XPost: sci.electronics.design, comp.arch.embedded, sci.electronics.basics   
   From: keinanen@sci.fi   
      
   On Sat, 23 May 2009 23:28:19 -0700, Robert Baer   
    wrote:   
      
   >   I my case, there is no connection between the MPU and AC Neutral (yet).   
   >   So, it seems you advocate a direct connectionfrom MPU ground to AC   
   >Neutral.   
      
   There are two ways of doing this.   
      
   You can isolate the MPU from the triac gate/MT1 (e.g. using an   
   optoisolator) and then you can connect the MPU to any potential you   
   like or even let it floating. This simplifies any MPU input signal   
   processing, since you can connect the MPU Gnd to the signal source   
   potential.   
      
   The other option is that the MPU board is directly connected to   
   gate/MT1, in which case the MPU signals are close to the MT1   
   potential. Please remember, that if the triac is powered through an   
   unpolarized mains plug, there is a 50 % chance that the gate/MT1 and   
   hence MPU input terminals are sitting on top of the mains voltage and   
   hence the input terminals can be momentarily 300-400 V above any   
   grounded objects. Thus each input must be separately isolated to   
   prevent any mains voltage from harming any people, depending on which   
   way the mains plug is inserted.   
      
   If there are many inputs that needs to isolated, it is easier to put a   
   single isolation barrier between the MPU and triac and using direct   
   connections to the MPU input pins rather than isolating each input   
   separately, when directly connected to the triac. However, as long as   
   all the inputs are from potentiometers with isolated knobs, well   
   isolated mechanical switched or an IR receiver, the MPU could as well   
   be directly connected and possibly sitting on the Live potential,   
      
   With the 2N6071 triac referenced in this tread, when using direct   
   connection, it might be better to connect the MPU Vcc directly to MT1   
   and MPU Gnd to -5 V relative to Neutral.   
      
   In the old days, tube TVs, at least in Europe, had a single rectifier   
   diode from one mains plug pin to form about +200 V anode voltage and   
   the other mains pin was connected directly to the chassis. Depending   
   on which way the  mains plug was inserted, the chassis had either   
   Neutral or Live 220 V connected to the chassis. Thus it was very   
   important to use a isolating wooden box as well isolated knobs on all   
   potentiometers and switches. Any electrical connections were very   
   rare, however, adding an external speaker connection was easy, since   
   the audio output pentodes required an audio transformer anyway, which   
   isolated the speaker terminals from mains and potentially from direct   
   Live connection.   
      
   When servicing such TVs and other devices with universal (AC/DC) power   
   supply, the first thing was to check which way the plug was inserted   
   in order to avoid having 220 V mains all over in the chassis and   
   getting nasty shocks from it.   
      
   Current regulations require that when servicing such beasts that an   
   isolation transformer must be used, so that when touching  the   
   chassis, the chassis potential will be the same as your potential.   
      
   These days when using MPU controlled systems controlling directly a   
   triac and hence sitting with 50 % likelihood directly on the Live line   
   that an isolator transformer should be used, when developing and   
   servicing such systems, if they are connected by an unpolarized plug   
   to the mains.   
      
   Paul   
      
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