Power scr driver circuit diagram

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This results in the supply to the battery getting cut off, ensuring the battery is not allowed to over charge. The above diagram shows a classic heater control application using an SCR. The circuit is designed to switch ON and OFF the watt heater depending on the thermostat switching. A mercury-in-glass thermostat is used here, which are supposed to be extremely sensitive to the changes in the temperature levels surrounding it.

However, since these types of thermostats are normally rated to handle very small magnitudes of current in the range of 1 mA or so, and therefore it is not too popular in temperature control circuits. In the discussed heater control application, the SCR is used as a current amplifier for amplifying the thermostat current.

Actually, the SCR does not function like a traditional amplifier, rather as a current sensor , which allows the varying thermostat characteristics to control the higher current level switching of the SCR.

We can see that the supply to the SCR is applied through the heater and a full bridge rectifier, which allows a full wave rectified DC supply for the SCR. During the period, when the thermostat is in the open state, the potential across the 0. This enables the SCR to conduct during these pulsed DC half cycle triggers, allowing the current to pass through the heater, and allow the required heating process.

As the heater heats up and it temperature rises, at the predetermined point, causes the conductive thermostat to activate and create a short circuit across the 0. This in turn switches OFF the SCR and cuts off power to the heater, causing its temperature to drop gradually, until it drops to a level where the thermostat yet again is disabled and the SCR fires ON. The next SCR application talks about a single-source emergency lamp design in which a 6 V battery is kept in a topped up charged condition, so that the connected lamp can be seamlessly switched ON whenever a power failure happens.

When power is available, a full wave rectified DC supply using D1, D2 reaches the connected 6 V lamp. C1 is allowed to charge to a level that's slightly lower than the difference between the peak DC of the fully rectified supply and the voltage across R2, as determined by the supply input and charge level of the 6 V battery.

Under any circumstances, the cathode potential level of the SCR is help higher than its anode, and also gate to cathode voltage is held negative. This make sure that the SCR stays in the non-conducting state. While the input power is present, the full wave rectified across the emergency lamp keeps it switched ON.

During power failure situation, the capacitor C1 begins discharging through D1, R1, and R3, until the point where the SCR1 cathode becomes less positive than its cathode. The SCR now fires and allows the battery to get connected with the lamp, instantly illuminating it through battery power.

When power returns, the capacitors C1 is yet again recharged, causing the SCR to switch OFF, and cutting off the battery power to the lamp, so that the lamp now illuminates through the input DC supply. The above circuit of a rain alarm can be used for activating a AC load, like a lamp or an automatic folding cover or shade.

The sensor is made by placing to metallic pegs, or screws or similar metal over a plastic body. The wires from these metals are connected across the base of a triggering transistor stage. Small voltage start leaking across the sensor metals and reach the base of the transistor, the transistor immediately conducts and supplies the required gate current to the SCR. The SCR also responds and switches ON the connected AC load for pulling an automatic cover or simply an alarm for correcting the situation as desired by the user.

We discussed in the previous section regarding a special property of SCR where it latches in response to DC loads.

The circuit described below exploits the above property of the SCR effectively for triggering an alarm in response to a possible theft.

Here, initially the SCR is held in a switched OFF position as long as its gate stays rigged or screwed with the ground potential which happens to be the body of the asset which is required to be protected. If an attempt to steal the asset is made by unscrewing the relevant bolt, the ground potential to the SCR is removed and the transistor gets activated through the associated resistor connected across its base and positive.

The SCR also instantly triggers because now it gets its gate voltage from the transistor emitter, and latches sounding the connected DC alarm. SCRs becomes ideally suited for making fence charger circuits. In ac circuits, the SCR can be turned on by the gate at any angle a with respect to the applied voltage. Power control is obtained by varying the firing angle and this is known as phase control.

In the phase-control circuit given in fig. The variable resistance R 2 limits the gate current during positive half cycles of the supply. If the moving contact is set to the top of resistor R 2 , resistance in the circuit is the lowest and the SCR may trigger almost immediately at the commencement of the positive half cycle of the input. If, on the other hand, the moving contact is set to the bottom of resistor R 2 , resistance in the circuit is maximum, the SCR may not switch on until the peak of the positive half-cycle.

This operation is sometimes referred to as half-wave variable-resistance phase control. It is an effective method of controlling the load power. Diode D is provided to protect the SCR gate from the negative voltage that would otherwise be applied during the negative half cycle of the input. It can be seen from the circuit diagram shown in fig. In the circuit shown here, the resistor R and capacitor C determine the point in the input cycle at which the SCR triggers.