[GreenKeys] TT-4C We have action!

[Ralph Mowery]

If you have just a simple loop with only printer coils and keyboard contacts in the circuit, the voltage is not critical, it is the current.  Most often you put the coils in parallel (many printers have two coils and must be phased correctly).  Then the current is adjusted for 60 ma for most printers.  Almost any voltage from about 20 to 200 volts can be used if the current is adjusted correctly.  Just adjust the series resistor for the desired current.   Low voltages may give more errors and if used with a keyboard may not keep the oil burnt off the contacts.   The printers usually operate with higher voltages.
For the older Modle 15 printers, the the demodulators that Hoff built used transformers with 125 volts on them.  The DC was much more.  Around 170 volts as you have noted.

  ----- Original Message ----- 
  From: Greg Arnoldussen 
  To: greenkeys at mailman.qth.net 
  Sent: Sunday, July 15, 2012 9:55 AM
  Subject: [GreenKeys] TT-4C We have action!


  With the assistance of my friend we managed to "manufacture" a DC voltage of 105 VDC for the loop supply.  The transformer we had handy had the choice of inputs of 400V or 230V and outputs involving combinations of 55 and 60V on two windings.  By hooking in 240 VAC to the 400V input we then chose 60V + 60V on the output and used a large capacitor with diode to rectify to DC.  As I have since learnt, AC is quoted as RMS but DC is at peak current and the difference between the two is about 1.4.  Therefore 120 VAC is not the same as 120 VDC, it is actually closer to 170 VDC.  Therefore, as the mains 240V was going into 400V that was a factor of 0.6 so that brings 170 VDC back to about 100 VDC.  Indeed, when I measured the output it came to 105 VDC, not quite 120 V but it seems to be OK (read below).  If required we can re-configure to achieve 140 VDC but I thought to play it on the safe side.  Also the breadboard is safely housed in a plastic box, and the transformer has no exposed wires.
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