Download SCR Over Current Protection

SCR UYGULAMA ORNEKLERİ
Dimer devreleri
We choose the following light dimmer circuit to study
Figure 1: light dimmer circuit
3.1 Battery charger based on scr
Figure 2: battery charger
How to protect an SCR using protection circuits ?
SCRs are sensitive to high voltage, over-current, and any form of transients. For satisfactory and reliable
operation they are required to be protected against such abnormal operating conditions. Because of complex
and expensive protection, usually some margin is provided in the equipment by selecting devices with
ratings higher (3 or 4 times higher) than those required for normal operation. But it is always not economi-cal
to use devices of higher ratings, hence their protection is imperative.
Over-voltage Protection.
SCR Over Voltage Protection
High forward voltage protection is inherent in SCRs. The SCR will breakdown and start conducting before the
peak forward voltage is attained so that the high voltage is transferred to another part of the circuit (usually
the load). The turn-on of SCR causes a large current to flow and poses a problem of over-current pro-tection.
Over-current Protection.
SCR Over Current Protection
Over-current protection can be provided by connecting a circuit breaker and a fuse in series with the SCR, as
usually done for the protection of any circuit. However, there are some reservations to their use. A
semiconductor device is capable of taking overloads for a limited period, so the fuse used should have high
breaking capacity and rapid interruption of current. There must be a similarity of SCR and fuse I 2t rating
without developing high voltage transients which endanger those SCRs in the off or infinite impedance
condition. These are contradictory requirements necessitating voltage protection when fast-acting fuses are
employed. Fuses when used, their arc voltages are kept below 1.5 times the peak circuit voltage. For small
power applications it is pointless to employ high speed fuses for circuit protection because it may cost more
than the SCR. Current magnitude detection can be employed and is used in many applications. When an overcurrent is detected the gate circuits are controlled either to turn-off the appropriate SCRs, or in phase
commutation, to reduce the conduction period and so the average value of the current.
If the output to the load from the SCR circuit is alternating current, LC resonance provides over-current
protection as well as filtering. A current limiting device employing a saturable reactor is shown in figure. With
normal currents the saturable reactor L1 offers high impedance and C and L are in series reso-nance to offer
zero impedance to the flow of current of the fundamental harmonic. An over-current saturates L1 and so gives
negligible impedance. There is LC?parallel resonance and hence infinite impedance to the flow of current at
the resonant frequency.
Protection against Voltage Surges.
There are many types of failure due to voltage surges as SCRs do not really have a safety factor included in
their ratings. External voltage surges cannot be controlled by the SCR circuit designer. Voltage surges often
lead to either malfunctioning of the circuit by unintentional turn-on of SCR or permanent damage to the
device due to reverse breakdown. SCR can be protected against voltage surges by employing?shunt
connected non-linear resistance devices. Such protective devices register a fall in resistance with the
increase in voltage and so develop a virtual short-circuit across the SCR when a high voltage is applied. An
over-voltage protection circuit employing thyrector-diode, which has low resistance at high voltage and viceversa, is shown in figure. Inductor L and capacitor C provides protection to SCR against large dV/dt and dI/dt.
DA Ayarlayıcısı ile DA motor kontrolü
Motor_speed_control_with_tachometer_feedback
This circuit shows a triac motor-speed control that derives feedback from a magnet-coil tachometer that is placed near
the motor fan (Figs. 8-17B and 8-17C). Motor speed is controlled by the 5-kΩ pot. The MAC210-4 triac is capable of
handling motor loads up to 10 A.
Basic_triac_control_circuit_that_uses_an_SBS
This figure shows the basic control circuit for triacs that use SBS triggers. The line voltage and load current depend
primarily on the triac characteristics. In this case, the MAC210-4 accommodates loads up to 10 A.
Electronic_crowbar
This circuit provides positive protection of expensive electrical or electronic equipment against excessive supply
voltage (resulting from improper switching, short circuits, failure of regulators, etc.). The circuit is used where it is
economically desirable to shut down equipment, rather than allow the equipment to operate at excessive voltages.
The circuit quickly places a short across the power lines (ac or dc), and thereby drops the voltage to the protected
device to near zero and blows a fuse. With the values shown, the crowbar operating point (set point) can be adjusted
over the range of 60 to 120 Vdc or 42 to 84 Vac. The values of R1 to R3 can be changed to cover different supply
voltages, but the triac voltage rating must be greater than the highest operating point that is set by R2. Lamp II (with a
voltage rating that is equal to the supply) can be used to check the set point and operation of the circuit, by opening
the push-to-test switch and adjusting the input or set point to fire the SBS. An alarm unit such as the Mallory Sonalert
can be connected across the fuse to provide an audible indication of crowbar action. Notice that this circuit cannot act
on short, infrequent power-line transients.
Temperature_controller
This circuit shows a CA3094B and triac that are connected to form a temperature controller.
800_W_soft_start_light_dimmer
This circuit shows the basic UJT building block (Fig. 9-1), which is used in a light dimmer with soft-start operation that
applies current to the light slowly enough to eliminate high surges (high inrush current). These current surges, found in
most cold-filament light dimmers, shorten lamp life. With this circuit, the lamp is heated slowly by a gradually
increasing voltage so that inrush current is kept to a minimum. R4 controls the charging rate of C2 and provides the
means to control or dim the lamp.
Half_wave_thyristor_control_with_average_voltage_feedback
This circuit shows a UJT used as a thyristor trigger (with feedback), where the average load voltage is the desired
feedback variable. R1, R2, and C1 average the load voltage so that the voltage can be compared with the set point
that is determined by RC.
250W SCR Inverter Circuit
Pulse DC Electronic Fishing Device Circuit (3)
The Pulse DC Electronic Fishing Device Circuit
Sanhe Songshi 15W vibrating massage stick circuit
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Electric fan stepless speed regulation circuit diagram
Electric fan stepless speed regulation circuit diagram is shown in the diagram: (View)