problem with triacs with neon transformer loads

The problem with switching transformer primaries with triacs is the high inrush current that can develop. For example, if the transformer magnetizing flux is strongly of one polarity when it's turned off, and the ac line is pushing toward the peak of the other polarity when the triac is turned on, the transformer core will saturate and the primary will look like a section of thick copper wire across the AC line. Very high currents will flow for a portion of an ac cycle. Your triac is designed to handle a little of this, but it's highly stressful and should be minimized.

There are several solutions, including zero-crossing switching, or a two-stage switch with a series resistor in the first path, or both.

--
 Thanks,
    - Win

HI Win, Thanks for ur replay. Here i will explain you more about the problem. The triac is capable of handling max 40A current. I have used the zero crossing triggering with the help of MOC 3083. Actually the problem not causing to damage of triacs. The same triacs i have used again. The neon transformer has one property such that it wont saturate the core. the windings is done in such a manner. Then wat will be the problem

HI Win, Thanks for ur replay. Here i will explain you more about the problem. The triac is capable of handling max 40A current. I have used the zero crossing triggering with the help of MOC 3083. Actually the problem not causing to damage of triacs. The same triacs i have used again. The neon transformer has one property such that it wont saturate the core. the windings is done in such a manner. Then wat will be the problem

Haha- write more- Genome is having too much fun here-)

Since your load(s) are inductive, you should use a snubber across the TRIAC.

--
John Fields
Professional Circuit Designer

Have you made any actual measurements? It could be the MOC3083, its drive, or the BTA41 that's the problem. The actual schematic would also be of help.

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It always works when returned to the lab?

At the installed location try removing all power for a few minutes, then turn it back on and see if flashing resumes.

You probably have some kind of "hang" mode in your flasher circuitry.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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[snip]

It is 'maximum voltage switching' that should be used in this case, isn't it? Switching on at zero crossing will start with a demagnetized core handing the full voltage * time integral, where in operation it would start out with the opposite magnetization.

Thomas

That's what I thought. But in any case, I don't believe his problem is a matter of destroying the triacs with excessive current since the thing works after the problme occurs when taken back to the lab or whatever. Possibly something in the driver circuit that's not turning completely off.

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In the early '80's, when I did disco stuff, I controlled neon transformers by feeding them _complete_ cycles only (using TRIAC's).

I found, from experiment, that partial cycles or an odd number of half cycles would ultimately smoke the transformer.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

"Jim Thompson"

** An automatic result of the large DC component inherent in half cycles or 1.5 cycles or 2.5 cycles etc.

But to minimise current surges - use whole numbers of cycles, commencing at a peak.

Least DC component x time.

Do the math.

...... Phil

Zak wrote in news:446f2627$0$8465$ snipped-for-privacy@news.tweakdsl.nl:

There are a number of descriptions on the web, some worse than others. Here's one where the scope figures bear no resemblance to the description:

All the photos show switching at or near the peak of the 120V waveform. The B graph is 50 Amp per division. It shows about 40 Amp peak current, the same value as in the A graph. The text claims the peak is 200 Amps.

Here's one that seems to do a good job simulating the result of the transformer inductive load while flashing neon signs. It shows the inrush current at various points on the AC waveform, and the effect of magnetic remanence. Unfortunately, a poor choice of colors for the traces makes them hard to read.

The article recommends a cooling period between power cycles to prevent destroying the transformer. It also recommends leaving the primary turned on for at least 30 cycles. This is somewhat different from Jim's recommendation to only use complete cycles:

Regards,

Mike Monett

Sam Goldwasser wrote in news: snipped-for-privacy@saul.cis.upenn.edu:

Maybe a marginal design where the sign is in direct sunlight and the extra heating in the triac increases the leakage so it won't turn off?

If so, bringing it back to the lab removes the sunlight and it now works.

One way to reduce the inrush current is to add a NTC resistor in series with the AC line. This is used in ordinary PC power supplies, where the inrush current charging the caps could be 50 times the normal operating current.

Here's an article that discusses different methods of limiting inrush current. It is slanted towards selling their new product, so some of the numbers are skewed. But otherwise not a bad overview.

Regards,

Mike Monett

Sure, it's possible. That's why I asked in a previous post for him to provide the schematic. Could be that the traics are very sensitive and there's enough leakage in the driver under some conditions.

How well would this hold up to continuous cycling?

Sure, but inrush current may not have anything to do with his problem.

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As the article describes, you need to allow enough time for the NTC to cool before starting another cycle. It also recommends waiting for the transformer to cool, so this may solve both problems.

Maybe it adds enough heating to the triac to make it fail in the application. Unfortunately, these are all just guesses as the OP never replied to your request for schematics. Regards,

Mike Monett

[...]

Good question. I did some followup and found some good NTC design articles on the web. This gives the design process to select the NTC resistor:

This shows the process for transformers:

The problem is cooldown of the NTC resistor. Most sites recommend 60 seconds, which may be too long for a flashing sign. One answer may be to bring the current up in steps. The following describes the process:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ There is a design technique engineers can employ to eliminate the problems posed by the cool-down/recovery time required for Surge Limiters to return to their initial level of resistance. Essentially, this involves designing inrush current protection to drop the Surge Limiters out of the circuit after they have performed their function. By removing them from the circuit once the initial surge has passed, the thermistors have an opportunity to cool down, so they are ready to respond to a subsequent surge after a power drop-out occurs.

This technique requires the addition of either a relay or a triac in parallel with the Surge Limiters, plus the circuits necessary to control it. All the components of the protection circuit would be in series with the input to the line. Once the inrush current has been absorbed by the thermistor, then either the triac starts firing or the relay closes. The simplest method of powering these components is from the power supply itself. Once the power supply starts, it closes the relay or fires the triac, dropping the thermistor out of the circuit and allowing it to cool down and regain its initial resistance, so it's ready to provide inrush current protection.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A simpler approach might be to use fixed resistors instead of NTC. This would eliminate any problems with ambient temperature and allow increasing the cycle time to a rate suitable for flashing signs.

Regards,

Mike Monett

Thanks, Thomas, I stand corrected.

--
 Thanks,
    - Win

"Mike Monett"

** The first pdf article seems to date from the late 60s or mid 70s.

The second is both desperate and dateless.

Nevermind.

The factual content is timeless anyhow.

....... Phil

OK, it's clear due to the phase lag of an inductor, switching an inductive load at a zero-cross will give high inrush current. It should be switched at the peak, or some other method should be used to limit the inrush current.

But I was still having problems understanding the inrush current due to remanence. Modern transformers use low-remanence steel, so why should there be a problem?

I finally found an article that offers an explanation. It shows the inrush current superimposed on the BH curve for two values of remanence, and describes the efforts needed to find a better low-cost steel:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Today's grain-oriented steels work with a fraction of the losses, at higher permeability, and they can be driven harder at elevated induction levels. But there are consequences. The magnetizing curve is no longer soft and round, because it has turned square and hard. However coincidental, the combination of high permeability and square loop comprise a major component of the formula for inrush.

Improved steels have enabled smaller, lighter and less costly transformers. Yet, those same improvements have created a generation of transformers that draw immense amounts of current at start up. Although it probably was not a problem in 1954, inrush current is definitely a problem today - one that concerns every primary circuit designer.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This makes sense. I just salvaged the transformers and magnetrons from a dozen old microwave ovens for a high power heating experiment. The transformers in the oldest ovens were huge and heavy monsters, but the later versions were much smaller.

Regards,

Mike Monett