problem with triacs with neon transformer loads

[Mike Monett]

Thanks, Thomas, I stand corrected.

Thanks,
- Win

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.

http://powerelectronics.com/mag/404PET20.pdf

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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

 

[Ken Smith]

The problem with switching transformer primaries with triacs is
the high inrush current that can develop.

Yes, but not, I think for the reason you suggest.

The secondary capacitance of the transformer reflects back into the
primary. There is a primary leakage inductance that is intentionally
built into the transformer to limit the current secondary to under (IIRC)
6mA. Between these two effects, you end up with a funny ring when the
switch closes. It could be that something in this is getting his triac.

Also the OP didn't say anything about snubbing. The triac may be refiring
on the dv/dt when it switches off.

 

[Mike Monett]

[.. neon sign xformer ..]

The problem with switching transformer primaries with triacs is
the high inrush current that can develop.

Yes, but not, I think for the reason you suggest.

The secondary capacitance of the transformer reflects back into
the primary. There is a primary leakage inductance that is
intentionally built into the transformer to limit the current
secondary to under (IIRC) 6mA. Between these two effects, you end
up with a funny ring when the switch closes. It could be that
something in this is getting his triac.

Also the OP didn't say anything about snubbing. The triac may be
refiring on the dv/dt when it switches off.

Here's the original post:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Hi,

I am designed a commercial neon lamp two channel flasher and
connected to my hotels name board. The circuit is having two
parts, one is flashing circuit and the other is the output stage,
wher the phase line is switched. In the out put stage i have used
BTA 41 triacs for switching the neon transformer, ie the phase to
the transformer is connected through the triac. The gate signal is
controlled through one MOC3083 opto triac. the output from the
flashing circuit is connected to the opto triac's(MOC3083) input
(LED).

I have installed the flasher in my hotel's neon name board. there
is two transformers connected to one flasher channel. it was
working for the last 4 months. last week one channel was not
flashing. only one channel is flashing the other channel is in ON
state, ie not blinking.

The next when i checked the flasher in my lab both of the channels
was flashing. again i connected the flasher to the same neon
board. After two weeks agin the same problem occured, ie one
channel was not flashing. Can anbody know the reason for that. Is
that problem with Opto triac(MOC3083) or BTA 41. Is there any
possibility of humidity or temparature cause the problem?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

He has two triacs connected to one flasher channel. It worked for 4
months. Then one channel failed while the other continued working.
Both worked when he took it to the lab. Returned to service, same
problem after two weeks.

Do you think a ringing problem or dv/dt could give these symptoms?

Regards,

Mike Monett

 

[Phil Allison]

"Mike Monett"

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.

** Absolute nonsense.

The inrush surge is caused by *core saturation* and consequent temporary
LOSS of primary inductance.

The core saturates because of the DC component ( really a low frequency
transient) generated by switching a sine wave.

It
should be switched at the peak, or some other method should be used
to limit the inrush current.

** A rapid "fade up" using a triac with phase control is one way.

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?

** Where it exists, it just makes the worst case inrush surge larger.

The peak surge current approaches the zero inductance situation in many
cases.

So, I pk = V pk / R primary

Eg.

A 1 kVA transformer ( R primary =1 ohm) can be expected to exhibit 300
amp peak surges on a 230 volt supply.





.......... Phil

 

[Mike Monett]

Hi,
I am designed a commercial neon lamp two channel flasher and connected
to my hotels name board. The circuit is having two parts, one is
flashing circuit and the other is the output stage, wher the phase line
is switched. In the out put stage i have used BTA 41 triacs for
switching the neon transformer, ie the phase to the transformer is
connected through the triac. The gate signal is controlled through one
MOC3083 opto triac. the output from the flashing circuit is connected
to the opto triac's(MOC3083) input (LED).
I have installed the flasher in my hotel's neon name board. there is
two transformers connected to one flasher channel. it was working for
the last 4 months. last week one channel was not flashing. only one
channel is flashing the other channel is in ON state, ie not blinking.
The next when i checked the flasher in my lab both of the channels was
flashing. again i connected the flasher to the same neon board. After
two weeks agin the same problem occured, ie one channel was not
flashing. Can anbody know the reason for that. Is that problem with
Opto triac(MOC3083) or BTA 41. Is there any possibility of humidity or
temparature cause the problem

Thanks in advance

Raseel.M

Try swapping the connections to the triacs and see if the problem stays
with the MOC3083 driver or BTA41 triac.

Here are the datasheets for the triacs:

http://www.st.com/stonline/products/literature/ds/7469/bta40.pdf

http://www.ortodoxism.ro/datasheets/motorola/MOC3082.pdf

Did you know the optocoupler is a zero cross? Here's the description:

"The MOC3081, MOC3082 and MOC3083 devices consist of gallium arsenide
infrared emitting diodes optically coupled to monolithic silicon detectors
performing the function of Zero Voltage Crossing bilateral triac drivers."

In the datasheet, there is a statement near Figure 8:

"*For highly inductive loads (power factor < 0.5), change this value to 360
ohms."

but I can't see what it refers to. Maybe there is a way to make it switch
at the voltage peak for inductive loads. This might help reduce the inrush
current and reduce heating in the BTA41 triacs.

Regards,

Mike Monett

 

[[email protected]]

Hi,
I am designed a commercial neon lamp two channel flasher and connected
to my hotels name board. The circuit is having two parts, one is
flashing circuit and the other is the output stage, wher the phase line
is switched. In the out put stage i have used BTA 41 triacs for
switching the neon transformer, ie the phase to the transformer is
connected through the triac. The gate signal is controlled through one
MOC3083 opto triac. the output from the flashing circuit is connected
to the opto triac's(MOC3083) input (LED).
I have installed the flasher in my hotel's neon name board. there is
two transformers connected to one flasher channel. it was working for
the last 4 months. last week one channel was not flashing. only one
channel is flashing the other channel is in ON state, ie not blinking.
The next when i checked the flasher in my lab both of the channels was
flashing. again i connected the flasher to the same neon board. After
two weeks agin the same problem occured, ie one channel was not
flashing. Can anbody know the reason for that. Is that problem with
Opto triac(MOC3083) or BTA 41. Is there any possibility of humidity or
temparature cause the problem

Thanks in advance

Raseel.M

Firstly as has been mentioned switch on at the AC maximum to reduce the
inrush. Triacs are not good for this application since they go off at
the current zero crossing point which is when the voltage is near
maximum. This sudden application can cause the triac to conduct during
the current "off" time so when the current tries to reverse you get
conduction once again. Large snubbers can help out but its marginal at
best, this is why sometimes it works and sometimes doesnt. The solution
is to chuck out the triac and use two thyristors instead.

 

[Ken Smith]

[.. neon sign xformer ..]

The problem with switching transformer primaries with triacs is
the high inrush current that can develop.

Yes, but not, I think for the reason you suggest.

The secondary capacitance of the transformer reflects back into
the primary. There is a primary leakage inductance that is
intentionally built into the transformer to limit the current
secondary to under (IIRC) 6mA. Between these two effects, you end
up with a funny ring when the switch closes. It could be that
something in this is getting his triac.

Also the OP didn't say anything about snubbing. The triac may be
refiring on the dv/dt when it switches off.

Here's the original post:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Hi,

I am designed a commercial neon lamp two channel flasher and
connected to my hotels name board. The circuit is having two
parts, one is flashing circuit and the other is the output stage,
wher the phase line is switched. In the out put stage i have used
BTA 41 triacs for switching the neon transformer, ie the phase to
the transformer is connected through the triac. The gate signal is
controlled through one MOC3083 opto triac. the output from the
flashing circuit is connected to the opto triac's(MOC3083) input
(LED).

I have installed the flasher in my hotel's neon name board. there
is two transformers connected to one flasher channel. it was
working for the last 4 months. last week one channel was not
flashing. only one channel is flashing the other channel is in ON
state, ie not blinking.

The next when i checked the flasher in my lab both of the channels
was flashing. again i connected the flasher to the same neon
board. After two weeks agin the same problem occured, ie one
channel was not flashing. Can anbody know the reason for that. Is
that problem with Opto triac(MOC3083) or BTA 41. Is there any
possibility of humidity or temparature cause the problem?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

He has two triacs connected to one flasher channel. It worked for 4
months. Then one channel failed while the other continued working.
Both worked when he took it to the lab. Returned to service, same
problem after two weeks.

Do you think a ringing problem or dv/dt could give these symptoms?

Yes:

The dv/dt problem prevents the traic from ever turning the transformer
off. Every time it switches off, the dv/dt turns it right back on again.
There may be some smallish change in the output voltage but the light
would remain on.


The ringing can be passing a large current spike through the triac. The
triac is now, in fact, damaged. When you damage a BJT or a triac, often
the first sign is that the breakdown voltage is reduced. The effect is
often progressive. This could cause a breakdown firing problem at high
mains voltage.

 

[Mike Monett]

Yes:

The dv/dt problem prevents the traic from ever turning the transformer
off. Every time it switches off, the dv/dt turns it right back on
again. There may be some smallish change in the output voltage but
the light would remain on.

The ringing can be passing a large current spike through the triac.
The triac is now, in fact, damaged. When you damage a BJT or a triac,
often the first sign is that the breakdown voltage is reduced. The
effect is often progressive. This could cause a breakdown firing
problem at high mains voltage.

[email protected] forging knowledge

So why does one channel fail in the field after some period of time, work
in the lab, then work in the field but fail again after another delay? And
the other channel apparently works fine.

That's what I was referring to when I asked if you thought a ringing
problem or dv/dt could give those symptoms. Somehow it deoesn't seem
possible that a ringing or dv/dt problem could heal itself, then fail again
after a delay. Especially when you describe ringing as a progressive
reduction in breakdown voltage.

Regards,

Mike Monett

 

[Ton Visch]

Hi
Look for SGS-Thomson application note "Control by a triac for an inductiv
load"
You will find the solution in using an alternistor instead of a triac for
this purpose.
Schematics included alternistor = TPDV 640 .... 1240
Ton Visch

 

[Ken Smith]

So why does one channel fail in the field after some period of time, work
in the lab, then work in the field but fail again after another delay? And
the other channel apparently works fine.

That's what I was referring to when I asked if you thought a ringing
problem or dv/dt could give those symptoms. Somehow it deoesn't seem
possible that a ringing or dv/dt problem could heal itself, then fail again
after a delay. Especially when you describe ringing as a progressive
reduction in breakdown voltage.

The OP hasn't stated whether it starts working again if he stands on his
left foot or the neighbor turns on his toaster. I assume the circuit is
right on the edge of the working/not working bondary.

 

[Mike Monett]

The OP hasn't stated whether it starts working again if he stands on his
left foot or the neighbor turns on his toaster. I assume the circuit is
right on the edge of the working/not working bondary.

[email protected] forging knowledge

Then any degredation should make it fail permanently.

Anyway, Fred thinks the post is a hoax.

Regards,

Mike Monett