A cool 0-400V variable transformerless power supply circuit can be built using just a single chip LR8, and a few resistors. The IC features a built in current control stage which makes the design extremely safe even for critical electronic circuits.
How LR8 IC is Designed to Work
The IC LR8 is quite similar to our very own LM317 or LM338 ICs except their maximum input voltage and the current delivering capacity specs which are wide apart, rest of the attributes are exactly similar.
Since the IC LR8 is designed to work with huge voltages upto 430V, its current handling capacity is consequently much lower at 20mA maximum, but nevertheless, at 400V this current could appear significantly useful.
Since the proposed 0-400V transformerless power supply circuit is rated to work with over 400V AC, implies that this circuit could be simply plugged in with our mains socket directly without having to worry about surge inrushes, or other related catastrophic situations.
How the Circuit Works
Referring to the circuit design of the 0-400V transformerless power supply above, we can see that it is exactly identical to the LM317 type voltage regulators, where R1 is used for setting up the reference voltage for the ADJ pin, while R2 is positioned for determining the intended output voltage across C2.
In the diagram the 18K resistor is supposed to produce a precise 5V at the output as long as the input voltage is 12V above the output value....meaning for acquiring 5V the minimum input supply voltage should be 17V. Similarly for ensuring a minimum 1.25V at the output, the input source will need to be around 13.2V. In short the differential voltage needs to be +12V over the desired output value.
For acquiring a smooth variable 0-400V or 0-300V DC output from a 220V mains rectified input source, the R2 could be replaced with a 100K pot.
For other fixed values the specified formula could be utilized as suggested in the diagram.
The pinout diagram for the LR8 IC can be learned from the following image:
Now since you know how to build a 0-400V transformerless power supply circuit, how do you plan to use it for your specific need?....think, and share if possible through the comment box.