Supercapacitors(SC) and Switching techniques

Discussion in 'General Electronics Chat' started by Marto, Oct 23, 2007.

  1. Marto

    Thread Starter New Member

    Oct 23, 2007
    Hi all,
    looking for some guidance. Have begun working on a project that uses an SC (in parallel with a battery pack) as a source of power, providing a load with its high start-up current before then switch back to the battery pack during the loads steady state, hence saving on the batterys capacity and increasing its lifecycle.

    Can anyone provide some 'search options' so that I can source possible switching methods of alternating between the SC and the battery as the soure of power for the load.
    voltage controlled switch??, time controlled switch?? SCRs??

    Thank you :D
  2. Dragon

    Active Member

    Sep 25, 2007
    There can be many options. What you havent specified is that should the switching be repeated after a specific time period? Or is the switching user controlled and random?

    Anyways, you can use 'transistor as a switch' with a PWM input to its base. The PWM input then decides when to 'switch on or off'.
  3. Marto

    Thread Starter New Member

    Oct 23, 2007
    the switching is very much a random event, but when called upon both sources will be capable of powering the load with the SC providing the initial burst and then the battery takes over.

    I suppose that I was hoping that people might be more aware (than myself) of SC technology and direct me on the type of switching they'd recommend or the issues that I need to take into account when designing my switching circuit.

    i.e. time based - would it be sufficient to measure the start-up time of our load and approximate from the time constant the discharge time of our SC and then switch from SC to battery at this point

    i.e. voltage based - monitor the SCs voltage and switch over to the battery once this drops below a certain value

    I need to research this a lot more myself but would welcome and appreciate any knowledge of the above.

    Au revoir.
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    Have you looked at a manufacturer's site to look for supercap application papers?
  5. nanovate

    Distinguished Member

    May 7, 2007
    Wouldn't having the SC in parallel "automatically" supply high initial current when the load changes? I guess you could also boost the SC voltage above the main supply then it will have more stored energy for a longer duration high current pulse.
  6. John Luciani

    AAC Fanatic!

    Apr 3, 2007
    A powerpath controller, like the Linear Technology LTC4412, will switch between
    multiple power sources.

    For an MSP430 uC board that I built I use a supercap and a coincell battery (CR2450).
    The uC board is only powered from the supercap. To increase battery life I power the uC from the supercap and the battery trickle charges the supercap with a current sink that never exceeds 150uA. The current drops to zero when the supercap voltage equals the battery voltage.

    A picture of my PCB is at

    (* jcl *)
  7. Jeepster

    New Member

    Jun 12, 2008
    Download SwitcherCad III from Linear tech and play with its simulation model for the LTC4412 powerpath controller... (adjust to your needs)
  8. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    As nanovate says, why do you need to switch the capacitor out at all? I assume it charges from the battery anyway. I'm not familiar with SCs, but would this be the best cap for the job? I would think very low impedance would be important but suspect that SCs aren't the best in this respect.