I have a circuit that takes an input voltage of 0.5-1.5 volts and uses it to charge a supercapacitor up to around 4 volts. However, I only need the capacitor to be charged up to 3 volts. What is the simplest way with commonly available discrete components (including resistors, capacitors, NPN and PNP transistors, power transistors/power MOSFETs, 2N6027 PUTs, and various diodes) to turn off power to the charging circuit when the capacitor's voltage goes over 3 volts and turn it on when the voltage drops below around 2.8 volts?


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ 3V Zener diode? \$\endgroup\$ Nov 26, 2015 at 0:30
  • \$\begingroup\$ 555 used as a schmitt trigger? \$\endgroup\$ Nov 26, 2015 at 0:34
  • \$\begingroup\$ That could work, but a 555 timer will not work at that voltage. \$\endgroup\$
    – 3871968
    Nov 26, 2015 at 0:36
  • \$\begingroup\$ Why dont you inhibit the cap charger because this can be done at low current. \$\endgroup\$
    – Autistic
    Nov 26, 2015 at 0:54
  • 2
    \$\begingroup\$ There are many circuit topologies that could be used to cut off the charge but most of those will require a supply voltage higher than what you have available as the input voltage rail (0.5V is too low to activate semiconductor junctions). So this leads to a question...does the capacitor charger circuit supply enough current at 2.8V and above at a steady voltage that it can be used for operating the cut off circuit? \$\endgroup\$ Nov 26, 2015 at 14:04

1 Answer 1


I would perhaps use a relay, with discrete components you won't necessarily get good accuracy. If you don't want a huge relay hanging around, I would use a comparator with voltage reference. I'd say using discretes in this case won't really cut it, unless you aren't looking for accuracy. Another option is to build the comparators out of transistors, but that sounds pretty painful.

  • \$\begingroup\$ Would I be able to use 2N6027 programmable unijunction transistors for anything? I have at least 25 of them lying around not being used for anything. \$\endgroup\$
    – 3871968
    Nov 28, 2015 at 18:34
  • \$\begingroup\$ Possibly. I personally have never used those. \$\endgroup\$
    – nkeck72
    Nov 28, 2015 at 18:47

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