Shunt Regulators

Shunt regulators of the simple type are often used as part of series regulators. We used them in parts 1-4 of this series as VR tube references. They are the classic "zener" regulators. However, these have a limited output capability. The final part of this tube regulator series investigates not only the classic VR tube references, but also single tube shunt regulators and feedback multiple tube shunt regulators. This appears to be not widely applied. I could not find much in the way of references to tube controlled shunt regulators, except for very high voltage and low current applications. (The old tube based color TV's used a shunt regulator to maintain the picture tube "anode" voltage; typically using a 6BK4 as the shunt regulator element. Ditto for oscilloscope HV supplies).

Shunt regulators have a different set of constraints than series regulators. The highest power dissipated in the regulating element occurs at zero external load, and the maximum available load current is thus determined by how much current you care to put through the shunting element. Their output voltage -vs- load characteristics tend to be very different as well. When the maximum output current is exceeded, rather than "following the unregulated supply" down in voltage with increasing current, the output impedance is very much higher: being the resistance of the current limiting element is series with the shunt. (More about this later). Also, shunt regulators are capable of sinking substantial current as well as sourcing it. This is useful if your load can kick current back into the supply (inductive loads can do this). The penalty for this characteristic is the current goes through the shunting device, so this must be taken into account when "sizing" components.

Shunt regulators tend to be much less efficient (get hotter) for a given output power. You can picture this relatively easily: in both cases, the required output current flows through the regulating device (either at no load or at full load). However, the voltage drop across the series pass device is only the input-output differential, whereas in a shunt regulator, the entire output voltage is across the shunting device. Because of these constraints, it is much more difficult to design a variable shunt regulator with a wide range of output voltage and wide range of output currents.

Power Supply used for all the examples in this part.

I used the simple rectifier-filter that was designed for the "example" in part 3; namely, the BFT1 with a capacitor -only- filter. This was the input to ALL the regulators shown in this part. The regulators we will examine are simple VR tube types, single control tube shunt regulators, and multiple tube shunt regulators. Output voltage picked for these examples is about 275 volts.

Simple VR Tube Shunt Regulators

This is probably the simplest form of shunt regulator. Lets us an oA2 and an 0B2 in series. This should produce about 265 volts. Since the power source produces about 490 volts when loaded with 20 or so mA, I'll use a 10k resistor to drop the voltage to the 2 series VR tubes, passing slightly more than 20mA thru the tubes. It looks like this:

One thing to note about the VR tubes. They generally have a few "extra" connections in their "basing". In the example shown, this is used to remove the output voltage if the 0A2 is removed. Likewise, I could have placed the output ground  in series with pin 7 of the 0B2, to remove the current flow path if the 0B2 were removed. This is sometimes used for protection.

The regulation characteristics are:

Notice that the effective output resistance is about 66 ohms. With only the C filter, the output ripple was about 10 mV. This is good ripple attenuation, but only because only 20mA can be drawn from the supply. With about 20 mA flowing thru the VR tubes at no load, you can also source about another 15mA FROM the load into this regulator and still maintain operation within all tube ratings.

Single Tube Shunt Regulator

It is possible to use a single control tube, like we did in part 2 of this series to achieve more output current and a bit better regulation characteristics. A typical circuit looks like this:

In this circuit, the top 0A2 drops the voltage to the screen of the 6KG6/EL509, and the second 0A2 further drops the voltage to the control grid. The 0A2 current is returned to a presumed -150 volt source. This establishes about -20 to -25 volts on the control grid. At no load, all the current flows through the EL509. As output current is drawn, the voltage would tend to drop, lowering (making more negative) the control grid voltage on the EL509, so it draws less current, compensating for the increase in load current. If current is SOURCED into the output terminals, the voltage would tend to rise, causing the control grid voltage to rise, causing the EL509 to conduct more heavily, thus "regulating" the output. With 40 watts available (plate + screen dissipation), and for about 275 volts output, about 150 mA can be drawn through the tube. An additional 5 mA can be provided due to the current through the 0A2s. Therefore, we can use a 1.2k dropping resistor from the source. In this example, I used a 1.5k resistor. The regulation characteristics are:

Note that the effective output resistance is about 40 ohms. Also note that the curves are notably different form the simple series regulator as I described above. (Specifically note that with series regulators, the regulation is "better" at high loads, and gets worse at low or negative loads. With the shunt regulator, the regulation is better at low or negative loads, and gets worse at high loads). Technically, since the tube is dissipating its rated power at zero load, "sourcing" current would over-dissipate the tube. For dynamic loads this is not a problem, since the power averaged over a load "cycle" would still be within limits.

The ripple output from this regulator was about 50mV. Even though the "regulation" is slightly better than the simple regulator shown above, the added output current caused the ripple on the output to increase. This could, of course, be improved by using a C-L-C filter before the regulator.

Shunt Regulators with multiple control tubes

It is possible to use multiple control tubes to provide more effective shunt regulation. This also makes it practical to achieve some level of output voltage control too. One such implementation that is usually "trouble free" to bet operating is:

Note that I have also "presumed" a -150v regulated source is available. I've also shown the "quiescent" voltages for this supply to make it more obvious how it operates. The added tube is a differential amplifier that amplifies the "error", sensed from the bottom of the 0A2 and referenced to the -150V source. Some level of output voltage adjustment can be achieved by altering the 33k/39k voltage divider ratio. Since I am placing about 13 mA thru the VR tube, and the 12AX7 draws an additional 2 mA, I have lowered the series resistor to 1k. (So I can actually get slightly more current from this supply without exceeding the EL509 tube limits.

Here's the regulation characteristics achieved:

Note that the output resistance is about 2 ohms, which is not too bad! The output ripple from this regulator was about 4mV at a 100mA load.

One other thing to note is the use of the multiple connections to the 0A2. If you remove the 0A2, the output is disconnected, and the screen voltage to the EL509 is removed, so that the EL509 is not over-dissipated, and the load is not "over-voltaged".

I hope you have enjoyed this series on voltage regulators.

-Steve