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

Linear and non linear

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

Postby chrismb » Mon Feb 13, 2012 1:41 pm

Hi guys, you may know the issue here.

I'm piecing together a high current 'mid-voltage' DC power source and I acquired a site transformer for the purpose (230V 1 phase to 115V 2 phase) and turned it into an isolation transformer by disconnecting the output centre tap to earth so the output floats. I have used two 3300uF 450V caps in a doubling configuration to take the 115 out and turn it back into 300V of DC.

(I tried loading a pic at this point, but the site still has this 'jpg size' bug, then it crashed 3 times loading a gif.)

OK, so in addition I added a 3800W voltage dimmer-triac type, off of Chinese ebay, in one of the two outputs on the site transformer. This dimmer works great with ohmic loads but I did try to 'dim' the input to the transformer and it didn't like it much. So I set it up to 'dim' the output. All works fine.

Now that I am running these two big caps, there are 'fluctuations' at certain voltages. So if I run a load off of the caps (to keep the triac switching) then I can control voltage smoothly from 50V to ~120V (across both caps), then the voltage goes 'bouncy' and unstable between 120 and 200 V, then it smooths for a while before going into a 10Hz or so oscillation at 'fully open' dimmer setting.

If I run the transformer directly into the caps (no dimmer) then no problem. Similarly, without the caps the dimmer works fine. So it appears to be an interaction between the dimmer and the caps. I figure this; the dimmer needs an actual current across it to switch, but the caps (being so big) end up pulling in all the current they need to reach some given voltage and the dimmer effectively turns off, then the voltage drops a little and the dimmer starts up again, etc...

I'd have thought the inductance in the transformer secondary would've kept current flowing.

Any thoughts/solutions? This voltage just isn't stable enough, but I would like (/need) voltage control.
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby Jerry » Mon Feb 13, 2012 8:34 pm

Dimmers should never be used to run inductive loads like a transformer. You need to use something like a variac.

What do you need two phase for? it is exceedingly rare. Some old spindle motors used it and back in the early days of power distribution there was two phase 25hz power.
Last edited by Jerry on Mon Feb 13, 2012 8:36 pm, edited 1 time in total.
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby chrismb » Mon Feb 13, 2012 8:35 pm

That's what I found.

So how about feeding a bridge/cap? Why the oscillations?
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby chrismb » Mon Feb 13, 2012 8:38 pm

Jerry wrote:What do you need two phase for? it is exceedingly rare. Some old spindle motors used it and back in the early days of power distribution there was two phase 25hz power.


I don't. A site transformer puts out 2 phase. By having a centre tap to ground, you end up with two phases wrt ground. The idea is that no electrical part ends up more than 75V (peak) or so away from ground, but you get 150V peak between the phases to drive your cement mixer, or whatever.
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby Jerry » Mon Feb 13, 2012 8:40 pm

chrismb wrote:That's what I found.

So how about feeding a bridge/cap? Why the oscillations?


No, its just not going to work. Dimmers are not what you need, You will eventually blow the thing out.
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby chrismb » Mon Feb 13, 2012 9:04 pm

Seems to work fine, for the most part, just that the charge/discharge cycle of the cap causes some sort of feedback to the triac circuit and causes the voltage control to drift a little more than I would like.

Sorry, I don't understand what you mean that it is not going to work. What is likely to blow? The one thing that does concern me a little about the circuit is that when the triac switches on, it will do so at some given voltage so there is the possibility of an excess surge current to recharge the caps. But I don't see why the inductance of the secondary would not naturally limit that, such that it'd not be that much worse than when it is under a regular AC load causing a V-I phase shift.
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby Jerry » Mon Feb 13, 2012 9:22 pm

chrismb wrote:
Jerry wrote:What do you need two phase for? it is exceedingly rare. Some old spindle motors used it and back in the early days of power distribution there was two phase 25hz power.


I don't. A site transformer puts out 2 phase. By having a centre tap to ground, you end up with two phases wrt ground. The idea is that no electrical part ends up more than 75V (peak) or so away from ground, but you get 150V peak between the phases to drive your cement mixer, or whatever.


That is not two phase. That is center tapped single phase. Two phase power has the phases out by 90 degrees. You can do this with a Scott-T transformer from three phase.

You will blow the triacs. Trust me, they try this at work all the time, running transformers off triac based dimmers. One of two things happen. The transformer overheats or the triacs blow, usually shorted. It may take a while but it WILL happen. I don't know how many channels we have lost from people trying this.
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby johnf » Mon Feb 13, 2012 10:58 pm

Chris
Jerry has it almost right.Most dimmer modules are designed for resistive loads and will oscillate / blowup when confronted with reactive loads.
There are more complex Triac circuits that do work with transformers --BUT this also comes with caveats. The waveform becomes unbelievably distorted and can result in enormous crest facors that will strain the rectifier diodes and the ripple conditions in the caps.
Do some Web browsing on the subject.
From memory it all depends on which quadrants the triac is driven in. Resistive loads dont care but reactive do and the correct quadrant is not that easy to drive

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

Postby Doug Coulter » Mon Feb 13, 2012 11:50 pm

The leakage inductance (not the full winding inductance) is storing energy, so when the triac tries to turn off - whammo, kickback. It's a nasty little high Q tuned circuit of very low inductance (remember, the primary is seeing a very low impedance from the power company - and that's reflected through to the secondary + a tiny leakage inductance) - hang a scope on there, and also try across a small series R to see the current waveform - it's going to be very ugly. Any fast kick can turn the triac back on via stray capacity inside the triac, they have ratings for that sort of thing. But to make the long story shorter, yeah, it's not going to work right. While a dimmer can change the average voltage, the peak will still be the same if it turns on during the first half of a zero crossing - it will always have a peak of full line peak volts. If it turns on after the line peak for that half cycle - then the attempted risetime of the turn on is very fast and you have the surge current issue.

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

In other words, it ain't gonna work right into a rectifier and filter that's going to try to charge to the peak, regardless. In the case where you'd think it might work, that of turning on after the input voltage peak, that peak isn't there very long at all - it's turning on into a waveform that's declining fast, so you'll have nasty regulation even if it works like you'd think the theory would predict. You could in theory have an inductance inserted between the rectifier and the cap - but then it can and will kick back into the triac. You can't win this game. Just use a variac, or make a variable DC supply some other way. There's a real good reason no one does this in commercial product. Normally you just brute-rectify the mains and then build a switcher to run off that. Or nowadays, it's fancier to give load factor correction in all substantial supplies - by law here so you don't just load the mains waveform at the peaks only.

That book by Pressman John F recommended covers all this in great detail, theory and practice both, it's worth the price.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Instability between a 16A triac 'dimmer' and 3300uF caps

Postby chrismb » Tue Feb 14, 2012 3:31 am

Jerry wrote: Two phase power has the phases out by 90 degrees.


So, what is the phase angle for 4 phase power, then?
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