[GreenKeys] Eico EC 700 - Re: Loop supply, or are we still beating out wifes and kicking the dog

[Jones, Douglas W]

From: drlegendre . [drlegendre at gmail.com]  Tuesday, May 31, 2016 7:21 PM
> Simply put, I don't see why this circuit should work. If you notice, both leads of +all+ caps are connected to what should be the same voltage potential. Both leads connect to B- with a 3M3 resistor in series with each lead. No functional level of current should flow into any capacitor, at any point.

The standard NE-2 implementation of an astable multivibrator uses 2 NE-2 lamps, with one lead of each lamp tied directly to the supply, and the other lead of each lamp tied to the opposite side of the supply through a high resistance, with a single non-polar capacitor bridging the two points where lamp ties to resistor.  That is, with fewer lamps, exactly what the circuit in question does.

With just two lamps and one capacitor, the two lamps alternate.  Initially, one or the other lamp will light first.  As it lights, the voltage across an NE-2 lamp drops abruptly from about 60 volts (the strike voltage) to about 40 volts (the forward voltage).  Whichever lamp lights first will drag down the voltage across the other lamp through the capacitor.  The RC circuit on the off-lamp then recharges until that lamp turns on, at which point it pulls the voltage across the other lamp down to 20 or so volts, turning it off.  The process then repeats with the roles of the two lamps reversed.

NE2 lamps light with pathetically small forward currents.  The standard electrician's test screwdriver has an NE2 bulb inside plus a 10 or so megohm resistor.  Stick it in a lamp socket with the electrician in the circuit, and the bulb will light.  That's the way it's intended to be used to see if the circuit is energized.  I have one and use it on and off when I need to see if a 110V line is hot.

So, I see no reason why the circuit for the do-nothing-box shouldn't work just fine.

          Doug Jones
          jones at cs.uiowa.edu