triac not triggering

I have a circuit with a triac that is used to control an EL wire via an Arduino sketch. The EL wire is powered separately from the Arduino via a 3V battery pack (through an inverter, since the EL wire needs AC power).

Circuit picture: here
Triac datasheet: NTE 5656

The triac is not triggering when I put +5V on the gate pin, as it should, I believe.

I know the EL wire part of the circuit works, because if I jumper the MT1 and MT2 pins on the triac, the wire lights up correctly. And I can measure 0/5 VDC on the gate pin of the triac (even by bypassing the sketch and just jumpering directly from the Arduino). And I have tried a few different triacs of the same part number.

So I think all is as it should be, but am at a loss as how to debug this circuit.

Any suggested strategies are much appreciated.

Thanks.

Without the circuit it is imposable to say what is wrong. You attempted to post it but nothing shows up. Can you try again.

But what are you really switching with the triac? The 3V supply, or the output to from the inverter?
I ask this because the Triac uses AC, and needs a minimum voltage to trigger in the "output".
And also, the output from te inverter might not be real AC, it could be a pulsed current from 0 to HV, instead of a negative/positive alternating current. Triacs need a negative current to properly "de-trigger"
Regards

Triacs need a negative current to properly "de-trigger"

Not strictly true. They need approximately zero current at zero voltage. If this is an inductive load then the phase lag on a system that does not go negative might hold it on but in a DC system reducing the input voltage to zero is enough to turn them off.

Thinking about it you have probably not got a common ground between the arduino and the triac. This is because the inverter circuit acts as isolation. I would think you need an opto isolator into the gate of the triac so that you can get current flowing through the gate.
However this is just a guess until I see the circuit.

The triac is not triggering when I put +5V on the gate pin, as it should, I believe.

If you connected without a resistor to limit a gate current, your triac is dead now :frowning:

You attempted to post it but nothing shows up. Can you try again.

Hmm, must have been a problem with my server. I changed the picture to a instead of . Try that, ok?

But what are you really switching with the triac? The 3V supply, or the output to from the inverter?

The inverter output. The triac should switch the AC on/off according to the voltage on the gate.

you have probably not got a common ground between the arduino and the triac.

True, but I didn't think it was necessary. All I read indicated that a triac should work. I'm new at the triac, but I looked at Speed Vest and the SparkFun El Escudo for background ideas.

If you connected without a resistor to limit a gate current, your triac is dead now

Nope, there's one there (680 ohm), which I hope shows in the circuit picture now. Is there any way to test the triac's vital signs?

My reading of the triac datasheet says that it should trigger at 2.2 volts DC on the gate.

Thanks.

.andy

True, but I didn't think it was necessary. All I read indicated that a triac should work.

Sorry wrong. It won't work because you have no path for the current to flow. You need to either connect one end of the triac to the arduino ground or better yet use an opto isolator.

See:- http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

My reading of the triac datasheet says that it should trigger at 2.2 volts DC on the gate.

It will but the voltage you are putting on the gate is just causing the whole circuit to float another 5V above where it is.

[head slap]

Ok, I see the problem, and pardon my ignorance here and praise your generous teaching spirit, but the MT1/MT2 side of the triac is switching AC current, not DC. Are you saying tie the Arduino ground to one side of the AC output of the inverter?

Yep... there is no point of reference for the gate pin... so you are having little effect on it from the Arduino. On top of that... you are providing little isolation from the high voltage (ok, I know it's low current) so you should think about what would happen in a failure condition. Hint: Triac fails and pops high voltage through gate to Arduino = bad.

Check this item below out for a clue on what your control circuit should look like. MOC301x chips are cheap and make your circuit safer.

http://computerchristmas.com/christmas/link-how_to/HowToId-7/How_to_build_a_triac_switching_unit_SSR

This circuit cannot work like this. The reference Potential from the arduino and the Triac is not the same.Use a MOC3022 for example (real Triac driver).

Then how do the SpeedVest and SparkFun board projects mentioned above work without opto-isolators?

SpeedVest schematic: here
SparkFun shield schematic: here

the voltage coming out of the inverter is floating against the ground. Maybe you should try to ground the - coming out of the inverter.

1st, I never said you HAD to have an opto-isolator... just that it's considered "safer:.

2nd, Sparkfun is great... but they are also known for being a bit minimalistic. In this circuit they DO have a GND common reference point between the drive pin and the triac... which is what were were saying was needed. Having a GND reference is also a good indication that HIGH VOLTAGE AC is not involved. Your circuit is not the same.

Why not just control the low voltage 3V side instead?

Thanks to all of you, I could now change the title of this thread to "triac triggering!"

It was, indeed, my faulty understanding of the ground and circuitry. With a common ground and the wires connected correctly from the EL wire to the triac, it is working as expected.

Grumpy_mike, chavotronic & pwillard, your comments about ground pointed me in the right direction.

pwillard, I understood that the opto-isolator was for a safer design. First, I just wanted to get the damn electrons flowing where they were supposed to go.

Also, not only was the SparkFun board made for a different purpose (which I knew, but was just using it to try to understand the principles), I found their schematic very hard to read. If that is not just my inexperience in circuit design and documentation, I would be interested to know why that is the case.

grazie a tutti!

.andy