JPS5858012B2 - temperature converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a temperature converter using a three-wire resistance temperature detector.

Generally, in an automatic control system, if a resistance temperature detector breaks due to fatigue or damage, it must be detected and appropriate measures taken to ensure safe operation of the control system.

Therefore, in order to detect disconnection of the resistance thermometer in a resistance thermometer type temperature converter, it is desirable to include a circuit that operates to swing the output signal outside the standard signal range, either the upper or lower limit. .

This is generally called a burnout circuit.

FIG. 1 shows an example of a conventional temperature converter composed of a three-wire resistance temperature detector and an arithmetic unit.

In Fig. 1, 1 is a three-wire resistance temperature detector consisting of a current supply lead wire A, a voltage detection lead wire B1, and a common lead wire B2. is the equivalent resistance of each lead wire.

Reference numeral 2 denotes a constant current source, which supplies a constant current i to the resistance temperature sensor 1 through lead wires A, B2 and bias resistor Rb as shown by the arrow in the figure.
is supplied.

Here, the bias resistor Rb is connected between the constant current source 2, the 00M line which is the reference potential of the arithmetic unit 3, and the common lead line B2.

The voltage generated through the current supply lead A with the 00M line as a reference potential is applied to the positive phase input terminal of the arithmetic unit 3 having a high input impedance via a low-pass filter circuit consisting of a resistor R1 and a capacitor C1. On the other hand, the voltage generated via the voltage detection lead wire B1 is applied to the anti-phase input terminal of the arithmetic unit 3 having a high input impedance via a low-pass filter circuit consisting of a resistor R2 and a capacitor C2.

Here, the arithmetic unit 3 sets the voltage applied to the positive phase input terminal as el, the voltage applied to the negative phase input terminal as e2, and the output voltage as e, using the 00M line as a reference potential.

, it is constituted by a differential amplifier that performs calculations as shown in equation (1).

In equation (1), G is the conversion gain of the arithmetic unit 3.

Further, ZD is a zener diode connected to both ends of the constant current source 2, and when the resistance temperature detector 1 is disconnected, the constant current source 2
This is to prevent an excessive voltage from being applied to the positive phase input terminal of the arithmetic unit 3 due to the load being opened, and to protect the constant current source 2 itself.

Here, the Zener voltage ■Z of the Zener diode ZD is set to ■7>(rl-+
-Rj +r2+Rb) i is selected.

R1 is the resistance value of the resistance temperature detector 1.

In addition, Senada, Iode 7. I) is not necessarily required depending on the circuit configuration of the constant current source 2 and the arithmetic unit 3.

In the temperature-nitriding converter shown in FIG. 1 configured as described above, the resistance value Rt of the temperature-measuring resistor 1 is expressed by equation (2).

Here, Rlb is a constant bias resistance value determined by the type of resistance temperature detector and the lower limit of the measurement temperature range, and ARl is determined by the type of resistance temperature detector and the measurement temperature range.411] Temperature from the lower limit of the constant temperature range This is the resistance change corresponding to the change.

First, since the zener diode ZD is in the cutoff state in the steady state, the voltage e1 applied to the positive phase input terminal of the calculator 3
Also, the voltage e2 applied to the negative phase input terminal is rr
"" r 2 "" r 3, (3) 1 (4)
Substituting the equation into equation (1) 19. In equation (5), if the constant bias resistance value R1b included in the resistance temperature detector 1 and the value of the bias resistance Rb are chosen to be equal to Rtb - Rb, equation (6) can be obtained. obtain.

As described above, in the steady state, the output of the calculator 3 is the resistance r of the lead wire, as shown in equation (6),
The resistance change ARt in any range of the resistance temperature sensor can be converted into an electrical signal in a desired range without being influenced by r2 and r3.

Next, in the temperature converter shown in FIG. 1, the operation of the resistance temperature detector 1 in various abnormal states will be explained using FIG. 2.

Note that the equivalent resistance r1. of each lead wire shown in FIG.
The values of r2 and r3 are the circuit resistance IR+, R2, Rl)
The resistances r1 to r3 of the lead wires are omitted in FIG. 2 because these values are sufficiently small compared to the values of , and there is no problem in explaining the operation in an abnormal state.

First, when the current supply lead A of the resistance temperature detector 1 is disconnected, the circuit shown in FIG. 1 is represented as shown in FIG. 2a.

At this time, the constant current i from the constant current source 2 flows only through the zener diode ZD as shown by the arrow in the figure, and the voltage e1 at the positive phase input terminal of the arithmetic unit 3 is equal to the zener voltage of the zener diode ZD.
It becomes equal to the value of 7.

On the other hand, the voltage e2 at the negative phase input terminal becomes zero.

Here, since the value of the Zener voltage ■Z is selected to be sufficiently large compared to the steady state signal voltage e1 shown in equation (3),
As is clear from equation (1), the output voltage e of the computing unit 3.

swings to the positive side. Below, I will write this as Burnara I...
It's called upscaling.

Next, when the voltage detection lead wire B1 of the temperature sensing resistor 1 is disconnected, the circuit shown in FIG. 1 is represented as shown in FIG. 2.

At this time, the constant current l from the constant current source 2 flows through the resistance temperature detector 1 and the bias resistor Rb as shown by the arrows in the figure, as in the steady state.
The voltage e1 applied to the positive phase input terminal of the arithmetic unit 3 is (
3) It is the regular voltage in the steady state shown in the equation.

However, the negative phase input terminal with high input impedance is in an open state, and the output e of the computing unit 3 is caused by line noise etc.
Q becomes indeterminate.

In the rare case that the common ground wire b2 of the temperature sensing resistor 1 is disconnected, the circuit shown in FIG. 1 is changed as shown in FIG. 2C.

At this time, the constant current i from the constant current source 2 flows only through the zener diode ZD as shown by the arrow shown in the figure because the human power impedance of the arithmetic unit 3 is very high, and a zener voltage 2 is generated.

This Zener voltage ■Z is applied to the positive phase input terminal of the computing unit 3 via the resistor R1, and to the negative phase input terminal of the computing unit 3 via the resistor Rt and resistor R2 of the temperature sensing resistor 1, respectively.

Here, the resistance value Rt of the resistance temperature detector 1 and the circuit resistance R,
, R2, the human power impedances at the positive-phase input terminal and the negative-phase input terminal of the computing unit 3 are selected to be sufficiently high, so that el = e2 = Vz.

Therefore, the output voltage e of the arithmetic unit 3.

From equation (1), e. =G(vz-IXVZ)=-
G・Vz and swings to the negative side.

Hereinafter, this will be referred to as burnout downscaling.

In this way, in a temperature converter using a conventional three-wire RTD, the burnout direction is not constant due to various abnormal conditions caused by disconnection of the RTD. The drawback was that it was not possible to select the direction of operation.

The purpose of the present invention is to eliminate the drawbacks of the conventional circuit as described above, and to ensure that the output of a temperature converter, for example, is switched to the positive side even when various abnormal conditions occur due to disconnection of a 3-wire resistance temperature detector. It is an object of the present invention to provide a temperature converter which realizes a burnout circuit with a simple configuration so as to perform a so-called burnout upscaling operation.

The gist of the present invention is to connect a first resistor between the input of the computing unit and the detection lead wire, and to connect a unidirectional semiconductor element between the input side of the computing unit of the first resistor and the common lead wire. connect and
A second resistor is connected between the detection lead wire side of the first resistor and the common lead wire, so that burnout and upscaling will always occur when the resistance temperature detector is disconnected.

Hereinafter, a temperature converter equipped with a burnout circuit according to the present invention will be explained in detail using an embodiment shown in FIG.

Note that in FIG. 3, the same parts as in FIG. 1 are designated by the same reference numerals.

In Fig. 3, the difference from Fig. 1 is that the connection point between the voltage detection lead wire B1 and the resistor R2 and the common potential 00M
A high-impedance resistor R3 is connected between them, and a unidirectional semiconductor element, such as a diode D, is shown between the connection point between the resistor R2 and the negative phase input terminal of the arithmetic unit 3, and the common lead wire B2. The only difference is that they are connected by polarity, and the rest is exactly the same as the circuit configuration shown in Figure 1.

Here, the value of resistor R3 is Rs> (r2 + rs +
Rb), (R2+Rb).

Also, the forward breakdown voltage VI) of diode D is VD>r
3. Selected as i.

Therefore, in the steady state, the diode D is in the cutoff state and the value of the resistor R3 is also high impedance, so it is the same as the state in which the diode D and the resistor R3 are removed from the circuit, and the measurement circuit is not affected.

That is, as in the embodiment shown in FIG. 1, the constant current i from the constant current source 2 flows as shown by the arrow in the direction <rl→Rt-+r3→Rb, and its operation and effect will be described in detail in the embodiment shown in FIG. It is the same as that.

That is, in a steady state, the output e of the computing unit 3.

is given by equation (6), and measurement conversion is performed accurately.

Next, referring to FIG. 3, the operation of the resistance temperature detector in various abnormal states will be explained using FIG. 4.

Here, the equivalent resistance r1 of each lead wire shown in FIG.
The value of ~r3 is a very small value compared to the value of the circuit resistance R7~R3°Rb, and there is no problem in explaining the operation in an abnormal state. Therefore, in Fig. 4, the resistance of the lead wire is r1 to r3 are omitted.

Note that the output e of the arithmetic unit 3.

The conditions for ensuring burnout and upscaling are:
As is clear from the relationship between equations (1) and (6), (7
) is shown by the formula.

Here, JRts is the resistance value change span of the resistance temperature detector 1 corresponding to the temperature change span from the lower limit value to the upper limit value of the measurement temperature range.

First, when the current supply lead A of the temperature sensing resistor 1 is disconnected, the circuit shown in FIG. 3 is represented as shown in FIG. 4a.

At this time, since the positive phase input terminal of the arithmetic unit 3 has a high input impedance, the constant current i from the constant current source 2 flows only through the zener diode ZD as shown by the arrow in the figure.

Therefore, the Zener voltage Vz of the Zener diode ZD is applied to the positive phase input terminal of the arithmetic unit 3.

On the other hand, the voltage expressed by equation (4) that was applied to the negative phase input terminal of the arithmetic unit 3 during steady state is also charged in the capacitor C2, but at the same time as the lead wire A is disconnected, the low resistance R2 and Rb , and the voltage e2 becomes zero.

Therefore, as in the case of FIG. 2a, the output voltage of the arithmetic unit 3 instantaneously becomes burnout and upscale.

Next, when the voltage detection lead wire B1 of the temperature sensing resistor 1 is disconnected, the circuit shown in FIG. 3 is represented as shown in FIG. 4.

At this time, the constant current l from the constant current source 2 is equal to the resistance Rt s of the hidden resistor 1, as shown by the arrow in the figure, as in the steady state.
The voltage e1 flowing through the bias resistor Rb and applied to the positive-phase input terminal of the arithmetic unit 3 is the same as the normal voltage in the steady state shown by equation (3).

On the other hand, the voltage e2 expressed by equation (4), which was applied to the negative phase input terminal of the calculator 3 during steady state, is also charged in the capacitor C2, and from the time of the disconnection of the lead wire B1, the resistor R
2 and R3, and finally the voltage of e2 becomes zero.

Note that since the value of the resistor R3 is selected to be much larger than the value of the resistor R2, the discharge time constant is almost equal to the input impedance of the negative phase input terminal.

Therefore, the output e of the arithmetic unit 3.

As is clear from equation (1), as e2 decreases, it gradually increases, and eventually swings out to the positive side, resulting in burnout upscaling.

The value of voltage e2 that causes burnout upscaling is (
7) From Eq.

In equation (3), the value of el in the case of the minimum input value (lower limit value of the measurement range), which is the worst condition for burnout operation, is
Since JR4=0, the equivalent resistance of the lead wire r1
．． If r3 is ignored, el==(R1b+Rb)i.

Here, since Rlb and Rb are chosen equally, e
1=2Rtb-i.

Substituting this into equation (8) yields.

That is, as is clear from equation (9), the output of the arithmetic unit 3 becomes burnout/upscale when the voltage of R18 e2 becomes smaller than the value of (Rt b - lower 1) i.

In addition, in order to satisfy formula (9), Rtb>JR(
Must be 8/2.

This is a condition that causes almost no problem in the normal temperature measurement range.

In addition, in the case of the condition of R1b≦ARt8/2, or when it is desired to quickly burnout or upscale the output, the voltage value (- If a voltage source 4 of Vb) is connected, the voltage will be charged with a time constant of (R3>R2, so R2 is ignored) from the time the lead wire B1 is disconnected, and the voltage will eventually be applied to the negative phase input terminal of the calculator 3. As is clear from equation (1), e2 becomes a twice as positive voltage whose polarity is inverted and is added to voltage e1.

Therefore, the output e of the arithmetic unit 3.

is forced to quickly swing to the positive side, resulting in burnout and upscaling.

What needs to be noted here is that by connecting the voltage source 4, the current of 1 b/R3 in steady state is r2→B1→B
In order to prevent measurement errors from flowing from 2 to r3 to Rb, -
(2) It is necessary to select the voltage (-Vb) and the value of the resistor R3 so that b/R3(i).

This condition is by no means difficult. For example, R3
= 560Ω, Vb = -IV, and i = 2mA, the above conditions are fully satisfied and burnout is ensured.
Upscaling can be achieved.

Finally, if the common lead wire B2 of the resistance temperature detector 1 is disconnected, the circuit shown in FIG. 3 is represented as shown in FIG. 4C.

The input impedance of the calculator 3 and the resistance R3 are compared to the values of the resistance R1 and the resistance (R2+Rb) of the resistance temperature detector 1.
Since the value of is selected to be very high, in this case, the constant current i from the constant current source 2 is R1-+R2 as shown by the arrow.
It flows as →D→Rb.

Therefore, the voltage e1 applied to the positive phase input terminal of the arithmetic unit 3 is el=(R1+R2+Rb)i+VI).

Here, since Rt=Rib+JRt and Rtb-9, the above equation can be expressed as equation (10).

Here, the zener diode Z
The value of the zener voltage Vz of D is chosen large and the zener diode is in the cut-off state.

Also, the voltage e2 applied to the negative phase input terminal of the arithmetic unit 3 is 01)
It can be photographed in the ceremony.

Here, the conditions for burning out and upscaling the output of the arithmetic unit 3 can be found by substituting the equation (10) into the equation (7).

That is, (JRt+R2) IV D> J R(s
・When i.

In the above equation, the worst condition for burnout upscaling is when the input is the minimum value (lower limit value of the measurement range), that is, 1. Since (R,=0), the equation 02 is finally obtained.

Therefore, the output e of the arithmetic unit 3.

To ensure burnout upscaling (1
2) A value for the resistor R2 that satisfies the equation may be selected.

In equation (12), ARt8 is several 10Ω to several 100Ω
Since VD is the forward breakdown voltage of diode D, it is approximately 0.6V.

Therefore, for example, if i = 2 mA, by selecting the value of the resistor R2 to be around 1Ω, the output e of the calculator 3.

You can reliably burnout and upscale instantly.

In the burnout circuit of the embodiment shown in FIG. 3, a case has been described in which a zener diode (ZI) is provided, but as described above, this is not necessarily required.

If the current supply lead A is disconnected when there is no Zener diode ZD, as shown in Figure 4a, the input impedance of the arithmetic unit 3 is very high, so the load on the constant current source 2 is substantially equivalent to an open circuit. Therefore, constant current i does not flow.

For this reason, a power supply voltage for driving the constant current source 2 (not shown) appears at both ends of the constant current source 2 and is applied to the positive phase input terminal of the arithmetic unit 3, resulting in burnout and upscaling of the output.

Lead wire B1. Figure 4 shows the burnout operation when B2 is disconnected.

As is clear from C, it is not related to the presence or absence of the zener diode ZD.

Furthermore, in FIG. 3, a case has been described in which the resistance temperature detector 1 has a constant bias resistance Rtb, and the measurement circuit is provided with a bias resistance Rb in order to subtract this bias resistance. is not necessarily required for the operation of the burnout circuit of the present invention.

When resistance temperature detector 1 does not include a certain bias resistance (R
tb=O) necessarily requires no bias resistor Rb (Rb=
O).

In this case as well, the burnout operation of the present invention is not restricted in any way.

However, if the voltage detection lead wire B1 is disconnected, the condition of equation (9) for burnout and upscaling will not be satisfied, so in order to perform upscaling, be sure to connect the voltage source 4 in series with the resistor R3. need to be connected.

(Refer to Figure 4.) Also, in Figure 3, the polarity of the arithmetic unit 3 is explained as the positive phase input terminal (G) on the voltage e1 side and the negative phase input terminal (→) on the e2 side, but it is completely opposite to that shown in the figure. This can also be applied to polar cases.

In this case, the output e□ of the arithmetic unit 3 is eo−G(2e2−
el), which has the opposite polarity to equation (1).

Therefore, if the resistance input JRi of the resistance temperature sensor increases, the output e.

increases on the negative side.

Similarly, in all burnout operations during various abnormal conditions due to disconnection of the resistance temperature detector 1, the output swings to the negative side, but since the output direction matches the direction when the input JRt increases, the burnout operation still occurs. It's upscale.

In addition, in FIG. 3, the polarity of the constant current source 2 is set to the negative side as the reference potential C! OM, but it can also be applied to cases where the polarity is completely opposite to that shown.

At this time, the polarities of the diode D1, the zener diode ZD, and the voltage source 4 (see FIG. 4) may all be reversely connected to those shown in the drawing.

In the embodiment shown in FIG. 3, one side of the voltage source 4 is connected to the reference potential 0.
Although an example in which the lead wire B2 is connected to 0M is shown (see FIG. 4), it may be connected to the intersection of the lead wire B2 and the bias resistor Rb.

However, it is more convenient to connect to the reference potential 00M side because the voltage source 4 can be shared with the power source for driving the arithmetic unit 3 (not shown).

In the embodiments shown in FIGS. 1 and 3, for convenience, the common lead wire B2 side of the resistance temperature detector 1 is set as the reference potential of the arithmetic unit 3.
Regarding the case where the arithmetic unit 3 is configured with a differential amplifier,
Although the operation of the burnout circuit according to the present invention and its effects have been described, it is not limited thereto.

For example, the voltage detection lead wire B1 side of the resistance temperature detector 1 (
Even when the reference potential of the arithmetic unit 3 is set to the negative phase input terminal (to which the voltage e2 is applied in the embodiments of FIGS. 1 and 3), the burnout circuit according to the present invention can be applied without departing from the gist thereof. It is applicable.

However, in this case, the voltage at the input terminal to which the voltage generated via the lead wire B2 and bias resistor Rb in FIG. When, as the computing unit 3, e.

-Gl el + e21, which has the same conversion gain G from the two input terminals.

As described above, according to the burnout circuit according to the present invention,
When various abnormal conditions occur due to disconnection of the resistance temperature detector,
Since the output can always be burnout/upscaled, the control system can be operated safely.

[Brief explanation of drawings]

Fig. 1 shows an example of a conventional resistance thermometer type temperature converter, Fig. 2 is an equivalent connection diagram of the circuit shown in Fig. 1 in the event of an abnormality,
FIG. 3 is an embodiment of a resistance temperature detector type temperature converter equipped with a burnout circuit according to the present invention, and FIG. 4 is an equivalent connection diagram of the circuit shown in FIG. 3 in the event of an abnormality. 1... Resistance temperature sensor, 2... Constant current source,
3... Arithmetic unit, A, B1, B2...
Lead wire, R2r... Resistor, C... Capacitor, ZD... Zener diode, D...
····diode.

Claims (1)

[Claims] 1. A resistance temperature detector having a supply lead wire at one end, a branched detection lead wire and a common lead wire at the other end, and a constant current that flows through the resistance temperature detector through the supply lead wire and the common lead wire. With the source and the common IJ wire at a reference potential, the voltage generated via the supply lead wire and the detection lead wire is manually applied to generate an output signal corresponding to the temperature from a change in the resistance value of the temperature measuring resistor. A temperature converter comprising a computing unit, wherein a first resistor is connected between the input of the computing unit and the detection lead wire, and the first resistor is connected to the input side of the computing unit and the common lead wire. A temperature converter, characterized in that a unidirectional semiconductor element is connected between the first resistor and the common lead wire, and a second resistor is connected between the detection lead wire side of the first resistor and the common lead wire.