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Traic Not Getting OFF

gauravkothari23

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Hi all.
I am trying to build a small circuit where i need to switch ON and OFF and 220V AC Light using 8051 Controller.
Circuit Diagram attached.
I am using BT134 Triac to control the Light and MOC3021 to drive the triac.
Have connected the MOC3021 to Controller Pin P0.5.
My Problem is when i try to switch OFF the lights, the output does not gets completely zero. it shows me output voltage of approx 50 to 60V AC and also the light stays dimming.
can anybody please let me know where the issue is.
The same triac BT134 when i replace it with BT136 Triac, the things work well.
 

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  • Triac (18.04.21).png
    Triac (18.04.21).png
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KlausST

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Hi,

Maybe the load is high impedance ... and stray current cuses voltage.

May I ask why you use the MOC ....the design is not isolated, thus the triac gate may be driven by the microcontroller directly.
(just because zero cross switching?)

Show a photo of the real circuit. Are you sure you connected the snubber correctly?

Klaus
 

gauravkothari23

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Th
Hi,

Maybe the load is high impedance ... and stray current cuses voltage.

May I ask why you use the MOC ....the design is not isolated, thus the triac gate may be driven by the microcontroller directly.
(just because zero cross switching?)

Show a photo of the real circuit. Are you sure you connected the snubber correctly?

Klaus
The load is normal incandescent bulb, but also i have tried using a small AC 220V Motor. but the problem remains the same.
yes, i agree, that the triac gate can be driven by the contoller directly, even i tried the same, using 330 Ohms resistor between gate and controller pin, but the problem remains the same. so i though to try by isolating the both.
Yes, the snubber is connected as mentioned in the circuit.
The actual circuit is attached.
--- Updated ---

Hi,

Maybe the load is high impedance ... and stray current cuses voltage.

May I ask why you use the MOC ....the design is not isolated, thus the triac gate may be driven by the microcontroller directly.
(just because zero cross switching?)

Show a photo of the real circuit. Are you sure you connected the snubber correctly?

Klaus
The snubber circuit is also connected properly,
No i am not using zero cross switching.
But the things work perfectly when using BT136 instead of BT134. have also tried replacing BT134 with a new one. But still dont understand where the issue is.
 

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  • Triac (18.04.21).png
    Triac (18.04.21).png
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Last edited:

KlausST

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Hi,

What wattage is the bulb?

Usually the snubber is across the triac. In your case it is across the load.
When it is across the triac, then it will cause current in the load.

Do some tests:
* circuit like shown. Bulb is dim even when OFF. Now disconnect the gate. What happens?
* circuit like shown. Bulb is dim even when OFF. Now disconnect the snubber. What happens?

Klaus
 

gauravkothari23

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Hi,

What wattage is the bulb?

Usually the snubber is across the triac. In your case it is across the load.
When it is across the triac, then it will cause current in the load.

Do some tests:
* circuit like shown. Bulb is dim even when OFF. Now disconnect the gate. What happens?
* circuit like shown. Bulb is dim even when OFF. Now disconnect the snubber. What happens?

Klaus
Disconnect the Snubber, Gate connected.
Removed the capacitor, still the same problem, even in OFF state the Light is DIM

Disconnected the gate, Snubber Connected.
Removed the Resistor R15 330 Ohms connected to pin 1 of MOC3021, Still the same problem,,
--- Updated ---

Hi,

What wattage is the bulb?

Usually the snubber is across the triac. In your case it is across the load.
When it is across the triac, then it will cause current in the load.

Do some tests:
* circuit like shown. Bulb is dim even when OFF. Now disconnect the gate. What happens?
* circuit like shown. Bulb is dim even when OFF. Now disconnect the snubber. What happens?

Klaus
Dsconnected the Snubber, Disconnected the Gate.
Still the same problem
--- Updated ---

Hi,

What wattage is the bulb?
Klaus
Bulb is approx 60 Watts.
--- Updated ---

Hi,

What wattage is the bulb?

Usually the snubber is across the triac. In your case it is across the load.
When it is across the triac, then it will cause current in the load.

Do some tests:
* circuit like shown. Bulb is dim even when OFF. Now disconnect the gate. What happens?
* circuit like shown. Bulb is dim even when OFF. Now disconnect the snubber. What happens?

Klaus
but why is BT136 working perfectly and not BT134
 
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KlausST

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Hi,

If the gate is disconnected an the snubber is across the load... then the current can only be avross T1 and T2 if the triac.
Makes no sense to me.

Klaus
 

gauravkothari23

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Got it,
Have added 220 ohms resistor between triac gate and GND.
It works perfectly.
But dont know the reason why
 

KlausST

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Hi,

O.K. solved.

Still is not clear to me. According datasheet the it needs some mA of gate current to trigger the triac.
So you disconnected the gate and still it gets conductive ... thus there need to be enough triac internal gate current to trigger the triac.
Resistive? I doubt it.
Capacitive? Needs to be rather big capacitance ... and the current is usually proportional to dV/dt .. which is the biggest at zero cross. If so I expect the triac to be almost fully ON .... not very dim.

Not clear to me. But if a simple resistor is the solution (BTW usually mentioned in triac drive application notes)... then it's simple.

Klaus
 

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