# Why must this DIY lab bench power supply be powered by an AC supply?

Technical Specifications:

Input Voltage: 24 V alternating (max)
Input Current: 3 A (max)
Output Voltage: 0 - 30 V, could be regulated continuously
Output Limit Current: 2 mA - 3 A, could be regulated continuously
Output Voltage Ripple: 0.01% (max)


The output of the transformer is single 24 V or dual 12 V (same as 24 V), and the power could be determined according to your need. If a full load output (30 V, 3A) is needed, the power of the transformer should be greater than 90 W.

The circuit must be connected to 24 V alternating current power, and direct current is forbidden. Why is this so?

• There's a charge pump for generating a negative voltage, for example as a supply for U2. This charge pump relies on AC as an input voltage. – sh- Feb 14 at 17:51
• Related, almost identical: Design questions regarding LM2735 (boost-converter) for DC-DC – Dave Tweed Feb 14 at 18:18
• I must say, I find "24VAC in, up to 30VDC out" to be pretty optimistic. The rectified AC is about 34V peak, minus 2V for the diode drops, that leaves less than 2V for the darlington pass element and main filter cap ripple... – marcelm Feb 14 at 21:03