# 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.

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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

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

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 : What is the use of SCR?

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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

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

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 : Compare SCR and TRIAC

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

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

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

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.

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

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 : Describe triggering of SCR using UJT relaxation oscillator.

Answer : Triggering of SCR using UJT relaxation oscillator:

Description : How GTO is advantages over SCR

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

Answer : Two Transistor Model of SCR or Thyristor

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

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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

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 : 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

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

Description : Why SCR is called as thyristor?

Description : Is a SCR a thyristor?

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.

Answer : Yes, SCR is a unidirectional switch.

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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

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

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

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

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

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

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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 : How do you test an SCR?

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Description : Leakage current flows through the thyristor in

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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

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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

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

Description : In a thyristor, holding current is   (a) More than latching current IL (b) Less than IL (c) Equal to IL (d) Very small

Answer : In a thyristor, holding current is Less than IL

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Description : A) Watt B) Volts C) Farad D) Ampere