Short-Circuit Protection in DC Low-Voltage Systems

aPowerSupply.com — Mon, 23 Apr 2012 22:17:16 +0000

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

Short Circuit Protection Diagram

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.

OverVoltage Protection

aPowerSupply.com — Thu, 19 Jan 2012 10:29:02 +0000

This overvoltage protection or crowbar protection circuit is used where we need protection against high voltage surge. The circuit has a few components, it is very easy to build and will protect your electric equipment againg overvoltages. The crowbar circuit must be mounted between power supply and the protected device.

The over voltage protection circuit is based on brute force: when the power supply voltage increases too much a thyristor shortcircuit the output. This mean that the overvoltage is quickly removed from equipment power terminals and F1 fuse will burn.

The voltage at which the crowbar protection starts is set between 5V and 25V with P1:
1. Adjust P1 at maximum resistance value.
2. Temporarily replace the fuse with a wire and connect the crowbar circuit at a variable power supply. Adjust the current limitation at 1 A si the output voltage at the desired value to activate crowbar protection.
3. Slowly rotate P1 until activation of the thyristor (when the current limitator engages)

The overvoltage (crowbar) circuit is now set. Replace the wire bridge with a fuse (5A). In repaos state the circuit take 1 mA.

Power supply circuit » Protection

Short-Circuit Protection in DC Low-Voltage Systems

Short Circuit Protection Diagram

OverVoltage Protection

Over Voltage Protection Circuit Diagram