voltage at triac no gate signal

Thread Starter

denison

Joined Oct 13, 2018
328
I have a triac coupled to a random phase opto-coupler. Even when there is no low side voltage signal to the opto the triac is showing an output of 16vac. The triac is coupled to a 240vac mains voltage. This low voltage would not be able to operate a mains device.
Is this normal for a triac. What would be the explanation for my meter showing this voltage?
 

Sensacell

Joined Jun 19, 2012
3,432
Just post a graphic image, like a JPEG or PNG file.
It's a good idea to consider how much effort others need expend to become familiar with your problem.
Make it easy for others to help you.
 

MisterBill2

Joined Jan 23, 2018
18,167
Here is the schematic. Hope its in the right form.
The attachment is a simulator file.
A switching triac is not like a hardware switch and so it will provide a small no-load output even when it is supposed to be off.And if you are measuring the output with a digital multimeter you are reading the no load voltage.
 

Thread Starter

denison

Joined Oct 13, 2018
328
The attachment is a simulator file.
A switching triac is not like a hardware switch and so it will provide a small no-load output even when it is supposed to be off.And if you are measuring the output with a digital multimeter you are reading the no load voltage.
Hi Mr Bill, I guessed as much. Had no problem with the load. Did not measure the voltage with a load attached. I was starting to think it might be capacitance at the gate terminal.
I have seen a transistor driven triac gate where there is a resistance from the neutral line to the gate terminal. The only reason would appear to be to drain off any residue capacitance.
I am attaching a schematic of the circuit from a text 'power electronics'. author B.W. Williams professor of electrical engineering in Scotland.
Notice that the ground of the driving power supply is also the neutral line.
Please note that resistances shown are not ones that would work. They just automatically came on. Also in the original '7406' was shown where I have put a 5v supply. I am going to try to find the resistor values I need to to get a working schematic for this Tina spice(Texas Instruments). If anybody would like to try this please post the result.
The attachment is a simulator file.
A switching triac is not like a hardware switch and so it will provide a small no-load output even when it is supposed to be off.And if you are measuring the output with a digital multimeter you are reading the no load voltage.
Thanks for that Mr Bill. I am attaching a schematic from a text book, author B.W. Williams professor of electrical engineering in Scotland. Title is Power Electronics.
There is a resistor from the gate of the triac to the neutral line. I believe this is to drain a capacitor effect at the gate terminal which might tend to hold the triac on when it should be off.
Also note that the lower power supply ground is common to the neutral line. I had considerable controversy previously on this forum when I did a similar thing with my zero voltage crossing circuit detector.
I don't have a triac model on my LTspice. I do on Texas Instruments spice. I am going to try this simulation with 5v at the 7406 terminal and do away with the 10v supply.
If anybody on this forum could come up with the necessary resistor values for this please post to the forum. I will probably be able to come up with the necessary values given enough time.
 

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