A transistor megger is a type of insulation resistance tester that operates directly from a DC power source, eliminating the need for a hand-cranked generator. The device uses a transistor oscillator to generate an alternating voltage, which is then rectified and doubled using a transformer to produce a DC output. This circuit is based on a magnetically coupled multi-resonant system. The DC voltage Ec is divided by resistors R1 and R2 to provide base bias voltages for transistors V1 and V2. When the switch S is turned on, a slight imbalance in the symmetrical configuration causes a small difference in the collector currents of V1 and V2. This imbalance generates magnetic flux in the transformer core, inducing a voltage in windings N1 and N2. The induced voltage reinforces the base bias of V1 and opposes that of V2, creating strong positive feedback that rapidly turns V1 on and V2 off. As a result, Ec is effectively applied across N3, while N4 acts as a non-conducting path, causing the core to gradually saturate. Once saturation is reached, the magnetic flux stops increasing, and the induced voltage disappears, reducing the base current of V1 and allowing the core flux to decrease. This change induces a reverse voltage, triggering V2 to turn on and V1 to cut off. This cycle repeats continuously, generating a pulse oscillation that produces a high-voltage square wave AC at the transformer output. After being stepped up by the transformer and rectified by a voltage doubler, the circuit can deliver up to 5000V DC, with a measurement range of 0–1000MΩ.
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