Circuit Description; Troubleshooting - Wahl ST2200 User Manual

ST2200 Soldering Iron Tester User Manual
8.4

Circuit Description

Figure 9 is a simplified block diagram of the ST2200. There are six distinct
measuring capabilities shown. During temperature measurement, the
thermocouple characteristics of the type K sensor are used and the microvolt
signals are amplified by the chopper stabilized amplifier, linearized, and
converted to a digital display of engineering units in degrees F or C, depending on
the model.
During the measurement of tip millivolts, the sensor is connected via the chopper
amplifier to a True RMS-to-DC converter which scales its output to 10 mV/mV of
True RMS input voltage. The tip millivolts are displayed in engineering units of
millivolts RMS.
The ohms circuitry consists of a precision 10mA DC current source and an ohms
converter circuit to provide the scaling and offsets necessary to display the tip
resistance to ground yet nulling out the resistance of the instrument and its power
cable. Ohms are displayed in engineering units of Ohms.
The analog to digital converter is a 16 bit dual slope converter which features auto
zero drift nulling capability and excellent long term stability.
The Display is a 0.7 inch, 4-1/2 digit, bright LCD display. In addition, peripheral
LED status indicators provide flashing go/no-go warnings at preset levels when
tip voltage or tip resistance exceed default levels.
The AC power inputs and sensor input are transient, static and surge protected for
the utmost in damage immunity. The output jack buffer is also protected against
shorts to ground and overvoltage inputs.
8.5

Troubleshooting

When trouble occurs, it will usually center around three main areas.
1. Defective soldering irons which act erratically.
2. High ground path resistance for the ST2200 or the soldering iron under test.
3. A faulty ST2200.
8.5.1 Troubleshooting - Temperature Measurement Problems
Following are some suggestions for obtaining accurate readings or to solve
temperature measurement problems.
1. Make sure the soldering iron tip is clean and tinned properly. A small amount
of clean solder will insure a good thermal connection.
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