Diagram for Reference

While many power supplies can be set to limit their output current to a defined level, to protect the circuit they are powering, no such protection is available if you are powering a circuit from a battery. If a fault develops, the circuit can blow before you have a chance to disconnect it. Of course, you can fit a fuse in series with the supply line to the circuit under test but it will blow if a fault develops. Or perhaps it won’t blow sufficiently quickly to protect the circuit. And repeatedly having to replace fuses becomes a nuisance as well.



The alternative is to use an electronic fuse. This circuit uses a relay to make and break the circuit. The current drain of the circuit under test is monitored by a 1O 2W resistor which is placed in series with the supply line. The voltage across this 1O resistor is monitored by op amp IC1a which has an adjustable gain of between 11 and 16, as set by trimpot VR1. The resultant DC voltage from pin 1 of IC1a is fed to pin 5 of IC1b which is configured as a comparator.

Trimpot VR2 provides an adjustable voltage reference to pin 6 of IC1b and this is compared with the amplified signal from IC1a. If IC1b’s threshold is exceeded, its pin 7 goes high and this is fed to Schmitt trigger inverter IC2a which then “sets” the RS flipflop comprising gates IC2c & IC2d. Pin 11 of IC2d then goes high to turn on transistor Q2 and LED1 while pin 4 of IC2b also goes high to turn on Q1 and the relay which then disconnects the load.

The circuit stays in this state until the RS flipflop is reset by pushing switch S1. Capacitor Cx, across the feedback resistance of IC1a, is used to simulate a slow-blow or fast-blow fuse and can be selected by trial and error. Changing the gain of IC1a or the value of the sensing resistor changes the fuse rating of the circuit.

Ic (constant current) is charge capacitor C, where Ic = Vsc/R.

The output reaches its nominal value after the time ton. Vo-Vsc=(Ic.ton)/C.

ton=C.[(Vo-0.45)/0.45].R = CVoR/0.45.

Vo follows the voltage in pin 2 at less than 0.45 volt. It is because voltage of more than 0.45 V can’t be produced between pin 2 and pin 5.

Chances are youll want this amplifier portable. Batteries do the trick fine, but you wont get much power out of a couple of 1.5V cells. Unfortunately the size of a decent amount of battery power will mean that the overall size of this amp will be much bigger and for that there are more benefits to be had using a device like the TDA7052 or TDA2822 for stereo.

Quick ref data of TDA1015T Chip

•     Supply voltage range: 3,6 to 12 V
•     Peak output current: 1 A
•     Output power: 0,5 W
•     Voltage gain power amplifier: 29 dB
•     Voltage gain preamplifier: 23 dB
•     Total quiescent current: 22 mA
•     Operating ambient temperature range: -25 to +150 °C
•     Storage temperature range: -55 to + 150 °C

The first IC used as a 1 second clock, which generates ON/OFF for the other ICs. Diodes help to cover the IC555 from the peak voltage. Take note that the relay used should have impedance more than 50 ohm.

Parts list :

•     Diode D1-D2 : 1N4001
•     Zener Diode D2 : 6V
•     R1,R5,R7 : 3k3
•     R2,R6,R8 : 68k
•     Resistor variable VR1 : 47k
•     Polar capacitor C1 : 10uF/16V
•     Polar capacitor C3,C5 : 2.2 uF/16V
•     Capacitor C2,C4,C6 : 0.01 uF
•     Transistor T1 : BC107/BC148
•     IC timer : NE555
•     Relay : 6-9 V
•     Power supply 6-9 V

It uses IC CD4047 to control the buzzer timing utilizing resistor and capacitor. When the voltage passing through the transistor, the buzzer would sound.

    Variable resistor R1 : 10K ohm
    Polar capacitor C2 : 4.7 nF/16V
    Capacitor C3 : 22uF
    IC1 : CD4047
    NPN transistor Q1-Q2 BC547
    PNP transistor Q3-Q4 BC557
    Buzzer K1 : Tweeter 8 ohm
    Power supply : 12V