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I'm pretty new to electronics, and I'm making a circuit to control EL wire with a DC logic control signal. Specifically, I would like to blink the EL wire with the output of a 555 timer.

My first pass at this was with an optoisolator. MOC3031 data sheet

The circuit looked like this:

circuit

(C1 is a stand-in for the EL wire, which I'm told is a capacitive element, and the 555 outputs 5V)

The good news is that if I turn the EL wire inverter on when the 555 outputs 0V, the EL wire will be off until the 555 jumps back to 5V. The bad news is that the EL wire never shuts back off, even when the 555 outputs 0V again (I've confirmed that the 555 circuit works w/ a multimeter).

I ripped out the optoisolator and replaced it with a triac, so now it looks like:

enter image description here

And everything works perfectly. I'm happy in the sense that I have a working circuit, but I'd like to know what changed. Can anyone explain why?

EDIT:

I want to switch some of the EL elements on and off while the inverter remains switched on and drives other EL elements continuously. The frequency is likely higher than 50-60 Hz and the voltage is probably not a clean sine wave.

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  • \$\begingroup\$ Is the EL-inverter you speak of normally powered by batteries or a DC adapter? EL-wire or plates are capacitive, a DC fed inverter is not. \$\endgroup\$ – Asmyldof Jul 14 '15 at 18:31
  • \$\begingroup\$ the EL inverter is powered by a separate set of 8 AA batteries and plugs into J1. \$\endgroup\$ – user358829 Jul 14 '15 at 19:26
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Something is being missed here. The optotriac is quite capable of switching that load current, however the triac in the optoisolator is handling the inverter's high frequency AC differently from your 'triac'. It's designed to control mains frequency AC - 50 or 60Hz, and is too slow to turn off at the very brief zero crossings when it is fed with ~1kHz AC current

Edit: there is a good app note from Vishay here which makes it a bit more clear. You cannot just use the static dv/dt spec - that is what prevents the opto from turning on when it is off, rather you need the (lower) commutating dv/dt, which may not be given.

In fact, perhaps it's not actually a 'triac' but an alternistor that you're using, which would tend to perform better because the semiconductor structure is different.

The answer in that case is to use a more appropriate switching element, such as a OptoMOS SSR.

enter image description here

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  • \$\begingroup\$ aha! I took a look at the datasheet (fairchildsemi.com/datasheets/MO/MOC3041M.pdf) and I can't find a reference to this anywhere. for the future, how would I have known that it's too slow? \$\endgroup\$ – user358829 Jul 14 '15 at 21:11
  • \$\begingroup\$ @user358829 The laziness of triaccy devices when it comes to turn off is not always mentioned in datasheets. Many similar elements have a "reverse recovery time", much like diodes. So it can be easily forgiven you didn't know. In fact the AC frequency seen by the triac on the HV side of the EL may look even higher because it might be more blocky than sinusoidal, and then the zero crossing is really, really quick. \$\endgroup\$ – Asmyldof Jul 14 '15 at 23:11
  • \$\begingroup\$ It's not in the data sheet because they did not anticipate your application. I don't know that there is much of a lesson for the future here except that you have to be careful when using devices outside their normal operating conditions. As &Asmyldof says not only is the frequency high but the dv/dt at the zero crossings is probably very hight. \$\endgroup\$ – Spehro Pefhany Jul 14 '15 at 23:43
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Based on your response to my question the answer is relatively simple.

A DC powered device draws DC current. A triac or triac-opto coupler stays on as long as the current through it is above its minimum holding-current.

Holding current is just "stay-on current". Once it's triggered as long as the current is larger than the minimum, it will stay on. This is why they are only used for AC voltages or at least some other voltage that goes to 0 regularly, to make them switch off.

But,... why does the Triac actually switch off?

Simple: The Triac is meant for a higher current, as such its holding current is higher. One of those MOCs can only handle about 100mA depending on type, so their holding current is probably in the single mA's (I'm too lazy to Google a datasheet now).

Your triac might be made for a current of 10A or such (you don't tell us), at which point its holding current can easily be up to 50mA.

If you EL inverter then uses 25mA DC, the MOC will not turn off, because that's much more than its holding current. The Triac will turn off, because you aren't at the holding current yet.

In general you want to not use Triacs and such for low voltage DC, because they also waste a lot of energy: They always have a comparatively large voltage drop, even when on.

You should probably just switch to a transistor or MOSFET:

schematic

simulate this circuit – Schematic created using CircuitLab

Or if you want opto-isolation:

schematic

simulate this circuit

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  • 1
    \$\begingroup\$ just to clarify -- J1 is the output of the inverter, so won't that be an AC signal? if I put a multimeter across the leads of J1, I get a 110V AC voltage. I used the triac because I want to shut that AC signal on and off (I can't shut down the DC input to the inverter because some of the EL wire will be always glowing and some will be blinking, and both strands are fed from the same inverter) \$\endgroup\$ – user358829 Jul 14 '15 at 19:53
  • \$\begingroup\$ @user358829 Holding current of MOC3031 is 400 uA typical fwiw. \$\endgroup\$ – Russell McMahon Jul 14 '15 at 20:39
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    \$\begingroup\$ @user358829 I misunderstood. Spehro gave you, in this case, the more likely answer. I'll leave this for posterity, in case someone else needs it. \$\endgroup\$ – Asmyldof Jul 14 '15 at 23:06
  • \$\begingroup\$ no worries! I appreciate the help =) \$\endgroup\$ – user358829 Jul 15 '15 at 18:43
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The datasheet for the MOC3031-M specifies that it's meant to be a trigger and not directly drive the load. As a result, I would imagine that the internal phototriac is much more sensitive than the eternal triac depicted in your second circuit and is probably why the phototriac was never turning off and your EL wire was always on. Instead, you should drive an external triac using the optocoupler to switch the load. See, for example, this circuit from the datasheet:

enter image description here

If you put the optoisolator in-between your 555 timer and the triac from your second figure, you should get the desired result.

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