# voltage differential vs max input of regulator?

so im trying to assemble a circuit to take my 120VAC wall line and ultimately power a small smart hub, small transformer i found somewhere that was actually labeled with which pins were input/output, 110VAC yields 11.7VAC, but because its a cheap transformer its putting out 21v unloaded, which is way higher than an 1117 can take, so i went looking at what else i had. find an old 1084 regulator i looked at the datasheet and instead of listing a maximum input voltage, it lists "Maximum Input to Output Voltage Differential" as 27V. so if the max differential is 27v, and the output is (to simplify math) 3v, my max input is 30v, correct?

for the sake of my understanding, would the component be able to take input floating between 30 and 40 VDC, and with the proper voltage divider on the adjustment pin, regulate to 24v?

lastly, is this unique to this device or is this just how voltage regulators worked and i never understood?

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• Note that if you are dropping 20V across a linear regulator, you'll almost certainly need to heatsink it.
– user16324
Mar 22, 2020 at 11:20
• @BrianDrummond i looked at the datasheet again, with a voltage differential of 25V it will self regulate to minimum output of 300mA, so it MIGHT work for the esp8266 i was planning, but im probably going to use the 1084 to regulate a 12v line instead(since i need 12v for the relays still), then an 1117 from there. Mar 22, 2020 at 17:45