From: jeffj@panix.com   
      
   >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!).   
      
   MOSFET based solid state relays   
   handle DC or AC and shut off immediately   
   whereas SCR or TRIAC turn off AC only at the zero crossing.   
      
   see:   
   https://www.ti.com/document-viewer/lit/html/SSZTB27   
      
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