I am trying to charge a supercap over USB. I understand I need to limit the current (ref: usb.org)

This has been discussed a lot here, for example: here and here, but I believe my requirements are a bit different and I honestly do not know why my circuit would not be sufficient.

I don't need to charge my supercap so fast, so what I have done is to place a 150 ohm resistor and then I use a small LDO that would take the voltage down to 2.8 volt which my supercap needs. If I understand ohms law, that 150 ohm resistor and 5 volt would give inruch current absolute maximum of 33.33 mA. (5 V / 150 R = 0.03333333 A)

My schematic

I would expect this to work, but when I connected it to my computer it restarts. I don't know for sure but I assume it is because of the current limit.

If I connect it to a wall plug it works great charging the capacitor.

Any ideas?

  • \$\begingroup\$ How many milliamps is the circuit drawing now? "My comp restarts" is not a voltmeter/ammeter, and USB voltmeter/ammeters are readily available from the cornucopia of cheap Cheese. \$\endgroup\$ Apr 19, 2023 at 22:01
  • \$\begingroup\$ Your schematic is upside down. Even if you connected a charged supercap in reverse this circuit shouldn't draw more than 50mA. Something else must be making your computer restart (wiring error, wrong value resistor, ESD?) \$\endgroup\$ Apr 20, 2023 at 0:08
  • \$\begingroup\$ @BruceAbbott OT but interesting that it is upside down. This i want to read more about. Do you have a reference? Wouldnt a typical computer (iMac from 2023) have decent enough ESD protection on its ports? \$\endgroup\$ Apr 21, 2023 at 9:53
  • \$\begingroup\$ "3. Basic layout and flow In general, it is good to put higher voltages towards the top, lower voltages towards the bottom and logical flow left to right." - electronics.stackexchange.com/questions/28251/… \$\endgroup\$ Apr 21, 2023 at 11:56

2 Answers 2


Use a current source after your LDO. One ancient one is a LM134 but there are many to choose from. You can set the voltage on the LDO to match the current source requirements, and set the current to whatever your USB handles.

enter image description here


There's no reason why that would cause the processor to restart. Maybe it's not wired right (the circuit is drawn upside-down compared with the usual conventions), or your power supply hasn't got 33mA spare capacity.

Be aware that With the 150 ohm resistor placed before the regulator, the voltage input to the regulator will collapse the moment you connect a large load, so it won't actually regulate until the current drops down again. But this may not be a problem for you.

  • \$\begingroup\$ Thanks. Think it should be ok as you say. Also if i put the resistor after the regulator i am affraid putting the resistor after the regulator could make it very slow or even prevent the voltage from reaching 2.8 volt? \$\endgroup\$ Apr 21, 2023 at 9:56
  • \$\begingroup\$ @www.jensolsson.se It should reach 2.8V eventually, but it may take some time. But you could probably decrease the resistor if it's after the regulator. \$\endgroup\$
    – Simon B
    Apr 22, 2023 at 14:58

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