Voltage Regulator Circuits using Transistor and Zener Diode

In this article we will comprehensively discuss how to make customized transistorized voltage regulator circuits in fixed modes and also variable modes.

All linear power supply circuits which are designed to produce a stabilized, constant voltage and current output fundamentally incorporate transistor and zener diode stages for getting the required regulated outputs.

These circuits using discrete parts can be in the form of of permanently fixed or constant voltage, or stabilized adjustable output voltage.

You may also want to read: Simple Regulator Circuits

Simplest Voltage Regulator

Probably the simplest type of voltage regulator is the zener shunt stabilizer, which works by using a basic zener diode for the regulation, as demonstrated in Figure below.

The zener diodes have a voltage rating equivalent to the intended output voltage, that may be closely match the desired output value.

As long as the supply voltage is below the rated value of the zener voltage, it exhibits maximum resistance in the range of many megohms, allowing the supply to pass without restrictions.

However, the moment the supply voltage increases over the rated value of 'zener voltage', triggers a significant drop in its resistance, causing the over voltage to get shunted to ground through it, until the supply drops or reaches the zener voltage level.

Due to this sudden shunting the supply voltage drops and reaches the zener value, which causes the zener resistance to increases again. The cycle then continues rapidly ensuring the supply remains stabilized at the rated zener value and is never allowed to go above this value.

To get the above stabilization, the input supply needs to be a little higher than the required stabilized output voltage.

The excess voltage above the zener value causes the internal "avalanche" characteristics of the zener to trigger, causing an instant shunting effect and dropping of the supply until it reaches the zener rating.

This action continues infinitely ensuring a fixed stabilized output voltage equivalent to the zener rating.

Advantages of Zener Voltage Stabilizer

Zener diodes are very handy where low current, constant voltage regulation is required.

Zener diodes are easy to configure and can be used to get a reasonably accurate stabilized output under all circumstances.

It only requires a single resistor for configuring a zener diode based voltage regulator stage, and can be quickly added to any circuit for the intended results.

Disadvantages of Zener Stabilized Regulators

Although a zener stabilized power supply is a quick, easy and effective method of achieving a stabilized output, it includes a few serious drawbacks.

• The output current is low, which may support high current loads at the output.
• The stabilization can happen only for low input/output differentials. Meaning the input supply cannot be too high than the required output voltage. Otherwise the load resistance may dissipate huge amount of power making the system very inefficient.
• Zener diode operation is generally associated with the generation of noise, which may critically affect the performance of sensitive circuits, such as hi-fi amplifier designs, and other similar vulnerable applications.

Using "Amplified Zener Diode"

This is an amplified zener version which makes use of a BJT for creating a variable zener with enhanced power handling capability.

Let's imagine R1 and R2 are of the same value., which would create sufficient biasing level to the BJT base, and allow the BJT to conduct optimally. Since the minimum base emitter forward voltage requirement is 0.7V, the BJT will conduct and shunt any value that's above 0.7V or at the most 1V depending on the specific characteristics of the BJT used.

So the output will be stabilized at 1 V approximately. The power output from this "amplified variable zener" will depend on the BJT power rating and the load resistor value.

However this value can be easily changed or adjusted to some other desired level, simply by changing the R2 value. Or more simply by replacing R2 with a pot. The range of both the R1 and R2 Pot can be anything between 1K and 47K, to get a smoothly variable output from 1V to the supply level (24V max). For more accuracy, you can apply the following volatge divider formula:

Output Voltage = 0.65 (R1 + R2)/R2

Drawback of Zener Amplifier

Yet again, the drawback of this design is a high dissipation which increases proportionately as the input and the output difference is increased.

To correctly set the load resistor value depending on the output current and the input supply, the following data can be applied appropriately.

Suppose the required output voltage is 5V, the required current is 20 mA, and the supply input is 12 V. Then using Ohms law we have:

Load Resistor = (12 - 5) / 0.02 = 350 ohms

wattage = (12 - 5) x 0.02 = 0.14 watts or simply 1/4 watt will do.

Series Transistor Regulator Circuit

Essentially, a series regulator which is also called series pass transistor is a variable resistance created using a transistor attached in series with one of the supply lines and the load.

The resistance of the transistor to current automatically adjusts depending on the output load, such that the output voltage remains constant at the desired level.

In a series regulator circuit the input current has to be slightly more than the output current. This small difference is the only magnitude of current that is utilized by the regulator circuit on its own.

Advantages of Series Regulator

The primary advantage of a series regulator circuit compared to a shunt type regulator is its better efficiency.

This results in minimal dissipation of power and wastage through heat. Because of this great advantage, series transistor regulators are very popular in high power voltage regulator applications.

However, this may be avoided where the power requirement is very low, or where efficiency and heat generation are not among the critical issues.

Basically a series regulator could simply incorporate a zener shunt regulator, loading an emitter follower buffer circuit, as indicated above.

You may find unity voltage gain whenever an emitter follower stage is employed. This means when a stabilized input is applied to its base, we will generally achieve a stabilized output from the emitter as well.

Because we are able to get a higher current gain from the emitter follower, the output current can be expected to be a lot higher in comparison to the applied base current.

Therefore, even while the base current is around 1 or 2 mA in the zener shunt stage, which also becomes the quiescent current consumption of the design, the output current of 100 mA could be made available at the output.

The input current is add up to the output current together with 1 or 2 mA utilized by the zener stabilizer, and for that reason the efficiency achieved reaches to an outstanding level.

Given that, the input supply to the circuit is sufficiently rated to achieve the expected output voltage, the output may be practically independent of the input supply level, since this is directly regulated by the base potential of Tr1.

The zener diode and decoupling capacitor develop a perfectly clean voltage at the base of the transistor, which is replicated at the output generating a virtually noise free volatge.

This allows this type of circuits with the ability to deliver outputs with surprisingly low ripple and noise without including huge smoothing capacitors, and with a range of current that may be as high as 1 amp or even more.

As far as the output voltage level is concerned, this may not be exactly equal to the connected zener voltage. This is because there exists a voltage drop of approximately 0.65 volts between the base and emitter leads of the transistor.

This drop consequently needs to be deducted from the zener voltage value to be able to achieve the minimal output voltage of the circuit.

Meaning if the zener value is 12.7V, then the output at the emitter of the transistor could be around 12 V, or conversely, if the desired output voltage is 12 V, then the zener volatge must be seleced to be 12.7 V.

The regulation of this series regulator circuit will never be identical to the regulation of the zener circuit, because the emitter follower simply cannot possess zero output impedance.

And the voltage drop through the stage has to rise marginally in response to increasing output current.

On the other hand, good regulation could be expected when the zener current multiplied by the current gain of the transistor reaches minimum 100 times the expected highest output current.

High Current Series Regulator using Darlington Transistors

To precisely achieve this this often implies that a few transistors, may be 2 or 3 should be used so that we are able to attain satisfactory gain at the output.

A fundamental two transistor circuit applying an emitter follower Darlington pair is indicated in the following figures exhibits the technique of applying 3 BJTs in a Darlington, emitter follower configuration.

Observe that, by incorporating a pair of transistors results in a higher voltage drop at the output of approximately 1.3 volts, through the base of the 1st transistor to the output.

This is due to the fact that roughly 0.65 volts is shaved off from across each of the transistors. If a three transistor circuit is considered, this could mean a voltage drop of slightly below 2 volts across base of the 1st transistor and the output, and so on.

Common Emitter Voltage Regulator with Negative Feedback

A nice configuration is at times seen in specific designs having a couple of common emitter amplifiers, featuring a 100 percent net negative feedback.

This set up is demonstrated in the following Figure.

Despite the fact that common emitter stages ordinarily have a substantial degree of voltage gain, this may not be the situation in this case.

It is because of the 100% negative feedback that is placed across the output transistor collector and the emitter of the driver transistor. This facilitates the amplifier to attain a gain of an exact unity.

Advantages of Common Emitter Regulator with Feedback

This configuration works better compared to a Darlington Pair emitter follower based regulators due to its reduced voltage drop across the input/output terminals.

The voltage drop attained from these designs is barely around 0.65 volts, which contributes to greater efficiency, and empowers the circuit to work effectively regardless whether or not the un-stabilized input voltage is only some hundred millivolts above the expected output voltage.

Battery Eliminator using Series Regulator Circuit

The indicated battery eliminator circuit is a functional illustration of a design built using a basic series regulator.

The model is developed for all applications working with 9 volt DC with a maximum current not exceeding 100 mA. It isn't appropriate for devices that demand a relatively higher amount of current.

T1 is a 12 -0 - 12 volt 100 mA transformer which supplies isolated protection isolation and a voltage step-down, while its center tapped secondary winding operates a basic push-pull rectifier with a filter capacitor.

With no load the output will be around 18 volts DC, which may drop to approximately 12 volts at full load.

The circuit that works like a voltage stabilizer is actually a basic series type design incorporating R1, D3 and C2 in order to get a regulated 10 V nominal output. The zener current ranges through around 8 mA without load, and down to about 3 mA at full load. The dissipation generated from R1 and D3 asa result is minimal.

A Darlington pair emitter follower formed by TR1 and TR2 can be seen configured as the output buffer amplifier delivers a current gain of about 30,000 at full output, while the minimum gain being 10,000.

At this gain level whn the unit operates using 3 mA under full load current, and a minimum gain i exhibits almost no deviation in the voltage drop across the amplifier even while the load current fluctuates.

The real voltage drop from the output amplifier is approximately 1.3 volts, and with a moderate 10 volt input this offers an output of roughly 8.7 volts.

This looks almost equal to the specified 9 V, considering the fact that even the real a 9 volt battery may show variations from 9.5 V to 7.5 V during its operational period.

Adding a Current Limit to a Series Regulator

For regulators explained above it normally becomes important to add an output short circuit protection.

This may be necessary so that the design is able to deliver a good regulation along with a low output impedance. Since the supply source is very low impedance a very high output current can pass in the situation of an accidental output short circuit.

This might cause the output transistor, along with a few of the other parts to get immediately burned. A typical fuse may simply fail to offer sufficient protection because the harm would likely occur quickly even before the fuse could possibly react and blow.

The easiest way to implement this perhaps by adding a current limiter to the circuit. This involves supplemental circuitry without any direct impact to the performance of the design under normal working conditions.

However the current limiter might cause the output voltage to drop quickly if the connected load tries to draw substantial amounts of current.

Actually the output voltage lowers so quickly, that despite having a short circuit placed across the output the current available from the circuit is a bit more than its specified maximum rating.

The outcome of a current limiting circuit is proven in the data below which displays the output voltage and current with regard to a progressively lowering load impedance, as attained from the proposed Battery Eliminator unit.

The current limiting circuitry works by using only a couple of elements; R2 and Tr3. Its response is actually so quick that it simply eliminates all possible risks of short circuit at the output thereby providing a fail proof protection to the output devices. The working of the current limiting can be understood as explained below.

R2 is wired in series with the output, which causes the voltage developed across R2 to be proportionate to the output current. At output consumptions reaching 100 mA the voltage produced across R2 won't be enough to trigger on Tr3, since it is a silicon transistor requiring a minimum potential of 0.65 V to switch ON.

However when the output load exceeds the 100 mA limit, it geneartes enough potential across T2 to adequately switch ON Tr3 into conduction. TR3 in turn causes some current fto flow towards Trl across the negative supply rail through the load.

This results in some reduction of the output voltage. If the load increases further results in a proportionate rise in potential across R2 to rise, forcing Tr3 to switch ON even harder.

This consequently allows higher amounts current being shifted towards Tr1 and the negative line through Tr3 and the load. This action further leads to a proportionately rising voltage drop of the output voltage.

Even in case of an output short circuit, Tr3 will likely be biased hard into conduction, forcing the output voltage to drop to zero, ensuring that the output current is never allowed to exceed the 100 mA mark.

Variable Regulated Bench Power Supply

Variable voltage stabilized power supplies work with similar principle like the fixed voltage regulator types, but they feature a potentiometer control which facilitates a stabilized output with a variable voltage range.

These circuits are best suited as bench and workshop power supplies, although they can also be used in applications that demand different adjustable inputs for the analysis. For such jobs the power supply potentiometer acts like a preset control that can be used to tailor the output voltage of the supply to the desired regulated voltage levels.

The figure above shows a classic example of a variable voltage regulator circuit which will provide a continuously variable stabilized output from 0 to 12V.

Main Features

• The current range is limited to a maximum of 500 mA, although this can increased to higher levels by suitably upgrading the transistors and the transformer.
• The design provides a very good noise and ripple regulation, which may be less than 1 mV.
• The maximum difference between the input supply and the regulated output is not more than 0.3 V even at full output loading.
• The regulated variable power supply can be ideally used for testing almost all types of electronic projects with require high quality regulated supplies.

How it Works

In this design we can see a potential divider circuit included between the output zener stabilizer stage and the input buffer amplifier. This potential divider is created by VR1 and R5. This enables the VR1's slider arm to be adjusted from a minimum 1.4 volts when it is near the base of its track, up to 15 V zener level while it is at the highest point of the of its adjustment range.

There exists roughly 2 volts dropped over the output buffer stage, allowing an output voltage range from from 0 V to around 13 V. Having said that, the upper voltage range is susceptible to part tolerances, like the 5% tolerance on the zener voltage. Therefore the optimum output voltage might be a shade higher than 12 volts.

A few types of efficient overload protection circuit can be very important for any bench power supply. This may be essential since the output may be vulnerable to random overloads and short circuits.

We employ a rather straightforward current limiting in the present design, determined by Trl and its linked elements. When the unit is operated with normal conditions the voltage produced across R1, which is attached in series with the supply uotput, is too little to trigger Trl into conduction.

In this scenario the circuit works normally, besides a small voltage drop genearted by R1. This produces hardly any effect on the regulation efficiency of the unit.

This is because, the R1 stage comes before regulator circuitry. In an event of an overload situation, the potential induced across R1 shoots up to around 0.65 volts, which forces Tr1 to switch ON, on account of the base current acquired from the potential difference generated across the resistor R2.

This causes R3 and Tr 1 to draw a significant amount of curent, causing the voltage drop across R4 to increase substantially, and the output voltage to be reduced.

This action instantly restricts the output current to a maximum of 550 to 600 mA despite of the short circuit on the output.

Since the current limiting feature restricts the output voltage to practically 0 V.

R6 is rigged like a load resistor which basically prevents the output current from getting too low and the buffer amplifier unable to work normally. C3 allows the device to achieve an excellent transient response.

Drawbacks

Just like any typical linear regulator, the power dissipation in Tr4 is determined by the output voltage and current and is at a maximum with pot adjusted for lower output voltages and higher output loads.

In most severe circumstance there may be possibly 20 V induced across Tr4, causing a current of around 600 mA to flow through it. This results in a power dissipation of around 12 watts in the transistor.

To be able to tolerate this for long durations the device must be installed on a rather big heatsink. VR1 could be installed with a sizeable control knob facilatating a scale calibrated displaying the output voltage markings.

Parts List

• Resistors. (All 1/3 watt 5%).
• R1 1.2 ohms
• R2 100 ohms
• R3 15 ohms
• R4 1k
• R5 470 ohms
• R6 10k
• VR1 4.7k linear carbon
• Capacitors
• C1 2200 µF 40V
• C2 100 µF 25V
• C3 330 nF
• Semiconductors
• Tr1 BC108
• Tr2 BC107
• Tr3 BFY51
• Tr4 TIP33A
• DI to D4 1N4002 (4 off)
• D5 BZY88C15V (15 volt, 400 mW zener)
• Transformer
• T1 Standard mains primary, 17 or 18 volt, 1 amp
• secondary
• Switch
• S1 D.P.S.T. rotary mains or toggle type
• Miscellaneous
• Case, output sockets, circuit board, mains lead, wire,
• solder etc.

How to Stop Transistor Overheating at Higher input/Output Differentials

The pass transistor type regulators as explained above usually encounters the situation of experiencing extremely high dissipation appearing from the series regulator transistor whenever the output voltage is a lot lower than the input supply..

Each time a high output current is driven at low voltage (TTL) it might possibly be crucial to employ a cooling fan on the heatsink. Possibly an severe illustration may be the scenario of a source unit specified of providing 5 amps through 5 and 50 volts.

This type of unit could have normally a 60 volt unregulated supply. Imagine this particular device is to source TTL circuits in its entire rated current. The series element in the circuit will have to in this situation dissipate 275 watts!

The expense of delivering sufficient cooling appears to be realized only by the price of the series transistor. In case the voltage drop over the regulator transistor could possibly be limited to 5.5 volts, without depending on the preferred output voltage, the dissipation could be substantially decreased in the above illustration this may be 10% of its initial value.

This could be accomplished by employing three semiconductor parts and a couple of resistors (figure 1). Here's how exactly this works: thyristor Thy is allowed to be conductive normally through R1.

Nevertheless, once the voltage drop across T2 - the series regulator goes beyond 5.5 volts, T1 begins to conduct, resulting in the thyristor to `open' at the subsequent zero-crossing of the bridge rectifier output.

This specific working sequence constantly controls the charge fed across C1 - the filter capacitor - in order that the unregulated supply is fixed at 5.5 volts over the regulated output voltage. The resistance value necessary for R1 is determined as follows:

R1 = 1.4 x V_sec - (V_min + 5) / 50 (result will be in k Ohm)

where Vsec indicates the secondary RMS voltage of the transformer and Vmin signifies the minimum value of the regulated output.

The thyristor has to be competent at withstanding the peak ripple current, and its functioning voltage should be a minimum of 1.5 V_sec. The series regulator transistor should be specified to support the highest output current, I_max, and should be mounted on a heatsink where it may dissipate 5.5 x I_sec watts.

Getting Fixed Voltage from Transistor Regulator

Using just a single transistor and few zener diodes, you can get different voltages ranging from 5 V to 10 V from a supply input of 12 V. The below shown diagram, and the chart shows how the transistor, the zener diode, and the biasing resistor can be configured for implementing the simple transistor regulator circuit.

Conclusion

In this post we learned how to build simple linear voltage regulator circuits using series pass transistor and zener diode. Linear stabilized power supplies provide us with fairly easy options for creating fixed stabilized outputs using minimum number of components.

In such designs, basically an NPN transistor is configured in series with positive input supply line in a common emitter mode. The stabilized output is obtained across the emitter of the transistor and the negative supply line.

The base of the transistor is configured with a zener clamp circuit or a adjustable voltage divider which ensures that the emitter side voltage of the transistor closely replicates the base potential at the emitter output of the transistor.

If the load is a high current load, the transistor regulates the voltage to the load by causing an increase in its resistance and thus ensures that the voltage to the load does not exceed the specified fixed value as set by its base configuration.

5V Transistorized Regulator circuit