In the days of vacuum tubes gas filled tubes were used as voltage regulators and they were effective for the need of the time. A tube normally runs at a much higher voltage then a solid state device and at a much lower current to perform the same operation.

The voltage regulator tubes did not have a filament so they were called cold cathode tubes.

The regulator tube was connected in series with a current limiting resistor and the two shunted the unregulated voltage.  The 0A2 was one very common regulator tube.  The 0A2 required an unregulated voltage of at least 180 volts.  The result was a 150 volt drop between the plate and the cathode of the tube.  The cathode would normally run directly to chassis ground and the +150 regulated voltage was picked off between the plate of the tube and the current limiting resistor.




Because solid state devices normally require a much lower voltage then the 150 volts of the 0A2 or even the 90 volts of the 0B3 a lower voltage regulator is required.  One way of doing this where a relatively small current is required is by using a zener diode.  If you look at the schematic diagram of the gas filled tube voltage regulator and the zener diode voltage regulator they look much the same; a current limiting resistor in series with the diode but with the zener diode the plus voltage is taped off the cathode side of the diode.
The reason the zener diode is reverse biased is because as a regulator it works on the principal of the zener effect also sometimes called avalanche point. There is a subtle difference between zener point and avalanche point but it is too involved to be described here.  The zener voltage is that voltage where the depletion region, that is the junction between the P and the N portion of the diode, is breached and current starts to flow. In most diodes when this region is reached the reverse current destroys the diode but in the case of a zener diode it is designed to work in this region up to a certain limit.  The size of the resistor that is place in series with the diode is easily calculated.  A zener diode is rated in voltage at which it works or its zener point and the maximum wattage it can sustain without destruction.
To calculate lets say we have a 12 volt unregulated source and we want a 6.1 volt regulated source.  I used 6.1 because that is a common voltage in which the zener diode comes and it is close enough to 6 volt that it can be used in a most 6 volt application and for sake of simplicity I am going to use 12 volts and 6 volts as my two levels. The diode we will be using will be 1 watt.  We know the voltage drop across the diode and the resistor will be 6 volts each.  With the diode’s maximum power level being 1 watt and the current through the resistor being equal to the current through the diode plus the current through the load so the resistor should be rated over 1 watt.  The maximum current that can flow through the diode is .167 amps (W/E=I) 1/6=.166666 which rounds up to .167.  If the regulated voltage is being used to supply power for an oscillator which switched off and on for CW operation that draws .06 amps in key down and 0 amps in key up positions the resistor will have to be large enough to dissipate the key down power without allowing excess current through the diode in key up.
A good engineering practice is to run a device at about half of its maximum rating.  Using this factor the maximum current that can pass through the diode is .0833 (Max. current = .167 amps times .5 equals .0833 amps).  That means the resistor’s resistance can not less then 72 ohms (6V/.0833A=72.03 ohms).  The voltage drop across both the diode and the resistor has to continue to be 6 volts so the current through the resistor must remain the same during both key up and key down.  The current will have to be split between the load, the oscillator, and the diode which means the current through the diode will decrease by the amount of the load.  The current through the resistor remains at .0833 amps when the .06 amp load is operating so the current through the diode will equal the total current through the resistor, .0833 amps, minus the current passing through the load, .06 amps thus when the oscillator is operating the current through the diode is .0233 amps (.0833 – .06 = .0233). This means the diode acts like a variable resistor so the total current through the resistor remains constant.  When the load is on the diode increases its resistance and when the load is off the diode decreases its resistance so the voltage remains constant.
That was all easy but if the unregulated voltage is a vehicle battery then the voltage swings between 12 and 14.8 volts. To calculate this the extremes must be taken into consideration.
The voltage drop across the current limiting resistor this time will very because the voltage across the diode will remain at 6 volts When the unregulated voltage is 14.8 volts the drop across the resistor will be 8.8 volts. To limit the current to .0833 the resistor must be 106 ohm (8.8/.0833=105.642).  When the unregulated voltage is at 12 volt the current trough the resistor will drop to .0566 amps which is insufficient to cover the necessary current of .06 amps necessary to run the oscillator.  In this instant the maximum power rating of the zener diode can be raised this will allow the resistance of the current limiting resistor to be dropped sufficiently to provide the current necessary for the oscillator and regulator to operate.
There is another way to provide sufficient current even if the load requires well above the .06 amps but this requires some extra circuitry.  It is done by controlling the output of a pass transistor with a Voltage Error Amplifier that compares the diode’s voltage drop and the pass transistor’s output voltage.  By doing this the voltage drop across the zener diode and the voltage drop at the output of the pass transistor will remain the same.

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