# What is holding current and latching current in SCR ?

What is holding current and latching current in SCR? Draw the gate characteristics of an SCR and explain the characteristics. What are the different signals which can be used for turning on SCR by gate control? compare them.

Holding current:

Holding current is the minimum current flow in SCR to hold SCR in on state. If the anode current fall below holding current SCR will turn off. And that voltage is known as holding voltage. Holding current is less than latching current. The holding current is in the range of 8 to 10 milliamperes.

Latching current:

Latching current is for turning on the SCR. Latching current is the current which is required to turn on the SCR and latch the SCR in conduction mode. Latching current is in the range of 10 to 15 milliamperes.

Gate characteristics of SCR:

Ig (min) and Vg (min) is the minimum current and voltage below which the SCR will not turn on. Ig (max) and Vg (max) is the maximum current and voltage above which SCR will damage. Means the shaded area is the safe operating area for SCR. For turning on SCR the gate current should be greater than Ig (min) but less than Ig (max) similarly, the gate voltage should be greater than Vg (min) but less than Vg (max).

Turning on SCR by gate control:

The turning on of SCR using gate control is the most commonly used method. The current is applied between gate and cathode of SCR. That current may be AC, DC or pulsating. Once the SCR start conducting because of positive gate current, the gate current is no longer needed to keep the SCR in conduction mode. The gate signal should not be applied if the SCR is in reverse bias condition. The gate terminal should be more positive with respect to a cathode for turning on of SCR.

The three different types of gate control signals are:

1. DC gate signal

2. AC gate signal

3. Pulsed gate signal

1.DC gate signal: In DC gate signal triggering the DC voltage of proper magnitude and polarity is applied between gate and cathode. In DC gate signal triggering the gate, power losses are more.

2.AC gate signal: In AC gate signal triggering the AC voltage is applied between gate and cathode. This method is used for fast turning on of SCR. Due to the positive half cycle of AC at a particular point of that positive half cycle, the SCR will be turn on.

3.Pulsed gate signal: In pulsed gate signal triggering the pulse signal waveform is applied to the gate of SCR. In pulsed gate signal triggering the gate, power losses are less. In this type of triggering the pulse transformer is used.

(i) Holding Current: Holding current is defined as the minimum anode current required to maintain conducting SCR in the on-state.

(ii) Latching Current: Latching current is defined as the minimum anode current required to maintain the SCR in the on-state immediately after the SCR has been turned on and the gate signal has been removed.

## Related questions

Last Answer : SCR turned off when the anode current falls below holding current.

Last Answer : Thyristor can be turn on by positive gate pulse and the current will continue to flow through the device even when the gate voltage is low hence thyristor is called latching device.

Description : Explain triggering of SCR using opto-coupler. State its advantages.

Last Answer : Triggering of SCR using opto-coupler:  A simple opto-coupler based triggering circuit for SCR is shown in the figure. An optocoupler or opto-isolator is a combination of light source ... is small size and light weight device. iii)Interfacing with logic circuits is easily possible.

Description : Draw the circuit diagram of battery charger using SCR and explain it’s working.

Last Answer : Battery charger circuit using SCR: The figure shows the battery charger circuit using SCR. A 12V discharged battery is connected in the circuit and switch SW is closed. The singlephase 230V ... not fired. In this way, after full charging, further charging is automatically stopped.

Description : Explain the thermal triggering method of SCR. Enlist different triggering methods of SCR.

Last Answer : Types of Triggering: 1) Forward voltage triggering 2) Thermal triggering (Temperature triggering) 3) Radiation triggering (Light triggering) 4) dv/dt triggering 5) Gate triggering ... generated leakage current increases. This current gets multiplied internally and thyristor is turned on.

Description : Explain working of AC circuit breaker using SCR with circuit diagram.

Last Answer : AC Circuit Breaker:  The circuit configuration of static AC circuit breaker using SCR is shown in the figure. When switch S' is closed, the SCRs T1 and T2 are fired in positive ... to non-availability of gate current. Thus the maximum time delay for breaking the circuit is one half-cycle.

Description : Temperature controller using SCR

Last Answer : Temperature controller using SCR The temperature control circuit is used to regulate the temperature. Figure shows the temperature control circuit using thermostat as temperature detector and SCR as ... half-cycle and heater gets connected to source to produce heat and raise the temperature.

Description : Compare SCR and TRIAC

Last Answer : SCR TRIAC It is an unidirectional device. It is a bidirectional device. It is turned-on by only positive gate current. It can be turned-on by either ... applications including lamp dimmer, heating control, zero voltage switched AC relay, small AC motor control etc.

Description : Describe triggering of SCR using UJT relaxation oscillator.

Last Answer : I need it's operation

Description : How GTO is advantages over SCR

Last Answer : i) It is turned-off by negative gate pulse. ii) No commutation circuit required, reducing the cost, size, weight and volume of the circuit. iii) As commutation choke is not used, the associated acoustic ... turn off time permits high switching frequency. v) It has higher di/dt rating at turn-off.

Description : Draw a neat labelling V-I characteristics of SCR and explain the region

Last Answer : (i) Latching current: It is the minimum anode current required to maintain the SCR in the conduction state just after turn-on and the gate signal has been removed.  (ii) Holding current: It is the minimum anode current required to hold the conducting SCR in the ON state.

Description : Draw construction of SCR using two transistor model. Explain its operation.

Last Answer : Two Transistor Model of SCR or Thyristor

Description : SCR is more utilized as compared to Triac because it is

Last Answer : SCR is more utilized as compared to Triac because it is available with higher ratings

Description : A 230 V, 50 Hz phase controlled single-phase full-controlled SCR bridge converter draws 15 A constant dc current. If the source inductance is 3 mH, the drop in dc output voltage is (A) 4.5 V (B) 6.75 V (C) 9 V (D) 13.5 V

Last Answer : A 230 V, 50 Hz phase controlled single-phase full-controlled SCR bridge converter draws 15 A constant dc current. If the source inductance is 3 mH, the drop in dc output voltage is 9 V

Description : In a 230 V, 50 Hz single-phase SCR bridge converter operating at a firing delay angle, α and with large R-L load, the input source current is (A) sinusoidal current (B) constant dc current (C) continuous rectangular pulses (D) alternating rectangular pulses

Last Answer : In a 230 V, 50 Hz single-phase SCR bridge converter operating at a firing delay angle, α and with large R-L load, the input source current is alternating rectangular pulses

Description : An SCR without any external connections is considered as (A) two diodes in series (B) three diodes in series (C) two n-p-n transistors in series (D) two p-n-p transistors in series

Last Answer : An SCR without any external connections is considered as three diodes in series

Description : Is a SCR a thyristor?

Last Answer : Yes, SCR is a thyristor. Thyristor is the family and SCR is the member of that family but SCR is universally known as thyristor.

Last Answer : Yes, SCR is a unidirectional switch.

Description : What is the use of SCR?

Last Answer : SCR are use in lamp dimming, power regulators, motor control, rectifications of high power AC, control of welding machines and also use as switch these are some uses of silicon controlled rectifier.

Description : Purpose of Selective Catalytic Reduction (SCR)

Last Answer : Purpose of Selective Catalytic Reduction (SCR) : 1. It reduces Nox 75% to 90% 2. Converts it in to molecular nitrogen and water vapor 3. It reduces HC emission up to 80% 4. It reduces PM emission 20 to 25%.

Description : An inductor is connected in series with SCR to protect it from A) Excessive high voltage B) Excessive high dv/dt C) Excessive high di/dt D) Excessive large forward current

Last Answer : An inductor is connected in series with SCR to protect it from Excessive high di/dt

Description : The no-load speed of a single-phase SCR bridge converter fed separately excited dc motor operating at a firing delay angle, α and flux, Φ (A) directly proportional to α and Φ (B) ... proportional to α and inversely proportional to Φ (D) directly proportional to Φ and inversely proportional to α

Last Answer : The no-load speed of a single-phase SCR bridge converter fed separately excited dc motor operating at a firing delay angle, α and flux, Φ directly proportional to α and inversely proportional to Φ

Description : In a 3-phase converter circuit during commutation when one SCR in one phase is turned on to turn-off an SCR in another phase, results in (A) voltage notching (B) harmonic distortion (C) voltage sag (D) voltage swell

Last Answer : In a 3-phase converter circuit during commutation when one SCR in one phase is turned on to turn-off an SCR in another phase, results in voltage notching

Description : What is latching current in SCR?

Last Answer : It is the minimum current required to latch(turn on) the SCR from forward blocking state to forward conduction state.

Description : Leakage current flows through the thyristor in

Last Answer : Leakage current flows through the thyristor in Forward blocking mode and Reverse blocking mode.

Description : A 3 -phase, fully controlled, converter is feeding power into a DC load at a constant current of 150 A, the rms value of the current flowing through each thyristor of the converter is

Description : When the firing angle a of a single-phase, fully controlled rectifier feeding constant direct current into a load is 30°, the displacement power factor of the rectifier is

Last Answer : When the firing angle a of a single-phase, fully controlled rectifier feeding constant direct current into a load is 30°, the displacement power factor of the rectifier is √3/2

Description : Give the circuit analysis of current source inverter with resistive load. What are the main difference between voltage source inverter and current source inverter?

Last Answer : Current source inverter (CSI): In current source inverter, the output current is independent of load. The input current of current source inverter is constant. Current source converter converts DC input current to ... are connected in parallel. In CSI diodes are connected in series.

Description : Explain hysteresis loss, eddy current loss and skin effect limitations with reference to magnetic material.

Last Answer : Hysteresis loss: Hysteresis loss is a loss which occurs due to the friction of magnetic domain due to the change in magnetic field in ferromagnetic material like iron. Hysteresis loss causes power loss ... loss in transformer is given by, We=Kef2Kf2Bm2watts Skin effect limitations:

Last Answer : No, IGBT is not the current controlled device. IGBT is a voltage controlled device. IGBT have combine qualities of MOSFET and BJT. The high switching speed of MOSFET and low conduction losses of BJT. Also, IGBT which stands for Insulated Gate Bipolar Transistor have high input impedance.

Description : Draw the structural diagram of GTO. Explain briefly the switching behavior of GTO with the help of appropriate voltage and current waveform.

Last Answer : Gate Turn Off Thyristor (GTO) Conventional Thyristor can be turned on with gate terminal but can not turn off from gate terminal. But in case of Gate Turn Off Thyristor (GTO), we can turn it on and off ... subdivided as storage period (Ts), fall period (Tp) and tail period (Tt).

Description : In a thyristor Latching current is ________ than Holding current a) Equal b) Less c) Greater d) None

Description : The typical ratio of latching current to holding current in a 20A thyristor is : (A) 5.0 (B) 2.0 (C) 1.0 (D) 0.5

Last Answer : The typical ratio of latching current to holding current in a 20 A thyristor is  2.0

Description : In a thyristor Latching current is than holding current: - a) equal b) less c) greater d) none of the above

Last Answer : In a thyristor Latching current is than holding current: - greater

Description : What is holding current in SCR?

Last Answer : When scr is conducting current in forward conduction state, scr will return to forward blocking state when the anode current or forward current falls below a low level called Holding current Note: ... the turn off process. In general holding current will be slightly lesser than the latching current.

Description : Holding current for an SCR is best described as:  (1) The minimum current required for turn-off (2) The current required before an SCR will turn on (3) The amount of current required to maintain conduction (4) The gate current required to maintain conduction

Last Answer : Holding current for an SCR is best described as: The amount of current required to maintain conduction.

Description : An SCR starts conducting with appreciable current when

Last Answer : An SCR starts conducting with appreciable current when anode and gate both are positive

Description : After firing as SCR, the gating pulse is removed. The current in the SCR will  1. remain the same 2. immediately fall to zero 3. rise up 4. None

Last Answer : After firing as SCR, the gating pulse is removed. The current in the SCR will remain the same

Description : Surge current rating of an SCR specifies the maximum  (a) Repetitive current with sine wave (b) Non-repetitive current with sine wave (c) Non-repetitive current with rectangular wave (d) Repetitive current with rectangular wave

Last Answer : Surge current rating of an SCR specifies the maximum Non-repetitive current with sine wave

Description : Analyze the effect of supply voltage on performance of LED as regards current, Lumen output, efficacy and life.

Last Answer : 1) Effect on Current:- In LED current depends upon supply voltage, if voltage get changes then current also change but this change in current will be control by resistors which are connected in series.  2) ...   4) Effect on Life:- If supply voltage is not stable then life of LED get reduced.

Description : With the help of N-channel JFET describe the effect of input voltage VGS on output current ID.

Last Answer : Working of N channel FET: * When a voltage is applied between the drain and source with a DC supply (VDD), the electrons flows from source to drain through narrow channel ... where the channel gets completely blocked and the drain current becomes zero is called pinch- off voltage.

Description : Define: i) Input Offset Voltage ii) Input offset Current iii) Input Bias Current iv) Output offset voltage

Last Answer : Definition of parameters of OPAMP is follows:  i) Input Offset Voltage: Input offset voltage is defined as the voltage that must be applied between the two input terminals of an OPAMP to null or ... to ground) and output voltage is obtained non zero such voltage is output offset voltage of opamp.

Description : Current division rule example

Last Answer : Current division rule example

Description : A linear network has a current input 4cos(ω t + 20º)A and a voltage output 10 cos(ωt +110º) V. Determine the associated impedance.

Description : Find current through resistor using nodal analysis

Last Answer : Find current through resistor using nodal analysis

Description : Nodal analysis problem two current source

Last Answer : Nodal analysis problem two current source

Description : Direct Current (DC) Next Topic : Alternating current (AC)

Last Answer : Direct Current (DC) is the unidirectional electric current. Uni meaning is one. And the flow of charge in same direction. We know that conventional current flow from positive terminal of battery to ... The magnitude of DC may constant or vary but direction do not change with respect to time.

Description : Alternating current (AC)

Last Answer : AC stands for Alternating Current meaning voltage or current that changes polarity or direction respectively over time. An Alternating Current (AC) is an electric current whose magnitude ... energy are known as alternators and are simpler construction than DC electromechanical generators.

Description : Does current flow through a capacitor ?

Last Answer : Imagine a pipe with a flexible diaphram in the middle separating two haves of the pipe. Now the pipe is filled with air. If you apply a pressure to one end the diaphram will deform ... capacitor, the effects on electrons will be recreated across the boundary. But theres no electrical connection...

Description : What kills, current or voltage?

Last Answer : Current would kill. You can actually have thousands of bolts going through you and not even know it. Yes you probably know already voltage is current times resistance. Since your body is a resistor it would take just a small amount of current to create a very high voltage