Battery Circuit Diagrams

Phantom Supply from Batteries Schematic Circuit Diagram

Professional directional microphones often demand a 48 V phantom supply, which is transmitted through the signal lines to the microphone and needs to be of high quality. Although a portable supply can be created using 32 AA cells in series, it isn’t very user-friendly. This circuit, however, operates with just four AA cells (or five rechargeable 1.2 V cells). We opted for a standard push-pull converter due to its ease of operation and predictable output voltage. Another advantage is the absence of a complex feedback mechanism. When designing the circuit, we assume a fresh set of batteries. We induce a voltage in the secondary winding slightly higher than required, ensuring a sufficiently high voltage to drive the linear voltage regulator even as the battery voltage begins to drop (see Figure 1).

Phantom Supply from Batteries Schematic Circuit Diagram

Astable Multivibrator Configuration

The circuit utilizes an astable multivibrator employing a 4047 low-power multivibrator configured in an astable free-running mode. The complementary Q outputs maintain a 50% duty cycle to prevent DC current flow through the transformer, preventing core saturation and potential damage to the FETs.

Oscillator and Dead-Time Control

The oscillator frequency, set by R1/C1, operates at approximately 80 kHz. R2/R3 and D1/D2 ensure that T1 and T2 conduct slightly later and turn off faster, establishing a dead-time and preventing short-circuit situations. The chosen BS170 FET with an on-resistance of 0.5 Ω can be substituted with other FETs with low on-resistance values.

Transformer Construction and Output Rectification

A larger toroidal core with a high AL factor (TX25/15/10-3E5 by Ferroxcube) is utilized. The secondary winding consists of 77 turns of 0.5 mm dia. enamelled copper wire. Two primary windings of seven turns each are created simultaneously. The transformer output from TR1 is rectified by a fast diode full-wave rectifier, and RF noise is suppressed by C4. An additional filter (L1/C5/C6) further reduces ripple. The clean voltage output powers regulator IC2.

Regulator and Protection Mechanisms

An LM317HV is recommended for the regulator due to its ability to handle higher voltage differentials. To prevent shorts, 6k81 resistors in the signal lines limit the current per microphone to 14 mA. D7 and D8 safeguard the LM317 from input shorts. Ripple is minimal, and any remaining noise above 160 kHz is usually negligible. The circuit can power up to three microphones simultaneously, with current consumption around 270 mA at 5.1 V input voltage.

Calibration and Enclosure Considerations

Adjust R4 to make the output voltage 48 V if there is deviation from the correct reference voltage. To minimize interference, especially in a switched-mode supply context, the circuit should be housed in an earthed metal enclosure.

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