the current detection resistor fluctuates and an error from the reference voltage occurs, the error amplifier increases or decreases the output signal accordingly. This signal is input to the control signal input terminal of the current control transistor via the optical isolation means, and the current flowing through the transistor is changed. Only the information content of the signal is transmitted to the control signal input terminal (the base terminal in the bipolar transistor) of the current control transistor by the optical isolation means, but the current flowing through the control signal input terminal is blocked by the optical isolation means. It does not flow into the error amplifier side. For example, when a photocoupler is used as the optical isolation means, the phototransistor on the light receiving side forms a closed circuit with the current control transistor, and the current flowing through the phototransistor is the collector current (or emitter current) of the current control transistor. ) To flow through the current detection resistor. Thus, for example, the current amplification factor of the current control transistor varies with a temperature change, even if the current flowing through the control signal input is varied, in this constant-current source, the current varies with full I over-back loop including an error amplifier In addition, since constant current control is performed, it is possible to flow a constant current with high accuracy.

  As described above, according to the deuterium lamp driving device according to the present invention, the current accuracy of the constant current source for supplying a constant driving current to the deuterium lamp is improved as compared with the conventional case. Even if there is, the amount of light emitted from the deuterium lamp can be kept stable and constant. Therefore, for example, if this deuterium lamp driving device is applied to a spectrophotometer, the detection sensitivity of absorbance can be further improved. Further, according to the deuterium lamp driving device of the present invention, the current control transistor and the error amplifier are electrically cut off, so that even if the current control transistor is electrically destroyed for some reason, the error amplifier is It is possible to reduce the possibility of following and destroying, and to improve reliability.

  Hereinafter, a deuterium lamp driving apparatus according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a circuit configuration diagram showing the constant current source 2 in detail in the deuterium lamp driving apparatus according to this embodiment, as in FIG. The same parts as those in the conventional configuration described above are given the same reference numerals to clarify the correspondence.

  A feature of the driving apparatus according to this embodiment is that a photocoupler 25 is inserted in a signal transmission path between the output terminal of the error amplifier 22 and the base terminal of the first transistor 26. That is, the output of the error amplifier 22 is input to the light emitting diode PD on the input side of the photocoupler 25 via the diode 24, and the emitter end of the phototransistor PT on the output side of the photocoupler 25 is connected to the base end of the first transistor 26. Has been. As a result, the feedback loop including the error amplifier 22 is electrically cut off at the insertion position of the photocoupler 25, but the error signal from the error amplifier 22 is converted from electricity → light → electricity so that the first transistor 26 It is transmitted to the base end.

The operation of constant current control by the constant current source 2 is as follows. That is, the error amplifier 22 compares the voltage generated in the current detection resistor 21 with the lamp current I _L and the reference voltage V _ref, and outputs a voltage corresponding to the difference. For example, when the voltage V _L is higher than the reference voltage V _ref , the output of the error amplifier 22 decreases. As a result, the drive current flowing through the light emitting diode PD of the photocoupler 25 decreases, the amount of light emission decreases, and the current flowing through the phototransistor PT also decreases. As a result, the current flowing through the base end of the first transistor 26 also decreases, and the collector current of the first transistor 26, that is, the lamp current _IL also decreases. On the contrary, when the voltage V _L is smaller than the reference voltage V _ref , the output of the error amplifier 22 increases. Then, the drive current flowing through the light emitting diode PD of the photocoupler 25 increases, the amount of light emission increases, and the current flowing through the phototransistor PT also increases. As a result, the current flowing through the base end of the first transistor 26 increases and the lamp current _IL also increases.

In such constant current control, the current flowing through the base end of the first transistor 26 is supplied by the phototransistor PT. The current merges with the collector current of the first transistor 26 and flows through the current detection resistor 21. Therefore, when the current amplification factor of the first transistor 26 and the light amount / current conversion efficiency of the phototransistor PT change due to a temperature change or the like, the current decrease or increase due to the change is reflected in the voltage detected by the current detection resistor 21. Accordingly, the constant current control by the feedback loop including the error amplifier 22 simultaneously corrects the current fluctuation caused by the temperature change and the like as described above. A lamp current I _L can flow.

  It should be noted that the above embodiment is an example of the present invention, and it is obvious that the invention of the present application is included even if appropriate changes, modifications, additions, etc. are made within the scope of the present invention. For example, in the above embodiment, a bipolar transistor is used as a transistor, but it can be easily conceived that an FET or the like can be appropriately modified. The same applies to appropriate modifications such as using a PNP type instead of an NPN type as a bipolar transistor, or further connecting a Darlington connection of a transistor to allow a larger current to flow.

The block diagram of the principal part of the deuterium lamp drive device by one Example of this invention. The block block diagram which simplified and showed the deuterium lamp drive circuit. The circuit diagram which described in detail the circuit structure of the constant current source in the conventional deuterium lamp drive circuit. The figure which showed the constant current source in the conventional deuterium lamp drive circuit by the equivalent circuit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Deuterium lamp 2 ... Constant current source 21 ... Current detection resistor 22 ... Error amplifier 23 ... Reference voltage source 24 ... Diode 25 ... Photocoupler PD ... Light emitting diode PT ... Phototransistor 26 ... First transistor

Claims (1)

A current detection resistor for detecting a lamp current flowing in the deuterium lamp so as to control the current flowing in the deuterium lamp to be constant under a condition in which a constant DC voltage is applied to the deuterium lamp; A reference setting unit for setting a reference voltage corresponding to a target value of current, an error amplifier that compares the reference voltage and a voltage detected by the current detection resistor, and outputs a signal corresponding to the difference; and In a deuterium lamp driving device comprising a constant current source including a current control transistor for controlling the lamp current according to an output signal from an error amplifier,
An optical isolation means for interrupting current while transmitting information on the signal is inserted on a signal transmission path between the output terminal of the error amplifier and the control signal input terminal of the current control transistor. A deuterium lamp driving device.

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