Need cheap solid state relays or alternative device

Discussion in 'The Projects Forum' started by Engineer-it-all, Sep 17, 2008.

  1. Engineer-it-all

    Thread Starter New Member

    Sep 17, 2008
    5
    0
    Good day Gents,
    This is my first post anywhere online so i would like to start by saying to you all "i am Proud to be an Engineer".

    Now here is my question, does anyone know a place i can get photo coupled MOSFETS / solid sate relays for less than the outragious price of £4 each?

    My University project requires 100 of them and £400 on relays is just not cool.

    Failing that, does anyone know of any other device that will act like a relay?
    I have investigated the use of Power MOSFETs and thyristors but due to the nature of the project they are unsuitable.


    Cheers for your help
     
  2. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
    304
    It would help us to answer you if you post some of the specifics about what you need. What voltage and current are you switching?
    What control voltage do you have, is it ac or dc?
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Search on e-bay, you will get many and you can see if one suits your application.
     
  4. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    I was going to suggest a discrete version, but that will probably end up costing about half of the 4 pound figure, that's if you're lucky anyways.

    Perhaps try to find some funding for the project, or find another, less expensive, project. My degree project last year cost about 1500$ canadian, which was a ton less than what I would have spent if I didn't scrounge.

    Maybe if you tell us what functionality, with specifics, you need, then we may suggest a cheaper alternative.

    Steve
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Do they have to be solid state? Does your class project really require more than a few hundred hours mean time between failures?
     
  6. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    thingmaker,

    There are a lot of disadvantages to electromagnetic relays.. Contacts arcing with high inductive load switching, slow time in comparison to SSR, power consumption?, etc.

    Reliability isn't the only factor, but yeah, it's pretty hard to justify 100 SSRs for a university project.

    Steve
     
  7. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    CP Clare makes Form A and Form B SSRs. I don't know the cost.

    If you could use a MOSFET I believe the IR makes photovoltaic gate drive ICs.

    (* jcl *)
     
  8. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    Locally I get them for around $5 each from surplus. I'd be surprised to get them cheaper.

    If we knew what you were switching we might be able to help more. I have seen SCRs used for a power supply that was pulsating DC (no filter cap) for example.
     
  9. AchMED

    Active Member

    Aug 5, 2008
    41
    0
    VO1263AAC $2.61 (Canadian dollars) in 100 quantities. You still need to buy mosfets so I don't know if this would be cheaper. It would help to know the circuit it would be switching.

    There are lots of totally integrated opto-moss relays but they have pretty limited current handling capabilities.

    I usually make my own using the VO1263AAC or similiar.

    Or completelly desighn your own using an isolated supply to drive the highside fet.

    Code ( (Unknown Language)):
    1. http://www.vishay.com/docs/84639/vo1263aa.pdf
     
  10. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Yet there are millions of them in service... A disadvantage which does not manifest is not a disadvantage. If the potential drawbacks of mechanical relays would come into play in the OP's project, then their use is contraindicated. If not, then economy dictates they are a viable choice.
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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    We still need the OP to let us know the voltage and current requirement for the loads plus the preferred control voltage.
     
  12. Engineer-it-all

    Thread Starter New Member

    Sep 17, 2008
    5
    0
    Cheers for your suggestions guys, I guess I will have to find some other method for switching power...
    just to clarify, my project is supposed to be a 'variable voltage battery array'.

    That’s to say I am building a rig where 16 AA batteries will sit and in between each battery, some Photo-MOSFETS in a special arrangement will be controlled by an AVR Mega128.
    I want it to arrange the batteries either in parallel, in series or a combination of these by making the correct path conductive.

    Not sure what the load is yet.. Could be a fan that speeds up as voltage is increased, could be a light that gets brighter or it could be an assortment of low power, different voltage devices.

    Because this is meant to be a source for some device, I need to minimize losses internally.
     
  13. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That lets SSR's out. They latch on with DC. You may need to use electromagnetic relays.

    There's always PWM for speed/lighting control.
     
    Madhav Kandukuri likes this.
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