5
\$\begingroup\$

schematic

simulate this circuit – Schematic created using CircuitLab

I have an atiny with an RF emitter which would detect the 220V AC off in the circuit and send a radio signal.

to do this I use 2 super capacitor (1.5F 2.7v each ) to send the signal before dying. I've done some tests and putting the two super capacitor in series in my 5V power supply and it is working. But It seems that super capacitor have special design with charging and discharging.

can I use them like this or it is more complicated?

Thank you

\$\endgroup\$
  • 9
    \$\begingroup\$ As a hint for the future: Please draw a proper schematic. These diagrams are less-than-optimally-clear at best, and plain annoying for people that just want to understand what you mean. This site has a schematic editor – use that! Schematics are the lingua franca of electrical engineering, and it really pays to practice drawing them once in a while :) \$\endgroup\$ – Marcus Müller Jan 2 '17 at 13:51
  • \$\begingroup\$ +1 for the interesting question, but I was hesitant to upvote because of the wiring diagram. For the future, heed what @MarcusMüller said in his comment. \$\endgroup\$ – Lorenzo Donati -- Codidact.org Jan 2 '17 at 14:02
  • \$\begingroup\$ How long and how much current do you need the caps to provide? You may be able to get away with a single electrolyte cap instead of two supercaps (plus balancing!). \$\endgroup\$ – JimmyB Jan 2 '17 at 15:07
  • \$\begingroup\$ indeed, I did not see this was available, it is great so here a new schema. I don't need a lot of current: the attiny + the transmitter should go below 100mA \$\endgroup\$ – bodtx Jan 2 '17 at 17:09
6
\$\begingroup\$

You can use them like that. The only drawback is the self-discharge of them internally. For a battery application this can be disastrous. For a mains powered application it's probably neglectable.

If you are using them in series to get 5+ V rating, you might need to consider the difference in leakage current though them and balance them off with external resistors.

| improve this answer | |
\$\endgroup\$
  • 2
    \$\begingroup\$ using only one super capacitor at 2.7V could be a good idea because the attiny can run at 1mhz from 1.8v and the transmitter is very tolerant also 1.5v to 12V \$\endgroup\$ – bodtx Jan 2 '17 at 17:13
  • \$\begingroup\$ @bodtx If that's possible you can definitly avoid the problem of unblanaced capacitors. \$\endgroup\$ – winny Jan 3 '17 at 8:31
0
\$\begingroup\$

I cannot understand the point of having these super caps, is your schematic missing some switching mechanisms? Without switches, you can think of the super caps as just another battery in parallel to the existing one.

Suppose your switch is placed like this:

schematic

simulate this circuit – Schematic created using CircuitLab

We have few states here:

  1. The caps have zero charge, the switch is open and the battery is fully charged.
  2. Now the switch closes. The caps will suck very high current (short circuit effectively). To overcome this, you need a pre-charge circuit: https://en.wikipedia.org/wiki/Pre-charge

There is few other possible states caused by switching depending on your design. Most important thing is not to make an effictive short circuit.

| improve this answer | |
\$\endgroup\$
  • 1
    \$\begingroup\$ The inrush isn't a problem. Energy hold-up when the external power is lost is. \$\endgroup\$ – winny Jan 3 '17 at 8:30
  • \$\begingroup\$ @winny Yes energy will hold in caps if sw1 opens (battery lost), but I cannot understand why the inrush isn't a problem? IMO it is a problem that will degrade battery life (might damage it or even cause it to burn at worst in case of lithium battery). \$\endgroup\$ – Chehadeh Jan 3 '17 at 11:01
  • \$\begingroup\$ Nothing says he is using a battery straight on the input. Besides, try simulating and or testing it and you will see what happens. \$\endgroup\$ – winny Jan 3 '17 at 11:43
  • 1
    \$\begingroup\$ @Chehadeh I'm on the home current 220V and transforming to 5V with this could super capacitor drain too much power for these little power supply? \$\endgroup\$ – bodtx Jan 3 '17 at 13:52
  • 1
    \$\begingroup\$ @bodtx Okay now it is clearer (there are other schematic elements for electronic voltage sources not batteries). From the link you have the maximum current this power supply could do is 5V/3W=1.667A . Assuming that the capacitor and power supply resistances are so small. These caps need about 9 seconds to reach full charge - that is really a lot (Use V=(I x t)/C. This might burn the power supply if it does not have overcurrent protection. My suggestion is to put around 1 Watt 3 Ohm series resistance in place of SW1. \$\endgroup\$ – Chehadeh Jan 3 '17 at 15:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.