Firing Circuits

Gate triggering is the most commonly used turn-on method employed to switch on the thyristors. Triggering circuits is also called firing circuits. There are various firing circuits available. R-Firing circuits is simple but suffer from limited firing circuits. Firing angle is limited between 0^o to 90^o. In actual practice firing angle can be varied between 3^o to 90^o. Limitation of the firing angle range of R-Firing circuit is eliminated by introducing a capacitor and a diode. Thus R-C firing circuits can increase the firing angle limitation range. Theoretically firing angle can be varied from 0^o to 180^o. However due to low voltage at 0^o and 180^o thyristor cannot be turn-on. Hence practically the range of firing angle is between 3^o and 177^o. 

Both R and R-C firing circuits suffer from following disadvantage:

• They can be employed in power circuits having only one thyristor
• They are capable of open loop control only
• Due to lower voltages near 0^o to 180^o, gate current is small. Especially in R-C firing circuit, near 180^o gate current is minimum due to maximum value of R. This will increase the turn on time, especially for R-L load, leading to higher turn on loss
• Higher frequency gate signal is desirable for reliable turn on. Both the circuits are not capable of providing the same
• There is no electrical isolation between control circuit and power circuit

However the circuits are simple and cheap. R-C firing circuits is widely used in low power thyristor controllers, such as solid state ac regulators for speed control of fans and blowers. R-C firing circuits can also acts as snubber circuit.

R-triggering and RC-triggering circuit notes : Click here to download

 

Turning off SCR [Commutation]

Commutation

• To turn ON a SCR, a low voltage, short duration pulse is applied to the gate (typically 4V, 100µs).
• Once the SCR is turned-on, the gate loses control and the SCR will only turn off when the load current falls virtually to zero, or the SCR is reverse biased.
• The SCR will turn off naturally with a.c. supplies as the voltage reverses (which is called as Natural Commutation), but no such reversal occurs with d.c. supplies and it is necessary to force a voltage reversal if turn-off is to occur. This process is called Forced Commutation.
• The process of turning OFF SCR is defined as "Commutation".
• In all commutation techniques, a reverse voltage is applied across the SCR during the turn OFF process.
• By turning OFF a SCR we bring it from forward conducting to the forward blocking mode.

      The conditions to be satisfied in order to turn OFF an SCR are:

• IA < IH ( Anode current must be less than holding current).
• A reverse voltage is applied to SCR for sufficient time enabling it to recover its  blocking state.

      There are two methods by which a SCR can be turned OFF.
                I. Natural Commutation
               II. Forced Commutation

Natural Commutation:

• In AC circuit, the current always passes through zero for every half cycle.
• As the current passes through natural zero, a reverse Voltage will simultaneously appear across the device. This will turn OFF the device immediately.
• This process is called as natural commutation, since no external circuit is required for this purpose. It is also known as line commutation.
• AC voltage controllers or phase voltage controllers are the example for the natural commutation.

Forced Commutation:

• To turn OFF a SCR, the forward anode current should be brought to zero for sufficient time to allow the removal of charged carriers.
• In case of DC circuits, the forward current should be forced to zero by means of some external circuits. This process is called as forced commutation.

1. Class A or Self commutation
2. Class B or Resonant commutation
3. Class C or Complementary commutation
4. Class D or Impulse or auxillary commutation
5. Class E or External pulse commutation

Power Devices Assignment I (Create Video) for SY B

G.S.Mandal's
Maharashtra Institute of Technology,Aurangabad
Department of Electronics and Telecommunication Engineering
Assignment I
CLASS:SY B                                                                                        Part II 2017-18
Group No.	Roll No.	Name of the Student	Topic
1	T2483	MALI ASHWINI CHHOTULAL	Resistor
	T2484	MHASKE SUNIL BHAGWAN	Capacitor
2	T2486	NAINAV PRIYANKA PRAKASH	Inductor
	T2487	DHUMAL PRATIKSHA DNYANOBA	Diode
3	T2488	KARPE PRATIBHA ASARAM	Schottky diode
	T2489	PATIL VIRENDRASINH RAMESHWAR	Zener diode
4	T2491	JEWALIKAR PRANESH ARUN	Power diode
	T2492	PURI SWATI KALIDAS	IR LED
5	T2493	RAKHE APURVA ANILRAO	LED
	T2494	RAUT SAURABH LAXMAN	7 Segment LED
6	T2495	SAHANE DIKSHA SANJAY	Tunnel diode
	T2496	SASANE PRAJAKTA DILIP	Varactor diode
7	T2497	JADHAV SATISH ARUN	LCD
	T2498	SEJUL SWATI DILIP	Transformer
8	T2499	SHAIKH SOHEB ANWAR MOHD YOUSUF ALI	Relay
	T24100	SHINDE TIRTHESH SANDHU	Battery
9	T24101	GHUGE SHRADHA RAMCHANDRA	Fuse
	T24102	TIWARI VYANKATESH SATYANARAYAN	Switches
10	T24103	UDAWANT RUSHIKESH RAJENDRA	MCB
	T24104	UMAP VIKAS MADHUKAR	Earthing
11	T24105	VAIDYA VISHNU NIVRUTTI	Insulator
	T24106	NAGRE VRUSHALI RAJENDRA	Connectors
12	T24107	WAGHMARE AISHWARYA BHASKAR	Types of wires
	T24108	WAMBLE ADINATH YASHWANT	PCB
13	T24109	ZINE PRAVIN VISHWAS	Potentiometer
	T24110	TANDALE DURGA UTTAMRAO	Thermistor
14	T24111	IGHE ADITYA GOVINDRAO	LDR
	T24112	KHANDARE RAVINA RAJU	BJT
15	T24113	KHURUD ANKITA VIKAS	MOSFET
	T24114	SHAH PRATIK PANKAJ	FET
16	T24115	INGLE TRUPTI VIJAY	JFET
	T24116	PATIL VAISHNAVI BHAGVAN	UJT
17	T24117	VETAL VAIBHAV PHULCHAND	Power BJT
	T24118	DANDGE CHITRAKALA PANDURANG	Power MOSFET
18	T24119	WADKAR ANJALI SANJAY	IGBT
	T24120	TORNE MANISHA VISHWANATH	SCR
19	T24121	DALE MAYURI GANESH	TRIAC
	T24122	KSHIRSAGAR SWAPNIL MAHADEV	DIAC
20	T24123	ANSARI AMER SK. JAFAR	GTO
	T24124	DALVI MAYURI JAYANDRA	IC 555
21	T24125	NIMBALKAR DIPALI KANCHANRAO	LM35
	T24126	KHAMAT GANESH SAHEBRAO	IC741
22	T24127	PANKHADE BIPIN RAVINDRA	IC7805
	T24128	DHARMADHIKARI SHITAL DEEPAKRAO	IC7905
23	T24129	SHAIKH SAIF ALI SAHEBAN	IC 7404
	T24130	DAUD SHUBHAM ASHOK	IC 7408
24	T24131	DANDGE NAGESH BABURAO	IC7486
	T24132	KOTURWAR VAIBHAVI SUHAS	IC7432
25	T24133	SOVITKAR SHANTIKUMAR RAGHUNATH	IC 7400
	T24134	GADEKAR SHIVPRASAD PRABHU	DC motor
26	T24135	KUBER RUSHIKESH KAKAJI	Solenoid Valve
	T24136	DALKE PRIYANKA KESHAV	Piezoelectric sensor
27	T24137	KAWARKHE GOPAL CHANDRAKAT	CFL
	T24138	SHIRSE MAYUR VITTHAL	LED Lamp
28	T24139	MOHAMMAD ATHAR MOHAMMAD AZHAR	Piezo buzzer
	T24140	SHIRALE TIRTHPAKSH AMBADAS	Speaker
29	T24141	CHAVAN SHIVAM BABASAHEB	Microphone
	T24142	RAUT SHUBHAM BHAGWAN	Optocoupler