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Dual Power Supply with 48V Phantom Power Output

Posted December 27th, 2013 by

Presented is another power supply schematic, but this time using adjustable regulators instead of the normal 78xx and 79xx series fixed voltage regulators. These typically provide lower noise output, and the output voltages can be changed by altering the values of resistors, rather than replacing the three-terminal regulators (which can be quite cumbersome).


Audio Power Supply Schematic
The output in this case is around ±15V, and the 500R potentiometers (R1 & R4) should be trimpots to allow fine adjustment of the voltages.

Also provided is a 48V phantom power supply, using an LM317 adjustable regulator. Note that the regulator is capable of providing any voltage output as long as the input voltage is no more than 40V higher than the output. Since the input voltage is around 55V (for 20VAC input, which is what I use) and the output is close to 48V, the input-output differential voltage is 7V.

Note that, however, under fault conditions, the output may be loaded such that the output voltage is over 40V lower than the input (eg. 12V), and in this case, the devices maximum ratings have been exceeded, and damage may occur.

An input-output differential voltage larger than 40V may also occur if the input voltage ‘suddenly’ appears. In normal start up, the input voltage ramps up from zero, and so the the output, resulting in a manageable input-output differential voltage. However, if the input voltage were to appear instantaneously while the output is still at zero, the input-output differential voltage would be too large, and again the devices maximum ratings have been exceeded.

The values of the capacitors are not critical, but obviously the voltage ratings must be appropriately selected. The 48V supply should have 63V capacitors as an absolute minimum (and in this case, an input voltage of 22VAC should be considered maximum), with higher preferred (I use 100V capacitors).

The 0.1uF capacitors (C3, C4, C7, C8, C15 & C17) should be placed as close as possible to the regulator IC pins.

The power supply should be capable of providing at least ±15V at 100mA. If more current is needed, heat-sinks should be placed on the regulators.

The 48V phantom power supply will probably not be supplying much current, but it may require a heat-sink anyway since it may have to drop a lot of voltage to get down to 48V.

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