As I just discovered, when a 3-terminal adjustable voltage regulator (LM317, LD1086) is operating with Vin less than setpoint + dropout, its output is unregulated Vin - dropout. I want it to output nothing, zero volts.

It occurs to me there might be a solution using an external zener and a PNP transistor that turns on when there is a greater than 0.6V difference between Vin and the zener setpoint, but then I would need a separate voltage divider to determine essentially the same setpoint, with two potentiometers in the case of an adjustable power supply.

Any more elegant solutions out there? Perhaps there is a more-than-3-terminal device I should be using instead?

  • 1
    \$\begingroup\$ What is your desire Vout? What is your worst case Vin? What kind of circuitry do you have as the load? \$\endgroup\$ Jul 28, 2017 at 19:44
  • \$\begingroup\$ Vin is a photovoltaic panel: 0V to 22V. Output primarily drives a 12V nominal relay that remains engaged with as little as 2V. \$\endgroup\$
    – M Anderson
    Jul 28, 2017 at 19:48
  • \$\begingroup\$ Do you want the output to be a low-impedance zero volts in the case you mentioned? (Crow-barred to ground?) Or can it be a high-impedance and therefore floating output (which you can tie to ground with a resistor if you want?) Saying "nothing" doesn't seem to specify, to me. Also, I think you may need to realize that if there's only this one source, there will be a minimum voltage below which there isn't enough headroom to do much and you'll lose management, anyway. \$\endgroup\$
    – jonk
    Jul 28, 2017 at 19:57
  • \$\begingroup\$ High impedence is just fine. The relay is driven by an N-enhancement MOSFET with a bleed-to-ground resistor. \$\endgroup\$
    – M Anderson
    Jul 28, 2017 at 19:59
  • \$\begingroup\$ It seems as though you need a voltage regulator with an enable pin, plus a brown-out circuit. Is that about it? \$\endgroup\$
    – jonk
    Jul 28, 2017 at 20:07

1 Answer 1


Any more elegant solutions out there?

Sure there are. Just look for a regulator with a pin called UVLO. It stands for under voltage lock out and has internally a comparator that switches off the regulator's pass transistor when the input voltage is too low as set by a resistive potential divider applied externally to that pin.


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