I'm creating power supply for controller that usually operates with very low power consumption, but sometimes needs high power. To make the situation harder there is "sometimes" external power present (mostly not). So the whole circuit must live from super capacitor.

The problem I have is that I do not know how to replace the 7 ohm resistor on left side with some 'intelligent' circuitry that will primary power the controller and use ONLY remaining power to charge super cap.

Controller works in two modes (using 2 to 5 V as input voltage):

  1. low consumption mode taking 1 uA to 20 mA (peaks of max 30 ms and minimum 5 s apart, usually few hours apart).
  2. High power mode consuming cca 200 to 800 mA, before entering this mode the controller switches on the main supply and checks if input voltage is more than 3.5 V, should the voltage be below the controller waits with mains on in low power mode and resumes high power operation when input is over 3.5 V

There is also push button for cold start, but normally should be not needed as in case the controller recognizes less than 2 V on input (shortly before dying) it switches the mains on, so as soon as mains will be present the operation will be resumed automatically.

Now the problem in detail: There is 7 ohm resistor that is 'necessary' as the super cap will burn the 5 V power unit on cold start. But on the other hand it prevents high power operation of controller as there is not sufficient power for the controller (big voltage drop). For this reason I need to replace the resistor with something that consumes 0 A (real 0 or less than e.g. 1 nA) in case there is no external power. In case there is external power it should primarily power controller, if there is any remaining performance of the power source it should be used to charge the super capacitor.

Any ideas how to create this kind of circuitry? Thans for any help.

Until now I was trying to use transistors to monitor voltage difference between cap and power source, but this is not sufficient (and has high power usage when no external power applied (10 uA) depleting the super cap too fast.

Block schematics below.

Block schematics

  • \$\begingroup\$ I think with a diode instead of the 7ohms resistor, you might get what you want. The diode will make sure that current is only flowing into the supercap and not in the other direction. You might need a second diode for the solar charger module. \$\endgroup\$ Mar 11, 2019 at 18:00
  • \$\begingroup\$ Diode will manage the direction, that is OK. The problem is in managing the power as controller needs 5V and 800 mA - power source can provide it, BUT there charging resistor (7OHM) will act as resistor divider reducing voltage. If I remove the charging resistor the power source will blow on cold start because it can not survive the capacitor charging phase... And still I can not find solution. \$\endgroup\$ Mar 11, 2019 at 21:04

1 Answer 1


This circuit might do what you want. (It's just an idea and not finished).

It has 2 charging rails for the supercap. If the supercap is empty, it slowly charges through R5 to prevent overload. If the supercap voltage is above 2.5V, it charges the supercap directly. (Of course you still need to fine-tune the circuit to your needs)


simulate this circuit – Schematic created using CircuitLab


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