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I have been playing around with a supercapacitor, 1F rated 5.5V. I charge it mostly through USB, which usually takes a couple of minutes to reach a good amount of energy. I have been thinking about a way to indicate when it is charged, and it doesn´t need to be super accurate. Anything with a transistor and a couple more components would do.

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schematic

simulate this circuit – Schematic created using CircuitLab

What about something like this? Adjust R1, R2 to taste. The LED turn on curve won't be too quick, but for rough purposes it might be alright. Make sure that you've got a current limiting resistor for your charging capacitor.

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  • \$\begingroup\$ While charging Vcap = Vinput which is 5.5V. That time how we can ensure wheather in full charge or not ? . During charging itslef the transister get into active region so the LED will TURN ON !!! \$\endgroup\$ – Photon001 Jan 21 '16 at 3:08
  • \$\begingroup\$ @Raj A current limiting resistor will take care of that (the comparator circuit is connected across the cap, not the 5V rail). You don't want to connect USB power directly to a supercap; the charging current will be limited only by the series resistance of the capacitor. This could easily damage the USB port. \$\endgroup\$ – uint128_t Jan 21 '16 at 4:44
  • \$\begingroup\$ Then it would be better if you have drawn the circuit accordingly. Here you have mentioned that, the voltage fed from Vcap. Is it correct? \$\endgroup\$ – Photon001 Jan 21 '16 at 5:58
  • \$\begingroup\$ @Raj V_CAP refers to the positive terminal of the capacitor. I have omitted the charging circuit (whatever it may be) and the capacitor. Seems correct to me. \$\endgroup\$ – uint128_t Jan 21 '16 at 6:00
  • \$\begingroup\$ Actually, I do plug the supercapacitor directly to the USB power. :o I like this design, if i got it right I just should add the limiting resistor between +5v and V_CAP. I assume something small, like 10 ohm or less, right? \$\endgroup\$ – Giordano Bruno Jan 21 '16 at 14:07
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use a TL431 as a voltage sensor.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Nice! I have a lot of these TL431 from old power supplies! I just mentioned on the other answer that I plug the supercapacitor directly to the USB port. It seems I would need a current limiting resistor between +5V and Vcap, right? \$\endgroup\$ – Giordano Bruno Jan 21 '16 at 14:16
  • \$\begingroup\$ Your USB port has some internal resistance. if it has survived this treatment thus far it's probably OK. but yes according to the rukes of USB you should be limiting the current, and a resistor is one way to do that. the circuit will work with either both positive terminals connected to the capacitor or with only the rightmost connected to the cap and the other connected to 5V. depending on wether you want top power the LED from the supercap or from the 5V input. \$\endgroup\$ – Jasen Jan 21 '16 at 20:07

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