Soft Start Circuit for Tetrode Filament using TRIAC

Soft Start Circuit for Tetrode Filament using TRIAC

Shyam Mohan V

Senior Research Fellow- DRDO, Bangalore

Abstract – This paper presents the creation of a soft start circuit that operates based on the current regulating property of TRIAC. TRIAC act as barrier in AC circuit preventing the current flow before being triggered. Supply voltage to the heater filament need to be increased step by step to rated value. By changing the firing angle (180-0) of triac, the voltage applied to the Heater filament can be varied. Using a zero crossing detector and mono-stable multi-vibrator circuit the firing angle is changed. Trigger pulse is given to the Triac using a photo-diac. The TRIAC is set into a conducting state and current flows through the Heater Filament soon after the trigger voltage is applied to the photodiac. The amount of voltage seen over the Heater Filament is determined by the firing angle of the triac which is fixed by RC time constant of the monostable circuit. Resistance is decreased step by step using the control circuit thus the firing angle is reduced and voltage across Filament is increased up to supply voltage. Using photo-diac and phototransistor load circuit is galvanic isolated from the control circuit.

INTRODUCTION

This paper describes the design, operation, and testing of a soft start circuit, using Device such as 555 timer, MOC3021, 74LS123 and triac, for tetrode

filament. Tetrode is a vacuum tube it has a Heater for Electron emission, its supply voltage has to be increased step by step from low value to its rated value for safety and long life of vacuum tube. In this topology power flow is regulated to the Heater Filament by applying a AC supply voltage intermittently across the load during each half cycle. At starting only 20% of the input sine wave is applied across the LOAD. Step by step as the Resistance of RC circuit decreases, the RC time constant of Mono-stable multi-vibrator decreases, thus pulse width of output decreases. At the trailing edge of this pulse, pulse generating circuit will activate and it will give a pulse to the MOC3021 it will trigger the triac. Triac will start conducting from that point to the starting of next half cycle. The proportion of the sine wave applied across the load increases as the pulse width of the mono-stable output decreases. Circuit design, operation, test results, and the interpretations that were made during the experiments are shown below.

Topology.

Main circuit.

In the main circuit realized using a zero crossing detector circuit, its output pulse is used as triggering pulse for mono- stable multi-vibrator. Its a simple electronic circuit using the capacitor and a PnP transistor. As the sinusoidal voltage wave form crosses the zero point in every half cycle, it will apply adequate base voltage to the PNP transistor this will in turn put the transistor in saturation region from cutoff region, it act as a switching circuit. Using a photo transistor this pulse is galvanic isolated from the main supply voltage

side to the controlling circuit; the isolated pulse is applied to the mono-stable. As the RC time constant of the mono- stable is decreased it will in turn reduce the firing angle, thus the overall voltage coming across the load will increase.

In the circuit diagram variable resistor is represented as AB with a resistance value of 60K(10+10+10+20)+10K initially full resistance is

coming across the RC circuit thus the pulse width will be

6.6 msec. step by step resistance is decreased to 10K thus the pulse width decreases to 1.1 msec, firing angle decreases to the minimum value. whenever the output of mono-stable goes from high to low, it will trigger 74LS123 it will give a pulse output of fixed width(<1msec).

It is used to give base voltage of a transistor, it in turn give pulse to the MOC3021 it will give gate triggering pulse to the triac. Triac will conduct from this point till the current through it goes below the holding current. After some time of starting the total resistance of the RC time constant will decrease to 10K from 60K, thus the triggering pulse will trigger the triac exactly at starting of every half cycle. Full voltage will obtain across the Load.

Photo-diac MOC3021 is used to trigger the Triac, but the Triac may also be triggered to on state accidentally. This can cause by exceeding the maximum blocking voltage VDRM or by applying very steep rising signals to the output of Triac. Such transient signals or noise may surpass the dV/dt rating of the triac driver and henceforth cause the device into on state. The TRIAC dV/dt ratings and its driver and hence cause the device to proceed into ON state. The dV/dt ratings of the triac and its driver are very important when switching inductive loads since load voltage and current are not necessary in phase. Since a triac turns off when the load current equal zero, load voltage is not equal to zero. As a result, the triac produces a sudden rise in load voltage as an output, which

could exceed its dV/dt rating.

To increase voltage rise time, a snubber circuit may be used. In majority of the cases, single snubber circuit shall protect the main triac and the photodiac. Comprehensive knowledge about the load is necessary while designing the RC snubber network for triac drivers. With the knowledge

of power factor PF, the maximum turn off voltage at the output can be predicted:

VT max = Vinmax *sin(arccos(PF)) Assume Vtmax = 240 V

Choose R1 to limit the current peak at maximum voltage R1=VinMax/Ifp,

339/1=339. Standard value close to this is 360.

As the peak current is limited by the resistor R1, time constant for limiting dV/dt has to be set using R2 and C: dv/dt=Vtmax/ = Vtmax/R2C.

Next, the value of R2 is set by determining the smallest trigger voltage requirement. Assuming a triac gate trigger current IGT = X mA and a required load voltage (Vint).

R1+R2=Vint/X , we already know the value of R1, so we can find out the value of R2.

Value of can be find out from dv/dt and Vtmax values.

From values of R2 and we can find out the value

of C.

The dV/dt rating of the main triac and the dV/dt rating of photo-diac shall be different. Thus, while designing the snubber circuit the worst case value must be selected.

Control Circuit.

Control circuit mainly consists of 4 relays and 555 timers. Its operated in mono-stable mode as the mono stable output goes low a voltage will come across the relay coil, it make the NO contacts to closed condition. It will remain in this condition it power off. Current sinking and sourcing property of 555 timer is used for this, each of the mono- stable timer output pulse width is set to 51.7 sec. Full voltage will be obtained after 3.5 minute at the Load side.

A 10k

RLY1 R5

10k

RLY2

R4 D1

2

NC1 A

COM1

NO1

NC1 COM1

NO1

R32

U5

0 120k

BAV101 D2

D8

NC2 COM2

3.3k

33k

R31700k

60k

NC2 COM2

3.3k

33k

100k

R34

1 33k

100k

8 BAS40

U4

NO2

A

8 8

B U13

NO2

U2 A R30 C22 U14A

V4

5Vdc

8 U7 750

V3 V1

2 VCC

4 TRIGGRE3R R7 3

V2 2

15Vdc 4

VCC

TRIG2GER

VCC 3

10k 15

1n 14

REXT/CEXT 13

VCC

MOC 3021

2

1. TRIGGER 3

   15VVdOcFF = 0

   BAV1015

   RESET OUTPUT

   B 5

   RESET OTURTIGPGUETR B

   CEXT Q

2. RESET OUTPUT

6 CONDT6R33OkL 10

QR1elay _DPDT_nb

6 CON4TRORLESET OUTPUT R3elay _DPDT_nb

6R33CONTROQL 2

VAMPL = 230 C11

7 THRESHOLD C9

7 THRE5SHOLD

1 4C17

7 THRESHOLD

CLR

FREQ = 50

C10

10D03n

DISCHARGE

DH3468CNU1 C8

100n

DISC6HACROGNETROL

2 A C1Q6

100n

DISCHARGE

100n

C6

GND

1

555B

1 PS2501

4 100n

C7

GTNHDRESHOLD

D1 ISC55H5ABRGE

R31

B

3

1000k

100n 3.3k

C12

GND

BC547555B

7

BAV101470u

R6 D4

0

C1 2

1k 470n

B

100n

3

10k

47C03u C2

100n

20k

GND 1

5k

555B

74LS123

470u

1

10k

120k

BAV101

0

100k

RLY4 0

NC1 COM1 NO1

100k

RLY3

NC1 COM1 NO1

Q3 BT136_600D

U12

8

VCC 2

33k

NC2 COM2 NO2

8

U9

4

VCC 2

33k

RN36C2

COM2 NO2

R35

4

5

3 TRIGGER OUTPUT RESET

6

CONTROL

THRESHOLD 7

C20

A 3 TRIGGER OUTPUT RESET

5

6

CONTROL

THRESHOLD 7

C15

56 A

C4

820

DISCHARGE

100n

C19 B

DISCHARGE

100n

C1L4OAD

10n B

GND

100n

Relay _DPDT_nb

GND

100n

Relay _DPDT_nb

1000k

555B

C18

1 470u

1000k

555B

C13

1 470u

1000k

the four relays connected in such a fashion that when output of the first 555 goes low the first relay coil will get energized thus relay NO(normaly open) contact become Closed voltage will come across the next 555 , after a fixed time output of 555 goes low it will energize the next relay. Simultaneously as each relay operates the total Resistance across the Mono-stable get decreased, each relay will bypass the resistance step by step.

Experimental result.

The voltage to the Triac was given using a variac. 230V, 50Hz supply was given as input, variac is set give 120V, 50Hz output voltage. As a load we have used 2K , 40w resistor is used. one by one relays are operated the firing angle is decreased , the corresponding graphs are given below. Graph contains output voltage waveform and triac firing pulse. The output voltages are shown below:-

Relay condition	Firing angle	Output voltage(V)
All Relay off	119	45
1relay on	99	72
1,2 relay on	79	96
1,2,3 relay on	59	112
1,2,3,4 relay on	19	119

Figure 1

Figure 2

Figure 1

Figure 3

Figure 4 Figure 5

CONCLUSION

Soft start circuit operation has been demonstrated successfully with triac, relays, 555 timers etc on a dummy load. With this circuit its observed that one can increase the intensity of a voltage across the Tetrode Filament step by step in an automatic way. The RC time constant used to determine the point when triac conducts the AC wave through the Filament during each half cycle. The firing angle of Triac decreases as relays in the control circuit operates one by one thus voltage across the filament increases, when all the 4 relays operates full voltage will come across the filament .For the protection of Triac and photo-diac snubber circuit is implemented.

ACKNOWLEDGMENT

I gratefully acknowledge support and facilities provided by Institute for Plasma Research (Dept of Atomic Energy), During my work period as a project Engineer (2013-2015) .

Figure 6

REFERENCES

1. EE362L, Power Electronics, Triac Light Dimmer,

   Rochelle Stortz and Brian Taraba, Team 277

2. http://electronicdesign.com/power/hard-facts-soft- switching