"Simple" Switching Circuit

Discussion in 'The Projects Forum' started by djoa87, Jun 9, 2011.

  1. djoa87

    Thread Starter New Member

    Jun 9, 2011
    Hello All,

    I am currently working on a prototype that utilizes a variable power supply with no external supply. The variable power supply is a extremely low powered device. I am needing a circuit that switches to 'on' (Voltage and Current are able to pass through) when voltage reaches a certain level. Since the power output is low, efficiency is a must. I have already put forth a lot of research in different switches and here is what I found (correct me if you know differently).

    1. BJTs - Not power efficient
    2. Comparators - Need a external power supply and voltage reference
    3. Voltage Detectors - Voltage too specific for my project

    I feel like the best alternative would be FETs but not sure which one and not sure how to implement. In short NEED HELP.
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
    What is the voltage range of power supply?
    What current can it supply?
    What is the trip point voltage?
  3. ifixit

    Well-Known Member

    Nov 20, 2008
    Hi djoa87,

    Your post doesn't say clearly what your requirements are, so I have questions.
    1. What is the input voltage range?
    2. What is the adjustable output voltage range required?
    3. What is the output load current minimum?
    4. What is the output load current maximum?
    5. What efficiency do you require?
  4. Dyslexicbloke

    Active Member

    Sep 4, 2010
    If you are interested in micro-power applications you should probably look up "Energy Harvesting"

    You will find all sorts of accumalate and use systems with miniscule quesent power requirements in standby.
    Commertial products that spring to mind are environmental sensors that harvest light vibration and stray RF, amongst others and collect enough energy to transmit temp and humidity data intermittently.

  5. djoa87

    Thread Starter New Member

    Jun 9, 2011

    Input Voltage Range:0-7V
    Trip Voltage: 4.2V
    Output Current:0-250mA

    High Efficiency is a paramount. Due to the already low power supply, too much current draw or too much drop in voltage would be result in a failed design.

    Thanks Al. I will look in Energy Harvesting.

    I still welcome more suggestions.
  6. russpatterson


    Feb 1, 2010
    Get one of the new super low power PIC's. Turn the clock down as low as it can go, and go into sleep state when waiting for changes your monitoring. They can run at 3V and something like 75uA and sleep at 30nA. http://www.microchip.com/en_us/technology/xlp/products.html

    They have comparators on board, a/d converters etc. All of which take more power to run but these devices are the cutting edge of low power stuff right now.
  7. #12


    Nov 30, 2010
    I don't know if this will work, but a programmable unijunction transistor comes to mind. I have a vague idea of the theory. Does anyone know if this is worth pursuing?
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
    Check out the LTC1540. Quiescent current is 300nA typical. You can use larger value resistors for the voltage divider if you want. I set it to trip on at 4.2V. You can also add hysteresis.