Methods and circuits for precise temperature measurement using microcurrent through LEDs or PN junctions

Abstract

This invention uses light-emitting diodes (LEDs) or semiconductor PN junctions as temperature sensing elements, analyzes the causes of errors in temperature measurement using PN junctions, and discloses a method that can forward conduction voltage U of the LED or PN junction. F A circuit that multiplies by 2 in analog operation. This invention discloses a circuit capable of constraining the forward conduction voltage U of an LED or PN junction. F With current intensity I F The ratio is a constant of resistance, thus eliminating the influence of ambient temperature nonlinearity on the reverse conduction current Is of the PN junction. This invention discloses a method for measuring the forward conduction voltage U of an LED or PN junction. F The method and technical solution for calculating temperature by executing a program on a microcontroller was developed, and a circuit device capable of accurately measuring temperature was manufactured.

Description

Technical Field

[0001] This invention belongs to the field of electronic technology. It relates to a method and circuit device for measuring temperature. Background Technology

[0002] The PN junction temperature sensor is based on the change in the forward voltage drop of the PN junction with temperature. The current flowing through the PN junction... In the formula I s It is the reverse saturation current, q is the electron charge, and U is the reverse saturation current. F Here, k is the forward voltage drop across the PN junction, k is the Boltzmann constant, and T is the absolute temperature. Existing PN junction temperature sensors operate by passing a small current I. F Electric power I F ×U F This heat is converted into thermal energy and added to the PN junction, causing the PN junction temperature to be higher than the ambient temperature, thus resulting in measurement errors. Additionally, the PN junction reverse saturation current I... s In fact, it changes nonlinearly due to temperature; it's not a constant, but a function of temperature T, I. s (T),

[0003]

[0004] Taking the logarithm of both sides of the equals sign:

[0005]

[0006] Publicly available literature on PN junction temperature sensor technology mistakenly assumes that as long as the current I passing through the PN junction is limited... F For constant current, the forward voltage drop U of the PN junction F It has a linear relationship with temperature T, but lnI was not noticed. S (T) is not a constant. This is a shortcoming of existing PN junction temperature sensor technology. This invention uses a microcurrent of less than microampere level through an LED or PN junction to achieve accurate temperature measurement. Summary of the Invention

[0007] Existing PN junction temperature sensor technology directly measures the forward voltage drop U of the PN junction. F This invention differs from the previous one; it includes reducing the forward voltage drop U of a PN junction (diode) or LED (light-emitting diode). F The analog computing circuit is multiplied by 2, and the circuit structure is changed from... Figure 1 As shown. Figure 1 D1 and D2 in the diagram are LEDs (Light Emitting Diodes), used as temperature sensing elements. As the ambient temperature changes, the forward voltage drop U of the LED (Light Emitting Diode) increases. F The change is converted into the output voltage V. X :

[0008] V X =U F +I F R (2)

[0009] I F The current passing through LEDs D1 and D2 is the same. When manufacturing D1 and D2, the parameters and performance must be the same, so the current passing through D1 and D2 is the same. Figure 1 The potential at the inverting input of the operational amplifier in the circuit is denoted as V. - The potential at the non-inverting input terminal is denoted as V. +

[0010] V + =U F =V X -I F R (3)

[0011] V - =I F R = Vx - U F (4)

[0012] The operational amplifier operates in a steady state, V + ≈V,U F ≈I F R, therefore

[0013] Vx = U F +I F R≈2U F

[0014] The output voltage Vx is approximately equal to the forward voltage drop U of the LED (light-emitting diode). F Twice that. This invention measures the output voltage V in order to measure temperature. X The value of the LED (light-emitting diode) is used, rather than directly measuring the forward voltage drop U. F This helps to achieve high-precision temperature measurement.

[0015] The invention also includes a function of automatically limiting the current passing through the LED (temperature sensing element). The current intensity I passing through the LED (temperature sensing element) is limited. F For a few microamps or less, the power I is significantly reduced. F ×U F The heat energy is converted into heat energy and applied to the PN junction, thereby making the PN junction temperature approximately equal to the ambient temperature, which helps to achieve high-precision temperature measurement.

[0016] V X =U F +I F R≈2I F R≈2U F

[0017]

[0018] For example, if a red LED is used as the temperature sensing element, at a room temperature of 20℃, U F ≈1.6V, resistor R=1000KΩ, limiting the current through the LED. (microampere).

[0019] This invention measures the physical principles related to temperature. According to equation (5),

[0020]

[0021] This is a constraint on the circuit of this invention: under different temperature conditions, the forward voltage drop U of the PN junction... F With current I F The ratio is the resistivity (R). This is essentially Ohm's law. Under standard atmospheric pressure, the temperature of an ice-water mixture is 0°C, T0 = 273.15K. In the environment of an ice-water mixture, the forward voltage drop of the PN junction is denoted as U0, and the current is denoted as I0; at the temperature T to be measured... x In this environment, the forward voltage drop of a PN junction is denoted as U. x Current is denoted as I x According to equations (6) and (1), the function I is eliminated. s (T) This nonlinear factor:

[0022]

[0023] Taking the logarithm of both sides of the equals sign:

[0024]

[0025] To determine The value of T is given. Under standard atmospheric pressure, the boiling point of water is 100℃. 100 =373.15K, in an environment where water is boiling, the forward voltage drop of the PN junction is measured and denoted as U. 100 Therefore, according to equation (7):

[0026]

[0027] Calculate according to equation (9) The value of is denoted as the g constant. Equation (8) can be rewritten as:

[0028]

[0029] Using the circuit of this invention, Vx is measured, and then... Then, according to equation (10), the temperature T can be calculated by using a microcontroller to execute the program.x This is the physical principle behind the high-precision temperature measurement of this invention. Attached Figure Description

[0030] Figure 1 This is a circuit diagram of the present invention, including the forward voltage drop U of the LED (light-emitting diode). F Multiply by 2 for the analog computing circuit, and the forward voltage drop U of the PN junction under different temperature conditions. F With current I F The ratio is the resistance constant (R), and this constraint condition is achieved in the circuit.

[0031] Figure 2 This is a circuit diagram of a high-precision thermometer made using this invention. Detailed Implementation

[0032] Measuring Vx using a specific circuit refers to sampling and converting Vx using an A / D circuit. Figure 2 In the process, the microcontroller chip includes an A / D circuit, and Vx is input to pin 1 of the microcontroller chip IC2. The temperature is calculated according to equation (10), and the difficulty lies in the calculation. The specific calculation method uses the McLaurin formula:

[0033] make The C language source code for calculating ln(1+x) is as follows:

[0034]

[0035]

[0036] Further to reduce redundant calculations When the thermometer is running, the program creates a two-dimensional array to record the corresponding numerical pairs (U) under different temperature conditions. x T x ), ..., (U n-1 T n-1 ), (U n T n For example, for a certain measured value U t Between (U) n-1 T n-1 ), (U n T n Between ) U n-1 t n According to equation (10), with (U) n-1 T n-1 Replace (U0, T0):

[0037] ​​

[0038] because Equation (11) simplifies to:

[0039]

[0040] Given U n1 t n The real-time temperature T can be calculated according to equation (12). t .exist Figure 2 In the middle, the real-time temperature T is displayed by an OLED display module. t .​​

Claims

1. A circuit device for accurately measuring temperature, characterized in that each temperature sensor uses two light-emitting diodes (LEDs) or two semiconductor PN junctions as temperature sensing elements to form a complete temperature-measuring device.

2. A circuit device for accurately measuring temperature, characterized by including a circuit that constrains the forward conduction voltage U of a light-emitting diode (LED) or a semiconductor PN junction. F With current intensity I F The ratio is equal to the resistance constant, thus eliminating the influence of nonlinear environmental temperature.

3. A circuit device for accurately measuring temperature, characterized by including an analog operational circuit that converts the forward conduction voltage U of a light-emitting diode (LED) or a semiconductor PN junction into a voltage value. F Multiply by 2 and output as temperature measurement data.

4. A circuit device for accurately measuring temperature, characterized in that U is known. x Value, according to the formula The microcontroller executes the program to calculate the temperature T. x .

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