From: dave@davehigton.me.uk   
      
   In message <106hurs$9ibr$1@dont-email.me>   
             Mike Scott  wrote:   
      
   > Hi - I'm working on a model railway project using pico boards, but have   
   > spotted something that ought to be improved.   
   >   
   > The power to each section of track is scheduled to be switched by a  relay.   
   > I've a bank of 8 traditional relays, but have just realized the  total   
   > relay current will be up to a half amp or so worst case. I'm  looking to a   
   > solid state solution.   
   >   
   > The relays are switching DC at 12V, but the polarity is determined  before   
   > the on/off relay. Most SS relays I've seen are triac-based for AC  only;   
   > one type would do DC, but not with the polarity changing (and at  10x the   
   > cost anyway!).   
   >   
   > Does anyone know anything solid state that might do this without  breaking   
   > the bank?   
      
   There are various solutions, each with its own advantages and disadvantages.   
      
   One is to use two MOSFETS with the sources and drains parallelled, the   
   switched path being that between the two drains.  You'll see a drop of   
   about 0.6V across it when it's on.  (If this is a problem you can connect   
   a Schottky diode in parallel with each MOSFET, which will reduce the drop   
   to nearer 0.3V.)  The principal snag is that the control voltage, which   
   is applied between gates and sources, has to float.   
      
   If you can drive relays momentarily but not continuously, consider   
   bistable relays.   
      
   David   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)