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?

  • \$\begingroup\$ We don't expect every post to be perfect, but posts with correct spelling, punctuation, and grammar are easier to read. They also tend to get read and upvoted more frequently. Remember, you can always go back at any time and edit your post to improve it. (From the site policy in Write to the best of your ability on the site's help pages.) \$\endgroup\$
    – Transistor
    Commented Mar 21, 2020 at 23:19
  • \$\begingroup\$ Note that if you are dropping 20V across a linear regulator, you'll almost certainly need to heatsink it. \$\endgroup\$
    – user16324
    Commented Mar 22, 2020 at 11:20
  • \$\begingroup\$ @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. \$\endgroup\$ Commented Mar 22, 2020 at 17:45

1 Answer 1


For the adjustable version of the LM1084, the input/output differential rating is needed because the regulator's only connection to Ground is through the voltage adjusting resistor network.

Yes, the variable output version could be used with a 40 volt input to provide a 24 volt output.

Quoting an input/output differential for the fixed voltage versions seems a bit odd to me. The LM78xx family of fixed voltage regulators show Input Voltage ratings rather than in/out differential.

The LM317 regulator also has an input/output differential specification.

  • \$\begingroup\$ i remember reading somewhere that the 'fixed output' versions are entirely identical to the adjustable versions, they just have the appropriate resistor divider baked in. with the 1084, given the max differential seems to remain the same for all of them compared to the internal reference voltage, i expect this to be the case as well. \$\endgroup\$ Commented Mar 22, 2020 at 1:04

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