Additional Current Limiter for Power Supply

Figure 1 

The circuit is designed for power supply protection by limiting the amount of maximum output current to prevent any further damage to take place inside the power supply as well as the external circuit connected to it.

• BD131 – a general purpose NPN transistor used for power applications and has low voltage at 45 V maximum and high current at 3 A maximum
• Buzzer – an electronic signaling device that produces light and buzzing sound when activated or triggered
• BC547 – NPN small signal transistors designed for general purpose switching and amplification due to its low voltage, low current and three different gain selections
• 1N1418 – a small-signal silicon epitaxial planar diode used for fast switching applications with a reverse voltage of 100 V and forward current of 150 mA

The operation of the current limiter is very basic for both circuits. The voltage on the base of the power transistor BD131 is preset by the two diodes made from 1N1418 which are fast switching types. The causes the voltage across R2 to become 0.7 V. To obtain the maximum amount of emitter current, the voltage across resistor R2 is divided by the value of the resistor. So given a 10 ohms value would result to the emitter current of 0.07 A or 70 mA. The same amount of current will be drawn from the output terminals because the collector current is always roughly equal to the emitter current. The collector current and base current are both derived from the emitter circuit and divided at the base with a small portion going out of the base and the rest as the collector current. When more than the collector current is is obtained, the output voltage will decrease.

In this circuit, any NPN power transistor may be used for as long as the specifications would fit with the required functionality. Some of these would include switch mode series, Silicon, high current Silicon, and other NPN power transistors. Similarly, the transistor may be of any value if the required current limit is needed to be changed. There is always proportionality between the current and the wattage wherein the increase in current would also cause the increase in power.

From the second diagram, a few components have been added. The combination of 4K7 and 10K ohm resistors at the base of transistor BC547 divides the voltage of the transistor. Also, other components may still be added like LED to indicate the decrease of output voltage where the LED would dim or turn off totally. This event would signify that the output has very excessive load. During the idle time of the current limiter, the buzzer can be useful wherein it will an alarm for any problem that arises like having the voltage decrease by 2 Volts or more. A heatsink would be suitable on this circuit especially planning to increase the maximum current available from the circuit. the heatsink can be made from an aluminum strip that is 3 cm tall, 6 cm long and 2 mm thick.

The circuit, as the title implies, would be mainly used in power supplies. This is to ensure that before the complete maximum level is reached, there will be a margin present by limiting the current from a simple power supply regulator. This is not similar with circuit breakers in which the current limiter only provides limits at a predetermined level. Simple current limiting circuit may be made of a single resistor. But with a resistor like the passive type, the voltage drop is changed with regards to the consumed current. The higher the current, the higher the voltage drops.

Source:www.zen22142.zen.co.uk/Circuits/Power/add-on.html