Instability between a 16A triac 'dimmer' and 3300uF caps.

Linear and non linear

Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby chrismb » Tue Feb 14, 2012 3:33 am

Jerry wrote: Trust me, they try this at work all the time, running transformers off triac based dimmers.


I'm not running the transformer off the triac. I'm asking about running a capacitor off of a triac-chopped sinusoid
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby chrismb » Tue Feb 14, 2012 3:39 am

Doug Coulter wrote:Of course, to see the current waveform, you might have to float your scope off ground or you'll fry it.


I've already explained that I removed the centre tap to ground. The outputs are now floating. I attached a 25 ohm load and a scope to the output and the waveform is perfectly as you would expect a chopped sinusoid to look, except for the fact that the average voltage floats around by 5% or so.

I ran the set-up for 4 hours yesterday at 100V (50V each) to make sure the electrolytics were OK as I have not used them for a few years.

I'll take pics and post the waveforms later to show y'all (if the software lets me!)
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby Jerry » Tue Feb 14, 2012 3:31 pm

No such thing as 4 phase power. Closest thing would be a unipolar stepper driver, but that is just pulsed DC.
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby chrismb » Tue Feb 14, 2012 3:50 pm

Doug, you'll have to fix that bug! I can't post resized images here, and if I don't resize them then they are too big.

Here are the traces, in a pdf..

dimmer_rippless.pdf
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby Doug Coulter » Tue Feb 14, 2012 4:42 pm

There's no bug I'm aware of. You use the upload attachment to upload an image (jpg's work best for a lot of things, gifs work well for b/w and text) - and there you go. Once it's uploaded (add the file button) you can then say "place inline" and that's where it goes. The board resized jpegs to a good display size, and if they're too big, you can click on them to go to a larger image view. Works for everyone else.

You can expect the volts to bounce all over, as this is a very terrible load for a triac/diac that works off a phase shift of the difference between its input and output terminals. Your cap waveform can look fine and that difference be absolutely crazy. As everyone else here and elsewhere says - don't do this, it won't work right, there is no simple fix - thousands of power supply designers, bordering on 100%, don't do this for good reasons even though it's super simple and super cheap. In other words, if something doesn't work for anyone else, don't complain it doesn't work for you either unless that motivates you to fix it yourself and enlighten us all on why we've all wasted millions if not billions of dollars world wide NOT doing this for power supply design - because it's too hard to make it work right.
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby chrismb » Tue Feb 14, 2012 4:53 pm

Doug Coulter wrote: don't complain it doesn't work for you either unless that motivates you to fix it yourself and enlighten us all on why we've all wasted millions if not billions of dollars world wide NOT doing this for power supply design - because it's too hard to make it work right.


No complaint being made, Doug. I didn't know it can't be done so was just asking if there was a solution. So, yeah, if you're all telling me there is no solution then it motivates me to fix the problem if it is a problem no-one else has resolved. The waveforms I've pinned up here look roughly OK to me, I'm sure I can find a way to smooth them out, yet further.
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby Doug Coulter » Tue Feb 14, 2012 6:28 pm

Remember, all the triac circuit sees is the difference signal between input and output - and you have to show us that (or see it yourself). Not a trivial thing to get a scope trace on. Typically, that difference is put through an RC network, which feeds the triac input (which is a series diac), and when the diac fires - delayed by the RC - that's when the triac turns on. Since that RC is integrating whatever shows up between the input and output terminals of the dimmer, any change in that waveform will change the on-time...so it's practically impossible to "regulate" that unless you come up with a from-scratch driver for the gate to main terminal signal (probably need to send pulses transformer coupled, off a separate AC reference with variable phase delay to get it constant).

Here's your pic. As you sent me the picture, your software had somehow destroyed the image sizing information in the jpeg header, or it didn't agree with the actual file data (you can't just change the header and not re-sample the image too) - it's not a bug here on the board, it's a bug with your photo editor or something. I simply opened it, then saved it in "the gimp", the linux tool I use, which figured that out and fixed the non-spec header in the image file. Most free jpg editing software ought to get that one right - but whatever you did, didn't.

ShortName.jpg
test upload
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby chrismb » Tue Feb 14, 2012 7:09 pm

Doug Coulter wrote:Since that RC is integrating whatever shows up between the input and output terminals of the dimmer, any change in that waveform will change the on-time...so it's practically impossible to "regulate" that unless you come up with a from-scratch driver for the gate to main terminal signal (probably need to send pulses transformer coupled, off a separate AC reference with variable phase delay to get it constant).


Well, what I was thinking was that a regular dimmer isn't 'doing the job' the right way around to feed a bridge/capacitor. I reckon it'd be better if it switched 'off' the AC cycle once it's put enough 'volts' onto the cap, rather than turn 'on' at some time-based point during the cycle. I don't know if that is still a 'dimmer' circuit, but I suspect that might work. I was also thinking that it might be better to run some sort of ramping circuit when it switches off too, a PWM-type ramp-down over a ms or so, rather than a hard switch-off. I guess there must be some 'simple' way to do that... I'll figure it... if I don't figure it then I won't have figured it :? ...

(No idea about the picture. Why does the website care whether the pic has a header or not - browsers can open the file without the header... The pic shows the dimmer control I am trying, and you can see it fitted into the top of the transformer.)
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby Doug Coulter » Tue Feb 14, 2012 7:22 pm

Jpeg files have a header in them that says how big they are, where each scan line begins and stuff like that. Some, more advanced software can eat non-spec stuff - linux stuff tends to be really good that way, but there is a standard, and that image didn't meet it.

You know you can't turn off a triac or SCR once it's on, any other way than reducing the current across it to zero for awhile. This will always happen sometime in an AC circuit with no rectifiers, so that's handled, kind of - so all they (can) control is when in each half-cycle it turns on. There are some tricks using resonant circuits to "commutate" SCR's when used in big inverters, again, in the Pressman book - but those things were very prone to failure - one failure to turn off with DC present and nothing stopped it but molten metal. I used to fix computers, and one of my customers was our FBI. One time, their huge UPS system which was a commutated SCR design blew - and so did every fuse in a quad processor IBM 390 and thousands of washing machine disk drives - while somewhere in there, the battery room / inverter melted down pretty badly and caught on fire. I helped the FBI guy scan the entire DC beltway area (this was a weekend) for all the AGC fuses of near the right size we could find at every Radio Shack, general store, hobbyist and so on over the weekend and was a hero for a day on that one.
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby chrismb » Tue Feb 14, 2012 7:27 pm

Doug Coulter wrote:You know you can't turn off a triac or SCR once it's on


I don't really know much about those. I was planning just to use FETs or IGBTs, with some sort of comparator/logic circuitry. Not planned it yet, but I've got a warm fuzzy feeling it's something I'm likely to be able to accomplish.
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