Difficulty level Moderate
Load regulation Excellent
Line regulation Excellent
Ripple & noise rejection Very good
Stability Good
Thermal drift Good
Output impedance Excellent

The TYPE 2 Zener series regulator differs from the TYPE 1 in that we now employ some feedback to make the output voltage regulation better.

The name I have given to this regulator (Type 2) is not it’s real name.  It is only a designation to show that I will be covering other types which all work on the basis of a series pass transistor with a Zener diode generated reference voltage.

For the purposes of this article, we will assume the use of the following components:

Transformer: Nuvotem Talema toroidal; 2 Output Toroidal Transformer, 300VA, 2 x 25V ac.  Spec sheet
Bridge Rectifier: Vishay GBPC3506W-E4/51, Bridge Rectifier, 35A 600V.  Spec sheet
Capacitors: Epcos 4700μF 63 V dc Aluminium Electrolytic Capacitor, B41303 Series.  Spec sheet
Transistor: ON Semi TIP140G NPN Darlington Transistor, 10 A 60 V.  Spec sheet
Zener diode: Fairchild BZX85C24 Zener Diode, 24V 5% 1 W.  Spec sheet

Input to the transformer is via an AC power source operating at 50Hz.  Voltage is shown as 325.22V peak to peak which is about 230V RMS.

We will be using only 1 of the secondary taps on the transformer.  Looking at the spec sheet you will see that the primary coil has a DCR of 4.7Ω.  This is represented by R_Prim in the figure below.  For a voltage rating of 25V on the secondary, we have a DCR of 0.1667Ω and output current of 6A per secondary.  This is represented by R_Sec in the figure below.

The bridge rectifier is chosen because it is readily available and capable of high voltage and current.

The capacitors are chosen because they display excellent characteristics with very low ESR of only 39mΩ (shown as ESR and ESR1 in the schematic below).
I1 in the schematic is a constant current load.  This components allows us to apply different loads to the power supply in order to review its performance.



How does it work?

Firstly we create a filtered version of the rail power supply.  The filter is made up of diode D2 and capacitor C3.  Diode D2 allows current to flow down into capacitor C3, but does not allow it to flow back out the same way.  This is important because under heavy load currents capacitor C3 could start helping to supply power to the overall circuit, and this is not what we want.

Transistor Q1’s BASE pin is fed by the filtered version of the rail supply via resistor R2.  The function of R2 is to limit the total amount of current which can be drawn.  The size of this resistor is chosen so as to allow sufficient base current into transistor Q1 and allow a bit of headroom while remaining within the thermal dissipation limits of transistor Q2.

Because we are pulling the BASE pin of transistor Q1 up (to near the incoming rail voltage), the transistor will turn on and start conducting through it.

We are using a 24V Zener diode (D1), so as soon as the output voltage on the EMITTER pin of Q1 rises to 24V, diode D1 will start allowing current to flow through it.  If the voltage on the anode of D1 (bottom of the diode as drawn in the schematic) reaches around 0.65V, it will start to turn on transistor Q2.

When transistor Q2 turns on, it allows current to flow from the BASE of transistor Q1 through to ground (0V).  In effect transistor Q2 is leaching (stealing) drive current away from the BASE of transistor Q1.

Because of the voltage drop caused by this current flow through resistor R2, the voltage as seen by the BASE of transistor Q1 will fall.  As the voltage on the BASE pin of Q1 falls, so too will the output voltage on the EMITTER of Q1 fall.  This will continue until such time that equilibrium is reached whereby transistor Q2 will always be leaching some power from the base of Q1, but Q1 will still have enough power on it’s BASE pin to allow current through it.

If the load demands a heavy current all of a sudden, the output voltage on the EMITTER of Q1 will start to sag.  As soon as this happens, the voltage and current flowing through Zener D1 will also change.  This in turn will lessen the current transistor Q2 is leaching from the BASE of transistor Q1, thereby allowing the BASE of Q1 to get a bit higher voltage via resistor R2, thereby maintaining the output voltage.

This is a beautifully simple way of seeing how negative feedback (or just feedback) works in real life.  The results are pretty good as well!

The expected output voltage of this regulator will be the Vbe drop of transistor Q2 which should be around 0.65V + the Zener voltage which in this case is 24V, thus we expect an output voltage of around 24.65V.

By changing the Zener diode voltage, we can therefore change the output voltage of the regulator to just about anything we want.

Load step response

The graph below shows the regulator output voltage when faced with a constant DC load stepped from 0 Amp up to 4 Amp in 1 Amp steps:


HEY!!!  For loads between 0 Amp and all the way up to 4 Amp there is hardly any change in the output voltage of the regulator – all the lines are on top of each other!  How great is this regulator?

Let’s zoom into that and take a closer look:


Note that the total scale we are looking at in the graph above is now only 0.1V – the difference between 25.5V and 25.6V.

As expected, the output voltage of the regulator is near our calculated mark of 24V + 0.65V with the no load output sitting at 24.595V

At a load current of 1 Amp the output voltage ripple is around 4.1mV P-P.  At the full 4 Amp load the ripple is only 4.9mV.  Wow!

Very importantly, the output voltage sag with increasing load is so little as to be nearly nothing for all purposes of normal electronic equipment.

Power dissipation

Transistor Q1 dissipates heat because it handles the voltage drop between the power supply output and the regulator output.  In this case our transformer via bridge rectifier and filter caps provide a voltage of 25V AC x 1.414 = 35.35 V.  Our regulator has an output of 25.6V, thus the voltage drop across transistor Q1 is 35.35V – 25.6V = 9.75V.

If we have 1 Amp worth of current flowing through Q1, then the dissipation will be:

P (Watt) = V x I
P (Watt) = 9.65V x 1A
P = 9.75 Watt.

For a load of 4 Amp, this will increase to 39 Watt.  This kind of dissipation requires a more substantial heatsink to keep the transistor cool – the standard little PCB mount type will not work here.

If we use a heatsink with a thermal resistance of around 5°C/W this will result in a temperature rise of 48.25°C above ambient for a 1 Amp load.  This is getting on the high side, but still manageable – bigger would be better to reduce the temperature a bit more.

The heatsink below is made by Fischer Elektronik – model number SK08, and has a thermal resistance of 2.3°C/W (°C/W is the same as K/W) for a length of 50mm.


Using the heatsink above, we can safely dissipate 15 Watt with a temperature rise of only 35°C above ambient.

There is a way to reduce the total dissipation through transistor Q1 for a given load:  If the voltage differential between the output of the power supply and the output of the transistor Q1 is made smaller then the dissipation will be less.  This does however mean that we need to ensure the maximum ripple in the power supply output is small enough that it doesn’t break through the regulation threshold.  Normally this can be done by using more or larger capacitors.

Output impedance of the filter only
(This excludes the transformer and bridge rectifier – power is supplied by a perfect voltage source with a current draw of 1 Amp)

This graph represents the output impedance including capacitor ESR and ESL but does not include trace or connection parasitic factors.


As usual, the low frequency impedance is dominated mostly by the transformer secondary and the bridge rectifier combination.

Output impedance of the filter only
(This excludes the transformer and bridge rectifier – power is supplied by a perfect voltage source with a current draw of 1 Amp)

This graph represents the output impedance including capacitor ESR and ESL but does not include trace or connection parasitic factors.


Output impedance is very good indeed!  Pretty much flat at around 1.1mΩ from DC all the way up to around 65kHz where it starts to rise.  This super low impedance is the reason why our voltage regulator has so little sag with increasing current loads.  The performance is down to the efficiency of the feedback system used.

Load regulation
(How well the PSU is able to maintain a steady output voltage with changes to the load applied to it.)

From the graphs above it can easily be seen that the output voltage is extremely stable with increasing load current with only minor voltage sag for increasing loads.  Therefore the load regulation of this type of supply is rated as excellent.

Line regulation
(How well the PSU is able to maintain a steady output voltage with changes to the input voltage.)

AC Mains voltage fluctuation will not impact the regulation as long as we are able to maintain enough headroom so that the power supply voltage does not drop below the threshold voltage.  This can be catered for during the design of the power supply so if taken care of it will not have any significant impact.  Therefore the line regulation of this type of supply is rated as excellent.


Any noise present or induced into the AC mains line will be damped by the transformer primary because it has resistance, inductance and a little capacitance.  The dominant factor which determine noise transfer in the transformer will be the large inductance of the primary coil.  This inductance causes rising input impedance with frequency, ie it is a low-pass filter.  Any high frequency noise on the line should therefore be reduced rather well.

On the secondary winding things are slightly different.  There are only 2 significant sources of noise: the noise the bridge rectifier introduces (switching noise) and the possibility of induced noise on the line.  When a power supply is housed inside a grounded metal case there shouldn’t be any significant amount of external noise which will affect the secondary winding or subsequent rectified supply line.

The potential exists for an AC power carrying cable which passes near-by the power supply to induce some 50Hz noise into the line.  With a bit of forethought and care in placement this shouldn’t be a problem.  In difficult situations you could always use shielded cables to cary the AC mains inside the case which will significantly reduce any stray magnetic coupling into the supply.

If some noise manages to be transferred onto the output stage, this will be heavily filtered by the low-pass filter created by components D2 and C3.  Because of this filtering effect, this supply is rated as “very good”.


Stability should be very good under most conditions.

Thermal drift

Temperature has no significant impact on the circuit.  Changes in temperature will have a small effect on the absolute voltage across the Zener diode D1, so there will be a little drift as things warm up to operating conditions, however in most circuits this will be small enough to not cause any real concern.

As usual, capacitors suffer most with high temperature which drastically shortens their life.  A rule of thumb is that for every 10ºC rise in temperature the capacitor’s life is halved.

Output impedance

Output impedance remains at around 1.1mΩ between DC and up to around 65kHz where it starts to rise.  It is therefore rated as excellent.

Scaling for different voltages and / or currents

In theory this type of regulator can be made to handle just about any output voltage and / or load current.

Always ensure that the transistors you use are capable of handling the voltages and currents to start with.  Next take the time to calculate the power dissipation through the pass transistor Q1 in order to work out heatsinking requirements, etc.

Be warned however that at significant currents the power dissipation through Q1 can become enormous and extremely difficult to keep cool.

Care needs to be taken with the dissipation of transistor Q2 as well.  It operates such that the higher the load on the power supply, the lower the dissipation through this transistor will be.  In the example used here, transistor Q2 is a small TO-92 type and should be fine for constant load currents of 1 Amp and more.  If the average load is less than 1 Amp we may need to attach a small heatsink to the transistor or alternatively use a transistor with a larger die size.

Having said that, if you can manage to keep everything cool enough then there is no reason why this scheme cannot work for any voltage and current you can think of.


The higher power zener series voltage regulator (type 2) performs exceedingly well with regards to load regulation, ripple reduction and noise.  In addition it is very easy to scale this power supply to meet nearly any output voltage and current requirements.

This type of power supply is highly recommended.