I want to power supply a board (variable load 1.6 W at 24 V) from a 150 VDC source (rectified AC line) through a 2 kΩ, 20 W series resistor.
Due to mechanical arrangement of the equipment the only available access is the second terminal of the 2 kΩ resistor and GND. No access to the main supply is possible. All needed power must be drawn from the supply through the 2 kΩ resistor (max 75 mA). The board can be simplified as three bucks (24 V to +10 V/+5 V/-10 V) each supply many devices.
Since the current is limited by the 2 kΩ source resistor, the board won't start operating because it needs a surge current at start-up (due to the large input capacitance). One could say a soft-start should be considered after reaching the desired voltage bucks can be enabled to operate. Good point, but the board could not be redesigned.
So, I came up with external circuitry to store sufficient energy in a large capacitor. After reaching the target voltage the energy should be released fast enough to the load. I tested the concept by connecting the board manually when the voltage across C1 is regulated through Q1D1 LDO. It works!
Where I got stuck is with making a fast switch to release the stored energy. I have tested the following circuit, but the board won't start; I think because it turns on slowly.
The second solution I considered is to use an SCR and a DIAC to connect the load after reaching the voltage threshold. The problem with this configuration is that if the source drops enough to power off the board, but is still delivering a small amount of current, the SCR stays ON and after the source reaches the nominal voltage again the boards won't restart operating (voltage divider between source resistor and load).
Maybe the fast solution is to use a comparator with a push-pull output and fast slew rate to drive the PMOS (1st solution) fast enough. It will be also easier also to define thresholds voltages by monitoring the source voltage to open and close the P-MOS switch.
I am looking for a simple solution with hand wired elements (1206, 0805, SOT23, SOT228, SOD80...). Any idea to suggest, even if it is for a complete solution, not only the switch?