The Smallest and cheapest SPST switch you can buy

Discussion in 'The Projects Forum' started by Matter45, Dec 13, 2012.

  1. Matter45

    Thread Starter New Member

    Dec 13, 2012
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    Hi Folks!

    This will be my first time posting on this forum (or any forum for that fact), I normally get all my information from already existing posts, however I have decided to come out from living under a rock and join the community, maybe help in answering some questions.

    Im a self taught electronics engineer, I have been at it for about 2 years now :)

    Anyways I am designing an analogue Balance charging system. I am using PNP transistors to transfer the current to the next battery if that battery in the series is overcharging then the rest. Now this is good and all but it still leaks a little bit of current when it is switched off because I am using a MOSFET to switch the PNP on and off.

    I also dont want the balance charging system to make calculations in the first few seconds when the charger is first turned on because the batteries take a while to get to their "charging settle state" voltage.

    So now I am going to use a combination of a SPST electromagnetic switch, PNP and MOSFET to do the job.

    SO! what is the smallest and cheapest SPST switch that you can buy? or is there a better way of doing this?

    Cheers!

    Mathew
     
  2. panic mode

    Senior Member

    Oct 10, 2011
    1,318
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    voltage, current, type (momentary, maintained), configuration (NO,NC)...?
     
  3. Matter45

    Thread Starter New Member

    Dec 13, 2012
    26
    1
    My apology. the voltage difference will depend on the state of charge of the battery. the actual voltage from neutral will go as high as 230v. I am mainly curious as to the smallest and cheapest SPST you can get. I think it would be beneficial to all who are looking to down sizing there circuits. I did find this on google, NL7WB66. its actually 2 switches in one. the downside is that it has a limit of 50mA. So for practical terms the ideal SPST would be extremely small, has unlimited current, unlimited voltage, requires almost 0 watts to operate and has almost 0 ohms. Im not sure what is (NO,NC)? and the type doesnt matter.

    Im sure this thread will become helpful to others if we develop a list and there limitations. anyone out there with good knowledge of SPST or relay switches in the current market?
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Need to narrow it down some more.

    These are the categories for SPST Switches at Digi-Key


     
  5. Matter45

    Thread Starter New Member

    Dec 13, 2012
    26
    1
    I dont know how specific I can be, except to explain how it works.

    it has 4 pins. 2 are contact pins (close/open circuit) the other 2 are to power the coil to create a magnetic field to close/open the circuit. thats alot of categories just for a simple electromechanical device!

    and looking into Digi-key, so many different products wow! Thanks for the Digi-Key reference, Ill find what im looking for in there.

    I suppose so many people have so many different requirements. but what if you just need a simple switch that turns on and off electronically that is tiny!!! How small can a electromechanical switch go before you have to use a solid state one?
     
    Last edited: Dec 13, 2012
  6. antonv

    Member

    Nov 27, 2012
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    27
    Sounds like you are looking for a relay.

    Smallest and cheapest often = short life.
     
  7. panic mode

    Senior Member

    Oct 10, 2011
    1,318
    304
    NO = normally open
    NC = normally closed

    so you need a relay, not a switch....

    you want to use electrical signal from control circuit to switch load in another (AC) circuit. beware that relay contact ratings are subject to scrutiny. some really compact relays are out there and for high current but they are only rated for very low voltage such as 12 or max 30VDC. you cannot use them in AC circuits.

    about smallest you can get that will switch AC loads is form factor like T7CV5D for example.
    http://ca.mouser.com/ProductDetail/...=sGAEpiMZZMtSzCF3XBhmWxLK563Vzrvb/u8kPNGGD8I=
     
  8. Matter45

    Thread Starter New Member

    Dec 13, 2012
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    T7CV5D is 1.6cm wide. not bad! at a rating of 10A @ 24vdc. NICE. so how much less should you actually rate if the contact rating is 10A @ 24vdc? whats the full story behind "scrutiny" ratings?
     
  9. panic mode

    Senior Member

    Oct 10, 2011
    1,318
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    easy - you need to read the datasheet carefully.

    for example that relay is referred to as "10A" which is quite in accordance with the rating in the datasheet (check graph for "electrical endurance"). but this is far from the norm. quite often you will see relay named "10A" while for some operating conditions you may need to limit current to 3-4A (or less!), and at some other conditions it may be 15A. main factors are reactive load (inductive such as motors etc.) and DC vs. AC. At DC contacts get burned more (even at much lover voltage) and therefore last less because there is no zero crossing like in AC which helps extinguish arc. physical size (distance or gap when relay contacts open) is what determines max voltage (just like with fuses, fuses for larger voltages have longer body).
     
  10. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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  11. DickCappels

    Moderator

    Aug 21, 2008
    2,647
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    For the cheapest, contact some large Chinese suppliers on Alibaba.com. Just be cautious.
     
  12. panic mode

    Senior Member

    Oct 10, 2011
    1,318
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    why bother unless you need 100s of them?
    relay i posted is $1-2, shipping will be more than that.
     
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