Sorry this post is so long. I have no formal electronics training and have actually surprised myself how far I have gotten in this project.
I have designed a fairly complex circuit that switches ac as well as having DC and logic components to it. It needs to fit in a small area so I do not want to use a transformer on the board, and since it also needs to switch ac, I do not want to run ac and dc to the board, so I needed a more unusual way to create the different voltages I need.
I designed a power supply circuit that used used a resistor voltage dividing circuit to get ~24V @ 0.6Ato run a standard sprinkler valve, a second to provide ~10v @ 1mA to operate the FET that controls the water valve and a third to provide ~18v @ 0.5A to supply a standard 5V regulator for the logic supply.
All of these voltage divider circuits were using the output of a standard 1.5A rectifier bridge with the standard 120v AC input so all resistor values were calculated with 168v expected output of the rectifier bridge (minus a volt or two from the diodes in the rectifier). After ordering all the parts and assembling the first prototype circuit, my output from the rectifier bridge is only 106V on the prototype board and 113v on a spare rectifier bridge. This huge change from what was expected caused problems in the circuit and needed to be addressed, but I was also afraid that if I start making bunches of these I will run out of bridge rectifiers and have to get replacements that will actually put out the ~168V I originally expected.
I have decided to re-design the power supply section to replace the resistor voltage divider circuits with a single resistor and Zeener diode arrangement to provide a set 24V or 30V signal that then uses one FET or BiPolar transistor to provide a 24V output that I can then use for all the other circuits, same principle as the first cicuit in this post
except I don't need an exact voltage and I haven't been able to find voltage regulators that can handle ~170V input so I would use a resistor & zeener instead of a voltage regulator. This will greatly simplify the circuit and save money also since just the most expensive resistor was $12.00 by its self.
I tested the theory with this FET
and it worked ok at low voltages with a 100mA load but when I increased the voltage the FET started getting very hot and I don't think it could handle the 1.1A @ 24V I need. I'm not sure if my problem is the voltage/current (specificlly the high voltage from dropping from 106V (or 168V) down to 24V) is out of the safe operating area or if the power rating is being exceeded because of the 24V @ 100mA
What I need is help understanding what minimum values I need in order to select the correct FET or BiPolar transistor for this supply circuit. My output requirements are 24V @ 1.1A with an input of up anywhere from 100V to 170V. I could also use two of them broken down to one 24V 0.6A supply and one 10-15V 0.5A supply.
I looked at this
part I think would work at a 106v input, but it looks like it would be out of spec for a 170V input.
Thanks in advance for the help!!!