Protection For Your Electrical Appliances

Here is a very low-cost circuit to save your electrically operated appliances, such as tv, tape recorder, refrigerator, and other instruments during sudden tripping and resumption of mains supply. Appliances like refrigerators and air-conditioners are more prone to damage due to such conditions. The simple circuit given here switches off the mains supply to the load as soon as the power trips. The supply can be resumed only by manual intervention. Thus, the supply may be switched on only after it has stabilised.

Protection For Your Electrical Appliances Circuit diagram :

Protection For Your Electrical Appliances

The circuit comprises a step-down transformer followed by a full-wave rectifier and smoothing capacitor C1 which acts as a supply source for relay rl1. Initially, when the circuit is switched on, the power supply path to the step-down transformer X1 as well as the load is incomplete, as the relay is in de-energised state. To energise the relay, press switch S1 for a short duration. This completes the path for the supply to transformer X1 as also the load via closed contacts of switch S1. Meanwhile, the supply to relay becomes available and it gets energised to provide a parallel path for the supply to the transformer as well as the load.

If there is any interruption in the power supply, the supply to the transformer is not available and the relay de-energises. Thus, once the supply is interrupted even for a brief period, the relay is de-energised and you have to press switch S1 momentarily (when the supply resumes) to make it available to the load. Very short duration (say, 1 to 5 milliseconds) interruptions or fluctuations will not affect the circuit because of presence of large value capacitor which has to discharge via the relay coil. Thus the circuit provides suitable safety against erratic power supply conditions.

Author : MALAY BANERJEE

Power Supply for Walkie Talkies Circuit Diagram

Power supply circuit that can be used for citizenband and VHF walkie-talkies of power rating up to 10 watts. The circuit uses a step-down transformer, followed by bridge rectifier, filter, regulator, and current booster stages. A pnp power transistor is added to the circuit to increase its current sourcing capabilities. Regulator 7812 can support around 100 mA current. When the current flowing through R1 nears 100mA value, the voltage (>0.65V) across the emitter-base junction makes transistor T1 to conduct and provide a path for additional current.

Circuit diagram :

Power Supply for Walkie Talkies Circuit Diagram

The circuit can source around one ampere of current at 12+1.4 volts=13.4 volts. Both the regulator IC and the power transistor must be mounted on heat sinks.

Versatile DC DC Converter Circuit Diagram

Here is a versatile power coupler that connects a device to 5V-19V DC generated from AC mains by a power adaptor. Power adaptors come in different voltage outputs like 5V (for mobile phones), 12V (for external hard drives) and 19V (for laptops). Sometimes the power adaptor may have a voltage rating higher than the required voltage. With the converter circuit given here, the adaptor can be used to power any device at a lower voltage. 

For instance, by using a 19V laptop adaptor, you can power a TTL circuit at 5V. There can also be other instances when one needs a 3V or 6V supply. All these and many other intermediate voltages are easily possible with this versatile converter circuit when used together with any off-hand power adaptor.

Circuit diagram :

Versatile DC-DC Converter Circuit diagram

Fig. 1 shows the circuit of the DC-DC converter. Smooth reduction in the voltage is achieved using the LM317 regulator IC. The complete unit can fit inside a piece of a glue stick tube.

Adjusting variable resistor VR1 gives the desired output voltage. The output voltage is read using a 0-100µA ammeter, whose series resistance R* is chosen such that the maximum desired voltage could be covered. For instance, if full-scale deflection (FSD) current of the meter is 100 µA and you need an output voltage of up to 15V, then R* = 15/0.0001 = 150 kΩ. The desired value of R* is obtained by using 150-kilo-ohm preset VR2. 

Use of a variable resistor which also has an on/off switch like the one in old radios is recommended. It will cut off the coupler from the input power supply without having to accomodate an additional switch. Also, use a heat-sink with LM317 to handle the desired amount of power.

Assemble the circuit on a small general-purpose PCB and enclose in a suitable case. Fit the entire PCB inside a glue stick tube as shown in Fig. 2. Affix the female and male connectors on the opposite ends and place the ammeter in between the stick tube. You can directly read the output voltage on the ammeter after due calibration.

Note. You can use a suitable VU meter instead of 0-100µA ammeter and calibrate accordingly.

Source By Streampowers