Adjustable Voltage, Current Power Supply Circuit using Transistor 2N3055

Building an efficient circuit for an adjustable power source is described in this article.

This circuits ability to modify both voltage and current makes it extremely versatile to almost any type of electronics project.

This circuit is designed using a common transistor the 2N3055 and other essential parts.

Caution: Building circuits with high voltage can prove dangerous.

Only do this experiment under an adults supervision.

What is a Adjustable Voltage, Current Power Supply Circuit:

An adjustable voltage current power supply circuit is a type of electrical circuit that allows direct control over the output current which is provided to a connected load and generates a variable output voltage.

This kind of power supply are often seen in electronics labs, testing facilities and workshops where power settings are required to be changed for various purposes and research.

Circuit Working:

Parts List:

This article presents a circuit for a variable power supply workstation that has both a constantly adjustable voltage control and a variable current control that provides overload protection.

It is clear from a closer look that this variable voltage current power supply circuit which makes use of the 2N3055 transistor behaves similarly to a normal steady power supply circuit.

However it effectively provides the desired features.

Using the preset P2 in a feedback setup with components D1, R7, T2 and P2 allows for voltage modifications.

D1s presence assures that the voltage gets reduced to as low as 0.6V which is the diodes forward voltage drop.

A Zener diode with the required value is used in place of the diode if a minimum value is needed.

Thus the output of this circuit for a variable power supply that uses a 2N3055 transistor and a 0 to 40V transformer goes from 0.6 to a maximum of 40V which is a very useful range.

T3, P1, R5 and R4 are used to carry out the current control function.

The maximum allowed output current is set in large part by the value of R4.

P1 is changed to choose the widest range of values that resistor R4 has indicated or identified.

Please feel free to ask questions in the comments section below if you have any doubts about this circuit for a variable voltage and current power supply that uses the 2N3055 transistor.

Formulas:

The following common formulas are used to apply to the circuits with adjustable voltage regulators:

Voltage Divider R2 and R3:

If R2 and R3 provide a voltage divider the voltage at the transistors base Vb is calculated using the formula below:

Vb = (R3 / (R2 + R3)) * Vin

where,

• The voltage at the transistors base is represented by Vb.
• The resistor values in the voltage divider are indicated by R2 and R3.
• Vin is the voltage input.

When constructing a circuit for a series pass transistor voltage regulator the following formulas are used:

Vout or voltage output:

This is the controlled voltage that the circuit will supply to the load.

Usually it is a set sum that depends on its requirements.

Voltage input Vin:

This is the source of uncontrolled voltage that powers the circuit.

A battery, wall adaptor or other source provide the DC voltage.

Voltage Dropout (Vdropout):

For the transistor to operate correctly in the active zone and effectively regulate the output voltage this is the lowest voltage gap of Vin – Vout that is required.

The dropout voltage depends on the transistors features and biasing method.

Datasheets often provide the minimum Vce (collector emitter voltage) for saturation mode which can be used as a reference for dropout voltage.

Current at Load Iout:

This is the maximum current that the load can get from the circuit.

One need to consider the current needs of the devices that require powering.

Choosing Transistors:

When choosing a transistor following factors like:

Current and Voltage Output:

The transistor must have a voltage rating higher than the output voltage and a current rating higher than the load current.

Power Loss:

The voltage difference between the transistors collector and emitter, Vce and the current passing through its Iout will lead it lose its power.

One must select a transistor that can tolerate this power dissipation without overheating.

The power dissipation is often calculated using the following formula:

Vce * Iout = P

Datasheets describe the maximum power dissipation and transistor heat problems.

The biasing resistor or Rbase:

The base current (Ib) of the transistor is controlled by this resistor which also controls the collector current (Iout).

The value of Rbase is calculated using the transistors required collector current gain (β or hFE) and the voltage difference between the transistors base emitter voltage (Vbe) and the voltage reference (often the input voltage).

This is a simple formula that assumes β is fixed:

R_base = (V_in – V_be) / (β * I_out)

Note:

Keep in mind that silicon transistors usually have a Vbe of 0.6 to 0.7V.

The β values are found in the transistor datasheet.

This is a simple formula more complex calculations that take transistor saturation and β changes into account are likely needed for a proper design.

How to Build:

To build a Adjustable Voltage, Current Power Supply Circuit Using Transistor 2N3055 follow the below steps for connections:

Rectification and Transformer:

• The transformers primary side should be connected to the main power supply.
• To convert AC to DC connect the secondary side to a bridge rectifier.

Control of Stabilization Circuit Voltage:

• Connect the T2 emitter to the junction of R7 and P2.
• Wire T2s collector to the positive supply and its base to P2s wiper.
• With the cathode pointing toward the base of T2 connect D1 and P2 in parallel.
• R7 should be connected between T2s base and collector.

Adjustable Variable Voltage:

• The base of the 2N3055 transistor should be connected to the output of the stabilization circuit.
• Connect the 2N3055s emitter to the negative power source.
• Wire the load to the 2N3055s collector.

Control of Current:

• Make a series connection between P1, R4 and R5 then connect this series to the T3 collector.
• Connect the base of T3 to where R4 and R5 converge.
• Join the 2N3055 transistors emitter to the T3 emitter.
• Connect P1s wiper to T3s base.

The output load:

• Connect the devices load between the 2N3055 transistors collector and the positive supply.

Heat Sink:

• To disperse heat produced during operation connect a suitable heat sink to the 2N3055 transistor.

Boost Your Power:

• Using a multimeter to track the output voltage and current slowly increase the circuits power.

Conclusion:

Always remember to compare the connections to the circuit diagram and the component datasheets.

Take care to handle electronic components carefully and to prevent short circuits.

If you are unsure about the abilities to construct this circuit think about getting help from an electronics expert.

References:

2N3055 Power Supply