Building a dark sensing circuit and using a Triac to control a lamp, how will the Triac turn off?

SLK001

Joined Nov 29, 2011
1,543
It won't, if the LDR is getting feedback from the light. Mounting the LDR above the light and pointing it at the sky may decouple it enough to give you an "OFF" when dawn arrives.
 

MaxHeadRoom

Joined Jul 18, 2013
19,964
The description is in the diag.
When the transistor is turned off the Triac will commutate off at the next zero crossing of the AC.
Max.
 

Thread Starter

Yami

Joined Jan 18, 2016
242
The description is in the diag.
When the transistor is turned off the Triac will commutate off at the next zero crossing of the AC.
Max.
Thanks for the answer, could you please explain it a little bit more would really appreciate it. Because it is my understanding that the triac remains "on" even when the "gate" is removed, and one would have to sort it to ground.

Thanks so much
 

MaxHeadRoom

Joined Jul 18, 2013
19,964
A SCR/Triac will conduct once the gate current has occurred, and will remain on as long as the controlled load voltage is present, in the case of a Triac switching a AC voltage the AC reverses every half cycle, which requires that it transitions through a zero point at this time, therefore when the zero point (zero voltage) occurs, the SCR will turn off and remain off until the next trigger occurs.
Max.
 

crutschow

Joined Mar 14, 2008
24,295
Thanks for the answer, could you please explain it a little bit more would really appreciate it. Because it is my understanding that the triac remains "on" even when the "gate" is removed, and one would have to sort it to ground.
That's true for controlling DC but not AC, as Max noted.
 

SLK001

Joined Nov 29, 2011
1,543
My original post was misleading. If the LDR is getting feedback from the light, it will cause the circuit to immediately shut off. If it is dark, the circuit will then come back on. Once back on, the LDR will cause the light to shut back off again. It will continue to oscillate like this until dawn, when it will shut off until nightfall. You will need to decouple the LDR from the light bulb to get this circuit to function properly.
 

ian field

Joined Oct 27, 2012
6,539
Thanks for the answer, could you please explain it a little bit more would really appreciate it. Because it is my understanding that the triac remains "on" even when the "gate" is removed, and one would have to sort it to ground.

Thanks so much
Look on the datasheet for: "holding current".

With no gate drive - an SCR or triac will unlatch if the current falls below that value. Sometimes an SCR is run on a DC supply, so sometimes arrangements have to be made to "commutate" the device. Triacs are most often used on AC, if the gate drive is not present; it gets an opportunity to commutate every time the AC waveform crosses zero.
 

Thread Starter

Yami

Joined Jan 18, 2016
242
So guys I've decided to use a 5V relay instead of the transistor to switch on the TRIAC. Couple of issues first I put together a voltage divider circuit and adjusted the variable resistor until I got 5V across the LDR, but it didn't energize the relay coil. So I opt for another circuit from the internet for the relay using two transistors- it works.( courtesy of http://www.buildcircuit.com/how-to-use-a-relay/). Now the big question. HOW TO TRIGGER THE TRIAC TO TURN ON?. Is there a way of knowing if the triac is on with out connecting the 230 AC bulb circuit. How would I make connection to the gate from the relay's output. Does everything have to be in the same common line??
 

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MaxHeadRoom

Joined Jul 18, 2013
19,964
To simply turn it on, i.e. no phase angle control, wire a 1.5k resistor from one of the N.O. contacts to the A2 of the Triac, connect the gate of the Triac to the relay common contact and it will switch on every time the relay closes and stay on when the relay opens until the next zero crossing.
Wire A2 to one side of the AC supply, the A1 goes to the load/lamp etc.
Example on Left.

Max.
 
Last edited:

ian field

Joined Oct 27, 2012
6,539
To simply turn it on, i.e. no phase angle control, wire a 1.5k resistor from one of the N.O. contacts to the A2 of the Triac, connect the gate of the Triac to the relay common contact and it will switch on every time the relay closes and stay on when the relay opens until the next zero crossing.
Wire A2 to one side of the AC supply, the A1 goes to the load/lamp etc.
Example on Left.

Max.
The resistor values in those diagrams are nearer than what you said.

Years ago I rescued a load of controller racks from the skip behind a bakery. The triacs were driven by triac output opto isolators which had 180R in series to the top of the power triac.
 

Thread Starter

Yami

Joined Jan 18, 2016
242
To simply turn it on, i.e. no phase angle control, wire a 1.5k resistor from one of the N.O. contacts to the A2 of the Triac, connect the gate of the Triac to the relay common contact and it will switch on every time the relay closes and stay on when the relay opens until the next zero crossing.
Wire A2 to one side of the AC supply, the A1 goes to the load/lamp etc.
Example on Left.

Max.
The triac I'm using is BT137, it needs 12V to trigger. Basically I thought of having two seperate circuits. Would the gate of triac have a pull down resistor?. Would I be able to check if the triac operates using a ohm meter?
 

MaxHeadRoom

Joined Jul 18, 2013
19,964
The triac I'm using is BT137, it needs 12V to trigger. Basically I thought of having two seperate circuits. Would the gate of triac have a pull down resistor?. Would I be able to check if the triac operates using a ohm meter?
The BT137 can be used as I mentioned, you do not need a 12v supply.
As @ian field use a 180 Ω res
Max.
 
Last edited:

ian field

Joined Oct 27, 2012
6,539
The BT137 can be used as I mentioned, you do not need a 12v supply.
As @ian field use a 180 Ω res
Max.
Its very important to put the resistors from the top of the power triac and not from the live - when the triac latches; it clamps the voltage that the resistor has to drop. The initial current spike is pretty high - but it only lasts microseconds.
 

tcmtech

Joined Nov 4, 2013
2,868
I'm confused here.

First, if you are using a relay to trigger the triac why not just skip using the triac altogether and just relay to switch the lamp on and off?
o_O

Second being a line powered circuit why haven't the mods went monkey poop all over this over site policy issues:rolleyes:
 

tcmtech

Joined Nov 4, 2013
2,868
This seems overly complicated being pretty much every basic dark sensor for controlling a basic light is nothing more than a 10 -50K photoresistor and a triac. Nothing else.

So is this for a more complex load besides an incandescent bulb like a CFL or LED type?
 
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