Short-Circuit Protection in DC Low-Voltage Systems

Here is a Short-Circuit Protection circuit to derive the additional power supply from the main circuit. The main circuit is protected from any damage due to short-circuit in the additional power supply circuit by cutting off the derived supply voltage.
The derived supply voltage restores automatically when shorting is removed. An LED is used to indicate whether short-circuit exists or not.In the main power supply circuit, 230V AC is stepped down by transformer X1 (230V AC primary to 0-9V, 300mA secondary), rectified by a fullwave rectifier comprising diodes D1 through D4, filtered by capacitor C1 and regulated by IC 7805 to give regulated 5V (O/P1). Transistors SK100 and BC547 are used to derive the secondary output of around 5V (O/P2) from the main 5V supply (O/P1).
Working of the ShortCircuit Protection circuit is simple. When the 5V DC output from regulator IC 7805 is available, transistor BC547 conducts through resistors R1 and R3 and LED1. As a result, transistor SK100 conducts and short-circuit protected 5V DC output appears across O/P2 terminals. The green LED (LED2) glows to indicate the same, while the red LED (LED1) remains off due to the presence of the same voltage at both of its ends.
When O/P2 terminals short, BC547 cuts off due to grounding of its base. As a result, SK100 is also cut-off. Thus during short-circuit, the green LED (LED2) turns off and the red LED (LED1) glows. Capacitors C2 and C3 across the main 5V output (O/P1) absorb the voltage fluctuations occurring due to short-circuit in O/P2, ensuring disturbance-free O/P1. The design of the circuit is based on the relationship given below:
RB = (HFE X Vs)/(1.3 X IL) where,
RB = Base resistances of transistors of SK100 and BC547
HFE = 200 for SK100 and 350 for BC547
Switching Voltage Vs = 5V
1.3 = Safety factor
IL = Collector-emitter current of transistors
Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet. Connect O/P1 and O/P2 terminals on the front panel of the cabinet. Also connect the mains power cord to feed 230V AC to the transformer. Connect LED1 and LED2 for visual indication.

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3 to 24V Variable Power Supply

This 3 to 24 volt variable-regulated power supply can be adjusted from 3 to 25 volts and is current limited to 2 amps as shown, but may be increased to 3 amps or more by selecting a smaller current sense resistor (0.3 ohm). The 2N3055 and 2N3053 transistors should be mounted on suitable heat sinks and the current sense resistor should be rated at 3 watts or more.

Voltage regulation is controlled by 1/2 of a 1558 or 1458 op-amp. The 1458 may be substituted in the circuit below, but it is recommended the supply voltage to pin 8 be limited to 30 VDC, which can be accomplished by adding a 6.2 volt zener or 5.1 K resistor in series with pin 8. The maximum DC supply voltage for the 1458 and 1558 is 36 and 44 respectively. The power transformer should be capable of the desired current while maintaining an input voltage at least 4 volts higher than the desired output, but not exceeding the maximum supply voltage of the op-amp under minimal load conditions.

The power transformer shown is a center tapped 25.2 volt AC / 2 amp unit that will provide regulated outputs of 24 volts at 0.7 amps, 15 volts at 2 amps, or 6 volts at 3 amps. The 3 amp output is obtained using the center tap of the transformer with the switch in the 18 volt position. All components should be available at Radio Shack with the exception of the 1558 op-amp.

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Adjustable Voltage Regulator

This Adjustable Voltage Regulator is made by combining a common 78L05 with an integrated audio amplifier of the type TDA2030, an adjustable voltage regulator can be constructed in a very simple manner that works very well. The output voltage is adjustable up to 20 V, with a maximum current of 3 A. Since the TDA2030 comes complete with a good thermal and short-circuit protection circuit, this adjustable regulator is also very robust.
As illustrated by the schematic, the design of this circuit is characterized by simplicity that is hard to beat. In addition to the two ICs, the regulator contains actually only two potentiometers and a few capacitors.

The adjustment is done by first turning potentiometer P1 to maximum (wiper to the side of the 78L05) and subsequently turning trimpot P2 until the desired maximum output voltage is reached. P1 is then used to provide a continuously adjustable voltage between this maximum and nearly zero volts.

At relatively small output currents there are no specific requirements regarding the cooling. However, when the output current exceeds 1 A, or if the input to output voltage difference is quite large, the amplifier IC has to dissipate too much power and a small heatsink is certainly appropriate.

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Switch Mode Power Supply

This Switch Mode Power Supply circuit use the IC from National Semiconductor has been producing and designing ICs for use in switch-mode power supplies for many years. The application of these devices is normally straightforward, helped by the excellent documentation that is available. A typical example of a switch-mode power supply is that based on the LM2671 or LM2674.The components for it are available for outputs of 3.3 V, 5 V and 12 V. There is also a version providing a presettable output voltage. Within the specified application, the supplies can deliver currents of up to 500 mA. Note-worthy is the high switching frequency of 260 kHz.

This has the advantage that only low-value inductor and capacitors are needed, and this results in excellent efficiency and small dimensions. In normal circumstances, the efficiency is 90% and may even go up to 96%. Both ICs provide protection against current and temperature overloads.

The LM2671 has a number of additional facilities such as soft start and the option to work with an external clock. The latter enables several supplies to be synchronized so as to give better control of any EMC (ElectroMagnetic Compatibility). The application shown in the diagram provides an output voltage of 5 V and an output current of up to 500 mA. Diode D1 is a Schottky type ((Uco≥ 45 V and Imax≥ 3 A).

 

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Homemade PCB

Printed circuit board or PCB is one of the important things to assemble an electronic circuit. It provides support to the components and makes electrical connection between the parts. In PCB assembling, the components are placed on one side of the Copper laminate passing their pins or leads to the other side through the holes. The pins/leads are then soldered to connect with the PCB tracks. Here explains the easiest method to make a homemade PCB for prototyping.To make the PCB, following materials are required


1. Copper clad board

This is available in different sizes. Select a suitable size to accommodate all the components. If the copper clad board is large in size, cut it to the required size using a Hacksaw blade. The copper clad board has a copper coated side which forms the soldering side. The other side is the component side on which the components are placed.If there is any dirt or copper oxide on the copper side, clean it throughly using a pencil eraser
Copper clad board

2. Ferric chloride solution

This is the Etching solution of Ferric chloride. It removes the unwanted copper layers from the copper clad board. The Etching solution can be prepared by dissolving 50 gms Ferric chloride powder in 100 ml Luke warm water.
3. PCB drill and bits
PCB drill is used to drill holes in the PCB. A hand drill with suitable bits is sufficient for the purpose. Use drill bits of the following size to make holes for different components
A. 1mm – for IC pins
B. 1.2mm – for Resistor, capacitor, transistor etc.
C. 1.5mm – for diode, LED pins, presets etc.
D. 5mm – for LED, nuts, screws etc.
E. 8mm – for switches, pots etc.
4. OHP Permanent Marker Pen, Tracing / Butter paper, Pencil Carbon paper, Varnish etc.
PCB Making
PCB making involves the following stages

1. Draw the circuit diagram as compact as possible on a paper. Mark the points (component pins) to be drilled. This diagram is used for Pattern drawing on the copper clad board.
2. Draw the same diagram in the tracing / butter paper using the OHP marker pen. Draw the diagram carefully without any overlapping or shorting of tracks or components. The neatness of the PCB lies in the Pattern drawing. After drawing, see the other side of the paper. There is a Mirror Sketch of the tracks. This is the actual pattern of the PCB.
Mirror Sketch of PCB tracks

3. Place the Pencil carbon on the copper side of the copper clad board. The ink side of the carbon paper should face the copper layer.
4. Place the tracing paper with diagram over the carbon paper. The diagram should be in the middle part of the copper clad board. Fold the sides of the tracing and carbon papers and stick it using cello tape. This prevents the movement while drawing.
5. Once again redraw the diagram using the OHP marker pen so that the carbon ink will create a mirror sketch on the copper clad board.
6. Remove the tracing paper and carbon paper. Using the OHP marker pen, redraw the carbon pattern of the mirror sketch on the copper laminate. So that the tracks will be created using the permanent marker ink. Keep it for 10 minutes to dry the ink.
7. Mark points to be drilled.
8. Take a Plastic or Porcelain tray and place the copper clad board with the track side facing upwards. Carefully pour the Ferric chloride solution over the copper clad till the copper clad immerse in the ferric chloride solution. Keep the tray in sunlight and shake occasionally. Etching will be completed in one to two hours.
Etched PCB

9. After etching, thoroughly clean the copper clad using tap water. This will remove the dissolved copper from the copper laminate except the copper beneath the OHP pen markings.
10. Drill holes using appropriate drill bits.
11. Remove the OHP pen markings using Petrol or Thinner so that the tracks will appear as copper lines.
12. If required, tin the tracks carefully using solder lead. Dip in varnish to prevent copper oxidation in tracks.
Tinned PCB

Commercial PCB
It is made using the following methods
1. Drawing the diagram in ORCAD or similar PCB drawing computer software. Diagrams include, Mirror sketch, Component values and symbols of components (Legend), diagram of pin holes.
PCB Design Software

2. Laser printing of the diagrams
3. Making Positive and Negative films of the diagrams
4. Screen printing of Mirror sketch and Legend on both sides of Copper clad board
5. Etching
6. Drilling of holes using machine drill
7. Tinning of holes in tinning machine
8. Masking using dyes
Commercial PCB- Legend and Track sides

Caution: Ferric chloride solution is toxic. It can cause skin burning or irritation. Use hand gloves during etching. Do not spill the ferric chloride on the skin. If this happens accidently, wash with water. Do not keep ferric chloride in places accessible to children.
D.Mohankumar