Search for faulty elements using a thermometer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The task of finding a faulty small-sized radio element, for example, a capacitor installed in the power circuit, which has a significant current leakage (tens and hundreds of milliamps), cannot be called very difficult. Such an element can be found by controlling its temperature with a finger. True, this method works if the faulty element is "pierced" thoroughly and its leakage resistance is a few or tens of ohms. But what if this resistance is a few kilo-ohms and the current is much less?

Such a task arose when repairing a faulty Samsung TAB 7.7 P6800 tablet computer, which, when turned off, completely discharged a 5 Ah battery in about three weeks. It is in the off, and not in the sleeping state. The external condition of the tablet was very good - it did not fall, did not fill with liquid, the battery was "fresh", and the case was not opened. Having tried all the possible and available troubleshooting methods - updating the software, software disabling the radio modules, removing the SD card, hard reset, etc., I came to the conclusion that the fault must be sought in the hardware on one of the boards. Most likely, the increased current consumption is caused by some element with low leakage resistance installed on the main board. Question - how to find this element? Obviously, it must heat up and its temperature, at least a little, but exceeds the temperature of the environment and neighboring elements.

There is information on the Internet about the successful and fast search for such elements by heating using a thermal imager. But the latter has a significant drawback - a very high price. Remote thermometers are also of little use for this purpose, since they measure the temperature over a certain area, and not at a point. WEB-cameras are also not suitable, even with a remote infrared filter, since their maximum sensitivity lies in a completely different range.

As a result, a multimeter with a temperature measurement function helped out. In stock was a Mastech MS8209 multimeter with a thermocouple included. A thermocouple is a ball about 1 mm in diameter. Its resistance at room temperature is about 14 ohms. Any other thermometer with a resolution of at least 0,1 would be fine. ^оC. Why not use such a thermometer instead of a thermal imager? And, of course, it takes a little patience. The main thing is that the dimensions of the temperature sensor be as small as possible, which speeds up the process and allows you to measure the temperature of small-sized elements. For better heat transfer between the element and the thermocouple, a small amount of KPT-8 thermal paste was applied to the latter.

The search consisted in sequentially checking all the "suspicious" elements for elevated temperatures with the battery connected. In my case, a ceramic capacitor with reference designation C504 (dimensions approximately 0,5x1 mm) with a nominal capacity of 10 uF, installed in the power circuit and located next to the power controller - the MAX8997 chip, turned out to be faulty. On all elements the temperature was 30 ± 1 ^оC, and this capacitor - 33 ^оC, which allowed him to "calculate". Approximate measurement time per element is a few seconds. In total, it took about 20 minutes to search and recheck the results. The faulty capacitor was removed from the board and measurements with an ohmmeter showed that its resistance was about 10 kOhm. After replacing it with a serviceable current consumption of the tablet computer in the off state, it decreased from 12 to 0,5 mA.

I would like to add that such a technique allows in some cases to carry out repairs even in the absence of a circuit diagram and without measuring the parameters of faulty elements, for example, if there are two identical printed circuit boards with different faults, when it is required to assemble one working board.

Author: E. Biryukov

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