Loudspeaker Protector Monitors Current

This circuit uses a 0.1O 1W resistor connected in series with the output of a power amplifier. When the amplifier is delivering 100W into an 8O load, the resistor will be dissipating 1.25W. The resulting temperature rise is sensed by a thermistor which is thermally bonded to the resistor. The thermistor is connected in series with a resistor string which is monitored by the non-inverting (+) inputs of four comparators in an LM339 quad comparator. All of the comparator inverting inputs are connected to an adjustable threshold voltage provided by trimpot VR1. As the thermistor heats up, its resistance increases, raising the voltage along the resistor ladder.

Circuit diagram:

Loudspeaker Protector Circuit Diagram

When the voltage on the non-inverting input of each comparator exceeds the voltage at its inverting input, the output switches high and illuminates the relevant LED. NOR gate latches are connected to the outputs of the third and fourth comparators. When the third comparator switches high, the first latch is set, turning on Q1 and relay 1. This switches in an attenuation network (resistors RA & RB) to reduce the power level. However, if the power level is still excessive, comparator 4 will switch, setting its latch and turning on Q2 and relay 2.

This disconnects the loudspeaker load. The thermistor then needs to cool down before normal operation will be restored. The values of R1-R4 depend on the thermistor used. For example, if a thermistor with a resistance of 1.5kO at 25°C is used, then R1 could be around 1.5kO and R2, R3 and R4 would each be 100O (depending the temperature coefficient of the thermistor). The setup procedure involves connecting a sinewave oscillator to the input of the power amplifier and using a dummy load for the output. Set the power level desired and adjust trimpot VR1 to light LED1. Then increase the power to check that the other LEDs light at satisfactory levels.

 

 

 

Source by : Streampowers

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How to Increase Current 7805 Voltage Regulator

7805 voltage regulator is one variant of the three terminal Positive Voltage Regulator. IC 7805 operated at maximum load current at a positive fixed voltage 5V 1A, but work on the maximum current will increase the thermal level. Aluminum heatsink is needed to reduce the level of heat, though internally IC 7805 has a thermal over-load protection.

The voltage regulator that works at the maximum rate is an unfavorable condition. Besides the heat level increases, it will likely be fatal if an increase in the load current due to short circuit. A good idea is to increase load current capability by adding an external transistor.

Based on the manual of IC 7805 has given a basic circuit to operate the IC at higher load currents. Download 7805 datasheet manual.

7805 Voltage Regulator Circuit 

In the following circuit to increase 7805 output current up to 3 amperes, must be added a complementary NPN transistor MJ2955. The advantage of this technique is to overcome when there is short-circuit as described above. When there is a voltage drop, then the circuit will reduce the maximum current consumption. So the problem can be solved.

7805 Voltage Regulator Circuit

Transistor BD240C in the DC voltage regulator circuit serves as a current limiting. The transistor is open when the voltage at 10R+0.22R be higher than 0.6-0.7 Volts, which leads to a reduction to zero of the T2 base current. The voltage at which the short-circuit protection starts to act, is given by voltage sum on 0.22R and 10R. Base voltage is determined by resistors 10R and 150R. In this circuit did not happen over-thermal when short-circuit occured, the maximum current was only 0.5 Ampere.

Hope this simple explanation may help in making a higher current-voltage regulator using IC 7805. You can open the manual datasheet of the 7805 voltage regulator for more information.

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Bar Type Current Transformers

 

In this type of current transformer, the primary winding is nothing but a bar of suitable size. The construction is shown in the Fig. below:

The insulation on the bar types primary is bakelized paper tube or resin directly moulded on the bar. Such bar type primary is the integral part of the current transformer. The core and the secondary windings are same in bar type transformer.


The stampings used for the laminations in current transformers must have high cross-sectional area than the ordinary transformers. Due to this, the reluctance of the interleaved corners remains as low as possible. Hence the corresponding magnetizing current is also small.

The windings are placed very close to each other so as to reduce the leakage reactance. To avoid the corona effcet, in bar type transformer, the external diameter of the tube is kept large.
The windings are so designed that without damage, they can withstand short circuit forces which may caused due to short circuit in the circuit in which the current transformer is inseted.
For small line voltages, the tap and varnish are used for insulation. For line voltages above 7 kV the oil immersed or compound filled current transformers are used.

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