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Soldering Iron Test Station


This project, a soldering iron test station from 1990,  could be a useful system to create as part of your own test and calibration equipment. It is well know that having a correctly calibrated soldering iron allows joints to be made correctly and hence your designs to function better and for longer.

This design of this soldering iron calibration system is for a three wire soldering iron and it measures three parameters when you touch the tip of the soldering iron to the sensor.

Of course with a modern microcontroller you may be able to pack more of the functions such as RMS conversion and use the internal ADC e.g. 18F series. You could also make use of the USB interface in a 18F2550 etc to act as a data recorder to check reliability of the soldering iron over time.

These are :

Tip temperature.

Tip voltage to ground.

Tip resistance to ground.

It performs the measurements utilising a thermocouple.



Executive Summary of the Soldering Iron Test Station

A method and apparatus for testing handheld soldering irons having a three wire soldering tip by touching the soldering iron tip to a single point sensor to measure tip temperature, tip voltage to ground the tip resistance to ground. A thermocouple sensor for use with a soldering iron testing apparatus is described.

Background of the Soldering Iron Test Station

To assure high quality solder joints and avoid damage to sensitive electronic components during production procedures, it is important that various soldering iron operating parameters be maintained within acceptable limits. This fact has been widely recognized and has motivated the adoption of several standards and specifications:

DOD-STD-2000-1B

DOD-HDBK-263

WS 6536E-2

MIL-S-45743E

MIL-STD-2000 1989.

The standards and specifications require measurement of soldering iron tip temperature, tip voltage to ground and tip resistance to ground.

One prior art device, Wahl, U.S. Pat. No. 4,878,016, discloses an apparatus for measuring all three parameters. Wahl uses a single sensor, a thermocouple, for making all three measurements. Because the sensor is a thermocouple, the temperature of the soldering iron tip causes a temperature induced voltage when measuring tip voltage to ground and tip resistance to ground.

This temperature induced voltage is typically 20 mV to 50 mV depending on the thermocouple type. The tip voltage to ground limit in several of the standard is 2 mV. It is not possible to measure tip voltage and ground without compensating for temperature induced voltage. The temperature induced voltage introduces errors of 10% or more when measuring tip resistance to ground.

The Wahl apparatus shorts the thermocouple wire ends to eliminate the effect of the temperature induced voltage.

Summary of the Soldering Iron Test Station

The present design is directed to an improved method and apparatus for testing three wire soldering irons and related heating devices such as desoldering devices, solder pots and solder waves (for soldering iron calibration). Electrostatic discharge devices such as wrist straps are also tested.

The soldering iron tip is touched to a single sensor to measure tip temperature, tip voltage to ground, and tip resistance to ground. In addition to numeric indication of the measured value, a pass/fail indication is provided.

The sensor is a Type K beaded wire thermocouple. The thermocouple junction is raised above the sensor mount to make the junction readily visible and accessible. The soldering iron tip is placed against the thermocouple junction from below the raised wire and a bead of solder is placed on the thermocouple junction from above.

The remote ends of the two thermocouple wires are selectively connected through an electronic switch to a temperature measuring circuit, a voltage measuring circuit or a resistance measuring circuit. A microcontroller is used to control the electronic switch.

In the temperature measuring mode, the differential voltage developed across the thermocouple wire ends is applied through input-protection resistors to an amplifier circuit. The resulting voltage is then converted to a digital signal. The microcontroller converts the digital signal to a normalized temperature signal.

In the voltage measuring mode, the common-mode voltage appearing at the thermocouple ends is applied through loading resistors to an amplifier circuit. The resulting voltage is then converted to a digital signal. The microcontroller converts the digital signal to a normalized voltage signal. The loading resistors are selected to eliminate the temperature induced voltage caused by heating the thermocouple junction.

In the resistance measuring mode, a current is placed through a precision resistor, one wire of the thermocouple, the soldering iron tip and then to ground. The voltage drop across the precision resistor is measured to determine the current magnitude. The voltage between the thermocouple junction and ground is measured through the second wire of the thermocouple.

By passing the current through one side of the thermocouple and taking the voltage measurement through the other side of the thermocouple, the effect of the sensor resistance is removed from the voltage measurement. The voltage present at the second wire of the thermocouple and the voltage drop across the precision resistor are measured with the current source turned off.

These voltages represent the temperature induced voltage and is substracted from the current induced voltage before calculating tip resistance to ground. This eliminates the effect of any temperature induced voltage on the tip resistance measurement. The resulting voltages are converted to digital signals.

The microcontroller calculates the tip resistance to ground using Ohm's Law.

The normalized signals from the microcontroller are numerically displayed using an LED display. Additionally, the LED display provides pass/fail indication.

Description of the Soldering Iron Test Station

Because electronic components are vulnerable to damage by electrostatic discharge, soldering equipment used by defense contractors must conform to Department of Defense standards. The purpose of these standards is to prevent a system failure caused by damage to a sensitive component during the manufacturing process. The present design, a soldering iron tester, was developed to test soldering irons for conformance with these standards.

The soldering iron testing apparatus allows a soldering iron to be tested by touching the soldering iron tip to a single point sensor 10. With the soldering iron tip in contact with the sensor 10, the three critical parameters, tip temperature, tip voltage to ground and tip resistance to ground, are sequentially measured by rotating a function select switch 76 for each desired parameter.

An auxiliary sensor 10' is provided for temperature testing of soldering pots and soldering waves. In addition, the soldering iron testing apparatus can also test the resistance of electrostatic discharge devices, such as electrostatic discharge wrist straps.


Figure 1 : Is a block diagram of the soldering iron testing apparatus signal processing circuit for the soldering iron test station



The sensor 10 is a typical Type K thermocouple. The block diagram in FIG. 1 shows resistance values used with Chromel-Alumel thermocouples.

A microcontroller 40 is used to control the testing of a soldering iron. The function select switch 76 is set to the desired parameter. The microcontroller 40 controls two electronic switches 20, 30 to select the appropriate measurements to be connected to an analog to digital convertor 70 where a voltage signal is converted to an equivalent frequency signal.

The electronic switches 20, 30 are CMOS multiplexors. The frequency signal is then processed by the microcontroller 40. The microcontroller normalizes the signal and compares the normalized signal to acceptable limits. An LED circuit 50 then displays the appropriate numeric value and indicates whether the measurement passes or fails the appropriate standard.

When measuring the soldering iron tip temperature, the microcontroller 40 uses an appropriate thermocouple equation and linearizes the temperature indicating frequency signal.

In addition to the measured parameters, tip temperature, tip voltage to ground and tip resistance to ground, an analog reference signal, a zero reference signal and a cold junction compensation signal 68 can be switched to the analog to digital convertor 70. The zero reference signal and the analog reference signal are used by the microcontroller to adjust system gain and offset.

The cold junction compensation circuit 68 responds to ambient temperature at the thermocouple 10 to generate an ambient temperature compensation signal.


Figure 2a : Is a schematic representation of the temperature measuring circuit for the soldering iron test station



During measurement of tip temperature, the function selector switch 76 is set to tip temperature. The microcontroller 40 controls the electronic switch 20 to switch the +TC and -TC inputs to an amplifier 62 FIG. 2A. Input-protection resistors 102 and 105 are placed in series with the thermocouple 10.

These resistors protect the electronic switch 20 from inadvertently applied overvoltage signals. The output of the amplifier 62 is connected through electronic switch 30 to the analog to digital convertor 70.

Input-protection resistors 101, 102, 103, 104, 108 and 109 are placed in series with the voltage (+MV, -MV), current (+IS, -IS), and auxiliary thermocouple 10' (+AUX, -AUX) signals.


Figure 2b : Is of the voltage measuring circuit, and fig. 2c is of the resistance measuring circuit for the soldering iron test station



FIG. 2B

During measurement of tip voltage to ground, the function select switch 76 is set to mV(AC) or mV(AC+DC). The microcontroller 40 controls the electronic switch 20 to switch the +MV and -MV inputs to the amplifier 62. Input-protection/loading resistors 103 and 104 are placed in series with the thermocouple 10.

The common-mode voltage potential at the ends of the thermocouple 10 includes any voltage to ground from the soldering iron tip and the temperature induced voltage from the thermocouple 10. Since the temperature induced voltage can be 10 to 100 times the magnitude of the tip voltage to ground, it is necessary to eliminate the temperature induced voltage from the measurement.

Thermal induced voltage error in Type K thermocouple sensors is minimized when the +ve and -ve leg outputs of the thermocouple are weighted by 0.5:1 respectively and summed. The temperature induced voltage of the +ve leg is approximately twice the temperature induced voltage of the -ve leg (these voltages are with respect to the copper tip of the soldering iron).

The value of input-protection/loading resistor 103 is twice the value of the input-protection/loading resistor 104. Resistors 103 and 104 are both inputprotection and loading resistors.

For other types of thermocouples, the ratio of the loading resistors should be the same as the ratio of the temperature induced voltages (the +ve and -ve leg outputs). Because the temperature induced voltages vary from thermocouple to thermocouple, the temperature induced error is essentially eliminated if the ratio of the loading resistors is between 0.75 and 1.25 of the ratio of the temperature induced voltages.

The output of the amplifier 62 is connected to an AC/AC+DC discriminator 64. If the function select switch 76 is set to mV(AC), the discriminator 64 removes any DC voltage from the amplifier 62 output. If the function select switch 74 is set to mV(AC+DC), the discriminator 64 passes the amplifier 62 output without change.

The amplified voltage then passes to a true RMS to DC convertor 66 which converts the AC or AC+DC voltage to a normalized DC voltage. The output of the true RMS to DC convertor 66 is connected through electronic switch 30 to the analog to digital convertor 70.


Figure 2c : Is of the resistance measuring circuit for the soldering iron test station



During measurement of tip resistance to ground FIG. 2C, the function select switch 76 is set to Ohms. The microcontroller 40 controls the electronic switches 20, 30 to make four measurements to calculate tip resistance to ground. The first two measurements are background measurements to remove any error signals present.

The first background measurement is the voltage drop across a precision resistor 107. The microcontroller 40 controls the electronic switch 20 to switch the +IS and -IS inputs to the amplifier 62. The second background measurement is the voltage to ground present at the second wire (B-) of the thermocouple 10.

The voltage is connected through electronic switch 30 to the analog to digital convertor 70. These background measurements are used to account for temperature induced voltages.

The microcontroller 40 then turns on a current source 60 which passes a current through precision resistor 107 to the first wire (B+) of the thermocouple 10 then through the soldering iron to ground. While the current is turned on, a second measurement of the voltage drop across the precision resistor 107 (+IS, -IS) is taken.

The background measurement of the voltage drop across the precision resistor 107 is subtracted from the second measurement of the voltage drop across the precision resistor 107 to obtain the induced voltage drop. Ohm's law is then used to determine the current being applied to the soldering iron tip.

While the current is turned on, the microcontroller 40 measures the voltage to ground present at the second wire (B-) of the thermocouple 10. The background measurement of voltage to ground is subtracted from the second measurement of voltage to ground present at the second wire (B-) of the thermocouple 10 to calculate the induced voltage to ground. Ohm's law is then used with the calculated current and induced voltage to ground to calculate resistance to ground.

Any alternating component of the four measurements (such as tip voltage to ground) is eliminated from these measurements by the analog to digital convertor 70. If the alternating component of the background signal is not removed by the analog to digital convertor 70, a separate discriminator must be provided.

Any errors due to temperature induced voltages are removed from the resistance measurement when the two background measurements are subtracted. Any error caused by the resistance of the thermocouple wires is removed by delivering the current to one leg of the thermocouple and measuring the voltage through the other leg.


Figure 3 : Is a perspective view of a sensor and sensor mount for use with a soldering iron testing apparatus for the soldering iron test station




Figure 4 : Is a cross-section of the sensor and sensor mount shown in fig. 3 for the soldering iron test station



The sensor shown in FIG. 3 and FIG. 4 consists of a substantially cylindrical body 80 formed of electrically and thermally insulating material such as ceramic or glass filled Teflon. Two passages 96 extend through the body 80. First and second wires 82 and 84 of dissimilar metal are joined together at a junction 94 to form a thermocouple 10.

Two support members 86 and 88 support the thermocouple 10 in a raised position above the body 80. These support members 86 and 88 are attached to the body 80 so that the support members 86 and 88 and the thermocouple 10 are held in a rigid fixed raised position. The lower end of the support members 86 and 88 are attached to elongated members 90 and 92.

The elongated members 90 and 92 extend through the passages 96. The elongated members 90 and 92 are shaped differently to assure the sensor can only be inserted in the correct orientation. The members 82, 86 and 90 are formed of the same thermocouple metal, Chromel. The members 84, 88 and 92 are formed of different thermocouple metal, Alumel.


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