# Firing Angle 0-180 Thyristor with UJT

Joined Nov 18, 2020
10
is my circuit correct? i'm trying to make firing angle 0-180 for thyristor

#### ericgibbs

Joined Jan 29, 2010
13,585
hi f,
Welcome to AAC.
Are you saying it does not work in simulation.?
E

Joined Nov 18, 2020
10
it works, but i don't know this is correct 0-180 or not because i tried another one 0-90 angle but it looks same, i don't know the difference between them

#### ericgibbs

Joined Jan 29, 2010
13,585
hi,
As you know180 degrees is the complete positive half of the mains cycle.
Try adjusting R1 on the sim until all of the 180 deg is controllable
Lets know what you see.
E

Joined Nov 18, 2020
10
still not the complete positive cycle sir

#### ericgibbs

Joined Jan 29, 2010
13,585
hi f,
Thats what I would expect.
Consider that the top of R1 is connected to the half wave rectified voltage of the mains.
So the voltage on R1 increases from 0deg degrees to 90 deg, after that the voltage falls, so the UJT never has sufficient voltage to trigger on the falling 90 deg to 180 deg voltage.

E
Check the UJT Gate voltage waveform on your simulation.

Joined Nov 18, 2020
10
Like this sir

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#### ericgibbs

Joined Jan 29, 2010
13,585
hi,
You can now see that with your circuit it only possible to control the UJT Trigger point, from 0 deg to 90deg.

E

Joined Nov 18, 2020
10
aahh i see now, how can i make it get trigger on the falling 90 to 180 deg

#### Dodgydave

Joined Jun 22, 2012
10,050
Remove D1, and replace it with a bridge rectifier, so you get both cycles of the mains.

Joined Nov 18, 2020
10
Hi D,
yes, i get both cycles of the mains but it's only for the V gate of the UJT. the thyristor triggering point still can't go beyond 90 deg

#### ericgibbs

Joined Jan 29, 2010
13,585
hi f,
How complex a circuit are you prepared to build, it can be done but it will not be simple.
How you heard of zero crossing detection of the mains cycle.?
E

Joined Nov 18, 2020
10
Hi E,
i never heard that before

#### ericgibbs

Joined Jan 29, 2010
13,585
hi

E

#### Dodgydave

Joined Jun 22, 2012
10,050
Here is a circuit for Zero crossing detection for micros...taken from the datasheet of 4n35,, this circuit produces Negative pulses on Zero mains.

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Joined Nov 18, 2020
10
okay sir, thank you so much, i will try that

#### Dodgydave

Joined Jun 22, 2012
10,050
Are you just wanting to make a simple light dimmer, if so I would just use a Triac and diac circuit.

Joined Nov 18, 2020
10
noo sir, it's actually i'm doing my practicum module and i'm just curious because all of my friend have the same problem and we're just stuck don't know what to do to find the trigger 90-180 deg

#### Dodgydave

Joined Jun 22, 2012
10,050
noo sir, it's actually i'm doing my practicum module and i'm just curious because all of my friend have the same problem and we're just stuck don't know what to do to find the trigger 90-180 deg
So is this homework for college.?