9V-12V Negative Voltage Regulator using PNP Emitter Follower

In this post, I am going to show two Negative Voltage Regulator circuit ideas. Before we use the just positive regulator as Simple DC Regulator using transistor and Zener. The output voltage of this circuit is 12V.

We use PNP Transistor be main electronic parts. By it is used as the character of emitter follower or voltage follower, make have a high current of about 0.7 Amp.

The working Principle
12.6V Negative Voltage Regulator using PNP Emitter Follower

Simplified schematic diagram of 12.6V Negative Voltage Regulator using PNP Emitter Follower

To begin with, the 220VAC or 110VAC(in the US) is fed to the power cord. It flows through the step -down transformer-T1 via S1(ON-OFF)switch and F1 (500mA fuse).

Then, we have about 12VAC output from the T1-transformer is fed to the D1-D2, full wave rectifier. Now, both diodes rectified the AC-Alternating Current to DC-direct current.

Next, the pulsating DC  is filtered by C1-2,200uF capacitor.

Recommended: 7805 regulator datasheet & pinout

And then, the electric current flow to a filtering regulator circuit. They include R1, ZD1, D3, R2, and Q1. Next, the R1-resistor will limit current to both ZD1-Zener diode and D3. Then, The Zener diodes are used as a specific reverse breakdown voltage across them is 12.6V(constant voltage).

Next, this low current flow base Q1 to increase current up, now ready to load.

The part that performs as fix voltage. The output is ZD1 from the circuit notes 6V

The capacitor C2-2200uF help give the circuit stability increasingly.
For other detail see in a picture.

Parts will you need

Q1 : TIP42 or equivalent (100V 6A, PNP transistor), Quantity: 1
ZD1, ZD2: 6V,0.5 watts Zener Diode, Quantity: 2
D1-D2 : 1N4007, 1A 1000V Silicone Diode, Quantity: 2
C1: 1,000uF 35V, Electrolytic Capacitors, Quantity: 1
C2: 2,200uF 35V, Electrolytic Capacitors, Quantity: 1
R1: 220 ohms 1/2W Resistors tolerance: 5%, Quantity: 1
R2: 1K 1/4W Resistors tolerance: 5%, Quantity: 1
F1: 0.5A Fuse, Quantity:1
S1: ON-OFF Switch(SPST Switch), Quantity:1
T1: 0.5A, 12V Center Tap Transformer

9V negative power supply circuit, low ripple

This is a simple 9V negative power supply circuit that low ripple unregulated circuit.  It has an output current of about 1A max. Often we make many positive voltage supplies.  But we sometimes need to make negative power circuit. See in the circuit below.


9V Negative power supply units

Simple negative power supply circuit

Both circuits have an opposite polarity supply, Diodes, Electrolytic capacitors. And in positive we use NPN transistor – TIP31, but in negative we use PNP transistor-TIP42.

The difference another one, it doesn’t have a Zener diode. Since it is an unregulated supply circuit.

But it is special, low ripple, low noise, and a constant voltage at 9V.

While unregulated circuit has too much a load current. It also has too much ripple or a noise.

In general, We will reduce the ripple by adding the large capacitor. It is a nice way.

Also a good idea, we use the gyrator effect.

The input capacitor C2 at the base of a transistor. It makes a stable voltage for the current gain of the transistor.

If transistor has a lot of gains such a 50 it can reduce a rate of ripple to low down.
The transistor will increase a high current to the output at its emitter.

The circuit is interesting because it is a smaller, cheap. We can this circuit is a high quality smoothed supply for a lot of loads, DC motor, Coils, Power lamps and more.

The output is full at 1A. Because of:

  • a transformer is 1A to 2A, 6.3V SEC.
  • And C1-1000uF 25V Electrolytic capacitor, you can change with 2200uF 25V, or add another one in parallel.

While the transistor is working, will have too hot. We should mount the big size heatsink. And add a fuse to protect the circuit from overload current.

But you want to builds easily circuit, you may use IC-7909 DC regulators

Keep reading: 9V supply circuit

This circuit requires enough power supply. Do you have this one? If you do not have it. Look:Learn Many Power supply circuits

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