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I'd like to power an Arduino using a supercapacitor. I already have that part of the circuit figured out. But I want to charge the supercapacitor using an external power source, without the supercapacitor discharging into the Arduino until its voltage gets to 5V. What can I use to do this? Sorry if it's a silly question - my knowledge of electronics is pretty basic. I've been playing with an SCR (datasheet here) but haven't been able to get it to "wait" until the supercap gets to 5V. The trigger current seems to be too low.

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  • \$\begingroup\$ What exactly do you want to achieve with the super capacitor? \$\endgroup\$ – Tom Kuschel Dec 15 '18 at 22:59
  • \$\begingroup\$ I want to use it as a power source for an Arduino. I'm looking for a way to power the Arduino without using a battery. I've already got that part of it figured out - I used a Buck Booster to get 5V for the time I need it to stay powered for (less than a minute) \$\endgroup\$ – alpha26 Dec 15 '18 at 23:02
  • \$\begingroup\$ I don't know your capacitance of your supercap and your power of the supply which drives the loading. Isn't it waste of time to wait until the cap is loaded up to a voltage where the booster starts driving the needed 5 V? Do I understand you correctly that you charge the capacitor on one side and want to supply the microcontroller via a booster at the same time? If not, please please show us a circuit/schematic or a sketch of the arrangement. For the last, did you look for BOR, supervisory circuits, reset IC on the market? Some of them are capable to adjust the reset voltage as well as delay. \$\endgroup\$ – Tom Kuschel Dec 15 '18 at 23:18
  • \$\begingroup\$ can yuo just put the arduino into a low-power mode ? \$\endgroup\$ – Jasen Dec 15 '18 at 23:39
  • \$\begingroup\$ The supercap/Arduino side of things seems to be OK but I haven't heard of BOR or supervisory circuits before so it's worth looking into. Regarding my applications, someone told me about SCRs - they seem like a good idea, since I can supply the "trigger" when the supercap gets to my desired voltage and then allow the supercap to empty into my arduino. Is there an alternative to SCRs? The gate voltage seems to be too low for my 5F/5.5V supercap \$\endgroup\$ – alpha26 Dec 15 '18 at 23:43
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An SCR won't do what you want - it will naturally drop about 1 volt across it when connecting the supercap to the arduino so 5 volt in becomes 4 volts out to the arduino. I'd consider using a buck-boost regulator where the supercap (and charger) is on the input side and, the output side connects to the arduino: -

enter image description here

Vin can work from 2.5 volts to well over 5 volts in so you can choose, if you want to activate the arduino at voltage lower than 5 volts.

How is this activated?

The "RUN" pin is connected to VIN by a potential divider and only when VIN rises above a level determined by the potential divider ratio does the RUN pin activate the buck-boost regulator. It's called under-voltage-lock-out.

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  • \$\begingroup\$ This seems very interesting and looks like what I'm looking for. I tried looking for breakout boards for the LTC3111 and unfortunately I couldn't find any, just the IC itself. I don't have the ability to make the full circuit (tried making one a while back, failed miserably). Is there a generic name for these types of buck boosters that have this type of control, similar to the "RUN" pin? Perhaps I can look for alternatives \$\endgroup\$ – alpha26 Dec 16 '18 at 13:33
  • \$\begingroup\$ @alpha26 If you can find a buck-boost that has an enable pin you should be able to "make" it have a UVLO feature using the enable pin but it might need to use a comparator to give it a precise threshold and some hysteresis. \$\endgroup\$ – Andy aka Dec 16 '18 at 16:20
  • \$\begingroup\$ Hi Andy, I just realized that the buck booster I've been using all along has an enable pin! I was searching for one without realizing I already had one. I'm using the Adafruit Powerboost 1000 Basic. My dilemma is that by default, this buck booster is "on" because the EN pin is pulled high. To turn it off, you'd simply pull it low by connecting the EN pin to GND. But I want it to by off by default, and to pull it high when my capacitor hits 5V - do you have any idea how I can do this? \$\endgroup\$ – alpha26 Dec 23 '18 at 18:39

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