# TRIAC Phase Firing Circuit Simulation using LTSpice

#### noobeestudent

##### Junior Member level 1
Hey, I need help with my assignment.
I was required too build a Phase Firing Circuit for a aquarium heater that I am going to use in a PCB Etching Tank.
However, I wasn't able to simulate it properly using LTSpice. I am not sure where i did wrong.
Attached below is the Phase Firing circuit in LTSpice and the output.

The green line shows the 240V AC mains, red showing the pulse that I send to the optocoupler (MOC3021) and blue line shows the output (voltage across the 100Ω resistor)

The MOC3021 optocoupler and Q8004L4 TRIAC was imported.

MOC3021 optocoupler Library
Code:
* OPTO TRIAC
* Helmut Sennewald 8/10/2004
* MOC3021 I_trig=8mA
* D+ D- MT2 MT1
.SUBCKT MOC3021 AN CT T2 T1
.PARAM Itrig=8m
.PARAM RH1=20k
.PARAM RH2=20k
.PARAM RH3=16.7k
Q2 vb1 vb1p vd1 0 PNP1
Q1 vb1p vb1 4 0 NPN1
R3 vb1 4 {RH2}
D1 1 2 DL
R1 ctrl1 4 1
C1 ctrl1 4 10
R2 ctrl1 vb1 {RH1}
R4 vd1 vb1p {RH3}
B1 ctrl1 4 I=-500*I(D1)*3m/Itrig
R6 vd2 vb2 {RH2}
D3 vd2 3 D1
Q3 vb2 vb2p 4 0 PNP1
Q4 vb2p vb2 vd2 0 NPN1
E1 vd2 N001 ctrl1 4 -1
R5 N001 vb2 {RH1}
R7 vb2p 4 {RH3}
D2 3 vd1 D1
R34 3 4 100MEG
.MODEL PNP1 PNP(Is=1e-15 BF=10 Cjc=10p Cje=20p Tf=0.1u Ise=1e-12)
.MODEL NPN1 NPN(Is=1e-15 BF=10 Cjc=10p Cje=20p Tf=0.1u Ise=1e-12)
.MODEL D1 D(Is=0.1u Rs=2 Cj0=50p) .MODEL DL D(Is=1e-20 Rs=5)
.ENDS
Q8004L4 TRIAC Library
Code:
*SRC=Q8004L4;Q8004L4;TRIACS;TECCOR;800V 4A
*SYM=TRIAC
.SUBCKT Q8004L4   T2 G  T1
*     TERMINALS: MT2 G MT1
QN1  5 4 3  NOUT OFF
QN2 11 6 7  NOUT OFF
QP1 6 11 3  POUT OFF
QP2  4 5 7  POUT OFF
DF  4  5  DZ OFF
DR  6 11  DZ OFF
RF  4 6  16MEG
RT2 1 7  0.123
RH  7 6  17.5
RGP 8 3  24
RG  2 8  11.6
RS  8 4  2.4
DN  9 2  DIN OFF
RN  9 3  12.2
GNN  6 7 9 3 0.166
GNP  4 5 9 3 0.352
DP 2 10  DIP OFF
RP 10 3  7.12
GP  7 6 10 3 0.112
.MODEL DIN  D (IS=229F)
.MODEL DIP  D (IS=229F N=1.19)
.MODEL DZ   D (IS=229F N=1.5 IBV=50U BV=800)
.MODEL POUT PNP (IS=229F BF=5 CJE=1.68N TF=76.5U)
.MODEL NOUT NPN (IS=229F BF=20 CJE=1.68N CJC=335P TF=5.1U)
.ENDS

#### wwfeldman

as i read the triac data sheet, everything is relaed to MT1, which you appear to call T1
it looks like you have the triac in (the simulation schematic) backward,
it also looks like the polarity of the gate pulse should be negative for quadrants 2 and 3

is the pulse time long enough and the voltage high enough to trigger the opto-couper?
if the output of the opto-coupler enough to turn on the triac
as i read the data sheet, an MOC3021 requires 15 mA to guarantee trigger
you are providing less than 17 mA - if the resistor is high, you may not be triggering the opto-coupler.

phase control requires the ability to change when the trigger occurs in the cycle,
so one can control (in your case) a heater.
i assume you'll be adding that

#### noobeestudent

##### Junior Member level 1
as i read the triac data sheet, everything is relaed to MT1, which you appear to call T1
it looks like you have the triac in (the simulation schematic) backward,
it also looks like the polarity of the gate pulse should be negative for quadrants 2 and 3

is the pulse time long enough and the voltage high enough to trigger the opto-couper?
if the output of the opto-coupler enough to turn on the triac
as i read the data sheet, an MOC3021 requires 15 mA to guarantee trigger
you are providing less than 17 mA - if the resistor is high, you may not be triggering the opto-coupler.

phase control requires the ability to change when the trigger occurs in the cycle,
so one can control (in your case) a heater.
i assume you'll be adding that
For the first part, sorry, I wasnt quite sure what you meant.

For the second part, Ive since changed the duration to switch on for 1ms.
I wasnt sure about the gate current, when i measure the current across the resistor, it still shows 0mA.

For the third part, Yes, Ive already designed a zero crossing circuit that works fine.

#### FvM

##### Super Moderator
Staff member
For the first part, sorry, I wasn't quite sure what you meant.
In a short MT1 and MT2 should be flipped.

Testing and correcting your simulation setup would be much easier if you zip the *.asc and *.lib files and append it to your thread.

#### FvM

##### Super Moderator
Staff member
I see several issues with your simulation.
1. The models are corrupted, You have apparently edited the subcircuit port names. I reverted to the original MOC3021 model from by Helmut Sennewald and restored the Q8004 port names to 1 2 3.
2. At least in my simulation, MOC3021 needs a small snubber capacitor to avoid self triggering during zero crossing.

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noobeestudent

### noobeestudent

points: 2

#### noobeestudent

##### Junior Member level 1
I have a separate question regarding the zero crossing circuit.

Attached below is my zero crossing circuit ASC file.

View attachment Zero Crossing Circuit.zip

Does anyone know if it is suitable to use the 4n25 optocoupler for the circuit?
If not, is there other recommendations that I can use or any change in the circuit that I can make.

#### FvM

##### Super Moderator
Staff member
Thanks for attaching the Ltspice file, you should also append a schematic screenshot for convenience. Current limiting resistor is surely missing.

A zero crossing circuit like this has threshold voltage set by the current limiting resistor, opto coupler transfer ratio and circuit at the collector. What's the expected threshold?

Problems you have to deal with are current limiting resistor power dissipation and type variation of transfer ratio.

#### noobeestudent

##### Junior Member level 1
I see several issues with your simulation.
1. The models are corrupted, You have apparently edited the subcircuit port names. I reverted to the original MOC3021 model from by Helmut Sennewald and restored the Q8004 port names to 1 2 3.
2. At least in my simulation, MOC3021 needs a small snubber capacitor to avoid self triggering during zero crossing.

View attachment 159679
Hi, based on the simulation that you have provided, I have noticed that there is a large spike at R2 which can go up to 800W, do you know if a normal 0.25W through hole resistor manage to cope with that power. The average power over R2 is only around 155mW.

#### KlausST

##### Super Moderator
Staff member
Hi,

we don´t know which resistor you use.
But every electronic part comes with a datasheet. There should be an information about pulse/peak power and voltage or energy.

Klaus

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