From: tnp@invalid.invalid   
      
   On 01/08/2025 09:45, 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?   
   >   
   > Thanks.   
   >   
   Absolutely   
      
   What you need is a switching MOSFET. And a couple of resistors to   
   control the gate and maybe a diode to absorb spikes.   
      
   I am using LB8721s.   
      
   Ground the gate to negative with 150k and feed it from your Pi with   
   something like 150 ohms. and connect the source to negative and the   
   drain becomes the switched negative output. You may want to put a 1N5187   
   diode across the load to calm any switch off spikes .   
      
   On a bit of metal to absorb heat these will do about 20A continuous -   
   they have very low 'ON' resistance so they don't get too hot.   
      
   These are only single pole and only able to switch the negative of course.   
      
   If you want to switch the positive (as well)  you will need a P channel   
   device and I  don't know what to recommend   
      
      
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