Leaky triacs question

1

1pet2_9

Active member
I'm confused about something: on my Renards (even SS24's), I have never had a problem with small lights never completely shutting off. My DMX dimmers have that problem all the time: if I stick a low-load LED on my high-end DMX dimmers, they never completely shut off. I have to add a dummy load to that channel (which is not uncommon). But I have never had that problem with a Renard. Why not? Aren't dimmable triacs supposed to be leaky?

Are the triacs on Renards just lower current (2 amps), as opposed to DMX dimmers (5 amps)--which therefore means the minimum load required on the Renard is much lower? Or maybe the Renard boards are shunted? I don't know. I'm curious if maybe some Renards in fact do have a minimum load requirement.
 
What p/n on your Triax? I had to replace some when moving from incans to LEDs.

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Now that it's daytime and I can spell triacs:

I swapped out all the the BTA04-700T triacs with a BTA06 or BTA08 suitable sub. Similar thing was happening on the Renards... Some random channel was flickering or staying on when another channel on that board was dimming. Didn't happen with the higher load incans; just the lower load LEDs.


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I don't know the triac P/N's, because I bought all my Renards used and those labels are worn off. But if it's the same and/or related issue to what I've seen on the DMX dimmers, the workaround is to mix in a small load of incan. They have "minimum load" requirements in the manual. The LED has a threshold voltage, below which it will not conduct (or bleed) current; and once it does, it emits photons. But incans behave more linearly like a resistor. They'll bleed current, but if they don't sink enough they won't get hot enough to glow. Which, as a dummy load, is desired.

I bought an incan night light at Home Depot for $.76, which is 7W. It immediately fixes the problem on my dimmers. From what you're saying, it sounds like yes, maybe the Renards do have the same problem.

I think people might be afraid of mixing incan and LED on the same channel, afraid that that might do bad things. But at least for this problem, it's actually quite lovely.
 
Sounds like a match for the symptoms and what people were doing to fix them before discovering how to fix the cause.

To be clear: replacing the Triacs worked for me. No dummy loads required anymore.

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A long time ago the triacs on the SSnn boards were changed from ones with more sensitive gates to less sensitive gates. The problem that this addresses was one where channels would flash on when neighboring channels were lit (by neighboring I mean channels with power cords running in close proximity). In addition to turning on when a gate voltage is applied, most triacs can turn on when sufficiently fast voltage transients (look up dV/dt ratings in the datasheets) from external sources are present across their output. Sensitive-gate triacs are more susceptible to this, which is why they were switched out. At the time I think that the aforementioned transients were induced from adjacent AC lines powering other lights when those other lights were being switched on and off. However, I never had appropriate equipment for observing those waveforms in sufficient detail to verify this, so my opinion is based mostly on triac datasheets and application notes.

That, of course, is a different problem from what 1pet observed, but may be present for similar reasons. It's possible that the 'fix' for spurious light flashing might be what makes the difference between the Renard and COTS DMX controllers, but I don't have a definitive explanation.
 
I didn't write that your boards had the dv/dt problem (or at least I didn't intend to write that), I wrote that the spurious flashing of LED strings seems to have been caused by that. The source of the problem that you're seeing is still up in the air.
 
I'm saying that I'm actually with you, that the dv/dt problem is real. I have seen that in a third set of boards: the cheap China DMX dimmers/triacs.

I'll keep an eye out for these possible problems in the Renards (I'm actually confused why I DON'T see a problem in the Renards, when I'm seeing behaviors in all my other dimmers with triacs BUT the Renards). I have a theory that mixing in a small load of incans will ameliorate the problem (the idea being that noisy currents will get eaten up by the incan before the voltage gets high enough to trigger the led's or neighboring TTL logic). But without experiencing the problem, I can't know. I opened the thread to find out if maybe the Renards are doing something right that $300 professional dimmers don't.
 
1pet2_9 said:
I'm saying that I'm actually with you, that the dv/dt problem is real. I have seen that in a third set of boards: the cheap China DMX dimmers/triacs.

I'll keep an eye out for these possible problems in the Renards (I'm actually confused why I DON'T see a problem in the Renards, when I'm seeing behaviors in all my other dimmers with triacs BUT the Renards). I have a theory that mixing in a small load of incans will ameliorate the problem (the idea being that noisy currents will get eaten up by the incan before the voltage gets high enough to trigger the led's or neighboring TTL logic). But without experiencing the problem, I can't know. I opened the thread to find out if maybe the Renards are doing something right that $300 professional dimmers don't.
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It used to be that some people added a snubber circuit or a 4w incan to the output to help make them stable. In my signing faces I find the leakage is related to humidity. On wet nights I get lots of flickering and dry cold nights I see none.

Martin
 
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