The transistor megohmmeter is a device that operates directly from a DC power source, eliminating the need for a hand-cranked generator. Instead, it uses a transistor oscillator to generate an alternating voltage, which is then rectified and doubled by a transformer to produce a DC output. This oscillator is a magnetically coupled multi-resonant circuit, designed to create stable and consistent oscillations.
When the DC voltage Ec is applied through switch S, a small imbalance in the symmetrical circuit causes a slight difference in current between the collector of transistor V1 and the winding N3 of the transformer compared to N4. This imbalance generates a magnetic flux in the transformer core, inducing a voltage in windings N1 and N2. The induced electromotive force (EMF) acts in such a way that the base of V1 becomes more negative, while the base of V2 becomes more positive. This creates strong positive feedback, causing V1 to turn on rapidly and V2 to turn off quickly.
As the core becomes magnetized, the magnetic flux increases until it reaches saturation. At this point, the induced EMF disappears, reducing the base current of V1, which causes the core's magnetic flux to decrease. This change induces an opposite EMF, leading to strong positive feedback again, but now causing V2 to conduct and V1 to cut off. This cycle repeats continuously, resulting in pulse oscillation at the transformer output.
The high-frequency AC voltage produced at the transformer’s output is then stepped up and rectified through a voltage doubler circuit, allowing the device to generate a DC high voltage of up to 5000V. This makes the transistor megohmmeter suitable for measuring insulation resistance with a range of up to 0–1000 MΩ. Its design ensures reliable performance and accuracy without the need for manual cranking, making it a convenient and efficient tool for electrical testing.
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