11-08-2016, 10:09 AM
Ignoring, for the moment, R15, 17, 18 & 19, the current through R20 is given as 47mA.
This give a voltage drop across each filament of 1.41mA but some of this is shunted to earth by R17, 18 & 19 and, possibly, R15 - but we can't see from the schematic snippet where this goes.
This would leave an even lower voltage drop across the filaments. If it ends up as 1.3V per filament, that would be an ideal voltage for a long life - remember that a 7.5V battery will only produce this voltage for a short period when fresh, dropping quickly to a nominal 1.4V per cell and then dropping slowly until it is exhaused (~1.2V/cell, say) so a continuous equivalent to 1.3V/cell will be ideal.
This give a voltage drop across each filament of 1.41mA but some of this is shunted to earth by R17, 18 & 19 and, possibly, R15 - but we can't see from the schematic snippet where this goes.
This would leave an even lower voltage drop across the filaments. If it ends up as 1.3V per filament, that would be an ideal voltage for a long life - remember that a 7.5V battery will only produce this voltage for a short period when fresh, dropping quickly to a nominal 1.4V per cell and then dropping slowly until it is exhaused (~1.2V/cell, say) so a continuous equivalent to 1.3V/cell will be ideal.
