or  spelled out
    Current = Voltage / Resistance.
Now with the resistance that connects to the maximum voltage, i.e. the B+ voltage, we start at the other end of the graph at the maximum voltage point and zero current. (This makes sense, because if the value of the resistance were 0 ohms, then the current would be zero and the voltage across the resistance would equal the B+ voltage minus the voltage across the other resistance. ) Moving to the next point, we find the value of current if this resistance had received the full voltage potential of the B+ power supply; once again, this equals I = V / R. the result of our efforts is two line crossing each other. The intersection of the two lines defines the voltage at the connection of the two resistances and it defines the maximum current  the circuit will see with the given B+ voltage.

the vacuum tube with the lower plate resistance.)
   Plotting different resistance values is of little value, as the formula is so simple. But if we place a limit on the maximum voltage, things become a bit more interesting. Say we limit the voltage to 100 volts by drawing a vertical line from the 100 volts marking on the X axis. Now the intersection of a resistance line with maximum voltage line marks the maximum current that would flow into the resistance at that voltage. Still, all is simple enough, but if we place two resistances in series and set a maximum voltage limit, then we can graphically see the voltage division between resistances.

Two Resistances In Series
  When two resistances are in series (and no resistance equals zero) and are place across a fixed voltage, one resistance will steal voltage from the other. If the resistances equal each other, then each will share half the available voltage. If one resistance twice the value as the  other, then it will hog 2/3 of the available voltage. Plotting the voltage ratio is easy enough. We start with the resistance that connects to ground, i.e. 0 volts. Fix the first point at o volts and 0 current and then place the second point at the intersection of the maximum voltage and the maximum current that resistance would see at that voltage based on the formula:
    I = V / R

     All of this may seem obvious enough, but what if we add a third resistance in series with the previous two? What are the voltage division points? What is the maximum current through this resistance string?

Three Resistances
   Doing the calculations without a graph is straight forward. If all the resistances are the same, then the voltage division could be very easily determined: just divide the maximum voltage (B+) by 3. But if the values are 27k,

< PREVIOUS

NEXT

pg. 7