Probe for testing printed circuit boards. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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When repairing imported or domestic equipment for which there are no circuits, it is often necessary to determine the connections of the elements on the board. And if the board has metalized vias, then you also need to make sure that they have not degraded, i.e. their contact resistance is less than 1 ohm. By the way, it has been noticed that under the influence of various factors, their contact resistance can increase up to 20 ... 100 Ohms over time, and this leads to the complete inoperability of the device.
Usually, radio amateurs use an avometer to determine the increased resistance of transitions on the board. However, with such a measurement, a voltage from 1,5 V to 9 V can be applied to the inputs of microcircuits, and even with incorrect measurement in the forbidden polarity for some outputs, which can lead to failure of the circuit elements. To avoid these inconveniences, a probe was assembled, when measured, the voltage applied to the board tracks does not exceed 5 millivolts. Such a probe can reliably determine the serviceability of "metallization".
The probe circuit is shown in the figure.
The device consists of a 1 kHz signal generator (VT1, VT2), a bridge (R7, R5, R6, R8, R9, R10) and an unbalance amplifier (VT4). The low frequency signal is fed to the resistor bridge. When the bridge is balanced, which occurs when the probes are closed, there is no sound in the telephone capsule. With any imbalance of the bridge, the telephone capsule emits a low-frequency signal.
With serviceable parts, the probe starts working immediately. It is only necessary to set the balance of the bridge with the resistor R10. Further adjustment of the probe is not required.
Any low-power bipolar silicon or germanium transistors with a gain of at least 100 can be used in the probe. The values of the resistors and capacitors may differ from those indicated by 30 ... 40 percent in any direction, it is only important to observe the condition:
R7=R8; R5+R6=R9+R10.
The design of the probe is any, although the traditional one, in the form of a probe, is most preferable. The printed circuit board and circuit board are shown in the figure.
See other articles Section Measuring technology.
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