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Bringing V and I in phase

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hhhsssmmm

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hello

I have a problem with an inductive load (230/12 transformer) running via TRIAC BTB16600BW. The TRIAC is controlled via PIC MCU optically coupled with MOC3020. Even though the TRIAC is a 'snubberless' version from ST...i have still added a snubber network across the TRIAC anodes. The TRIAC is controlling the phase angle on the transformer HT (input side) and the transformer LT side is powering a 12V halogen lamp.

The above setup described is actually a PIC dimmer and works well if an electronic transformer is used instead....but i need to use simple transformer.

The problem happening is that with the SIMPLE transformer....it is heating up very quickly even though the rated VA of the transformer is powerful enough and my halogen lamp is only 25W. Of course this is happening due to the inductive load nature and V and I are not in phase. In order to bring the V & I in phase i connected accross a 100W/230V normal light bulb accross the transformer HT. This has fully solved the problem and now every thing is running smoothly with no over heating anywhere in the network circuit.

However having a 100W bulb ballast is not practically feasible for long term...So please can someone advice on a better alternative to correct the V and I phase out problem? I hav also tried power resistor to simulate the same 100W bulb ballast but it over heats! I hav also tried to remove the 'snubber' network, but that made no difference.

Please can someone help?


hhssmm
 

tcmtech

Banned
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Post a schematic!
Are you using an optical coupled triac driver to trigger the triac itself?
 

hhhsssmmm

New Member
Hello

here it is.....my diagram web link for the actual circuit arrangement......

Microchip


...and YES im using non zero opto TRIAC coupler...MOC3020.


So please can you help?

hhhsssmmm
 

tcmtech

Banned
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The connection going from the 100 ohm resistor to MT2 needs to go to the top of the inductor or transformer as you have it shown.

That way the triggering voltage is not affected by the inductance.

the 220 ohm resistor will need to be changed out to one that will limit the gate current to what ever the triac requires though.
 
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hhhsssmmm

New Member
hi

thanks so much for replying.

i hav tried exactly as u mentioned....but no success.

the 220 Ohm resistor simply over heats too much and burns out....i hav tried other higher power resistors but the current is too much and hence the excess heat dissipation leads to total destruction of the 220ohm resistor....and other resistors.

the only best way so far as been the same approach....that is the addition of the 100W bulb accross the inductive load just as per the diagram i posted....which for longer term is not feasible.

can u please suggest something else?.......is it possible to dim inductive load with phase angle changing method?

thankyou

hhhssmm
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

Did you try using the triac on the SECONDARY side instead of the primary side?
I assume you have a light bulb connected to the secondary side right?

Inductive loads are known to be problematic when used with triacs as control
element.

Also, just curious, do you have some way of sync'ing your control pulse to the
line frequency?
 
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hhhsssmmm

New Member
Hi

My design for this project will be an enclosure box which will hav 2x 230V output dimming channels to connect either a normal ligth bulb or an electronic transformer or a simple wire wound transformer. Both transformers are used for the 12V AC halogen lamps.

Now this dimming project works excellently with normal light bulb and with electronic transformer........the challenge remains with wire wound transformer.....thats where the trouble is.....V & I are not in phase due to the highly inductive load resulting from the wire wound transformer.

I can not connect anything on the secondry due to what i hav just above explained and plus im making this for a customer who will be connecting both wire wound and electronic transformer etc.....so the project has to be universal...

i hav seen lots of ideas on the internet and all hav the same design....for inductive load they simple involve the use of a snubber circuit and which i hav already included......

so its confusing to me and a mystry that this type of challange is nothing new and can be achieved....but is not working for me.....therefore i need expert opinion in order to get this project running smoothly with wire wound transformer...or basically a highly inductive load dimming.

Please can you help?




NOTE: Sync is achieved via zero crossing



hhhsssmm
 
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MrAl

Well-Known Member
Most Helpful Member
Hi again,

Ok, i just wanted to get an idea what you are doing here.

I took a look at the schematic you linked to, and i believe tcmtech was on the
right track. The only problem with that exact mod is that the resistor overheats as
you said but even more serious is that the triac gate gets overpowered.
One of the goals in triac circuits is to minimize gate power, and that isnt
going to do it. That idea however can be implemented in a slightly different way
though as i will explain.
It looks like the problem right now is that although the gate appears to be
'triggered' by the PIC output (via the opto) it is not actually synced with that
pulse, but with that pulse AND the voltage at MT2. The voltage at MT2 is
of course dependent on the inductor voltage and that is not good.
The idea then is to develop a new trigger signal, but one that is dependent
solely on the PIC output pulse, and virtually nothing else.

The main way to do this is to create a DC power supply that is common to MT1,
and use that with say a normal opto (transistor output) to trigger the triac.
Of course to test this circuit first, you can use a battery as the dc supply.
Keep in mind however that when you use a dc supply you can not still use the
triac opto, it has to be a transistor type.

What i would do is use a battery, use a transistor opto, and test the circuit
again and see how it works out. I would bet it would be better.
One thing though, you may hear the transformer hum a bit more because it
will be being banged by higher than usual harmonics.

I hope you can try this soon, and let us know of the outcome.

I also have to ask:
Does the PIC circuit really have to be electrically isolated from the
power line? Many circuits can run with a common line from the
PIC ground (or +supply) to the line power neutral.
 
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tcmtech

Banned
Most Helpful Member
you need to change the 220 ohm resistor to one with much higher resistance for 230 volt operation!
I will dig through my tech info files and find the schematic from a welder. Many welders use a similar circuit to what you have in order to get infinitely variable voltage with voltage regulation from simple triac or back to back parallel SCR's.
 

MrAl

Well-Known Member
Most Helpful Member
Hi tcmtech,

Then how does the triac turn on near zero crossing?
That's why i suggested a different power supply for the control triggering
in my previous post.
 

tcmtech

Banned
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If I recall it correctly the control circuits on the welders are synchronized with the mains from the small control circuit transformer. That gives a zero crossing reference point.
After that its just a matter of waiting so many milliseconds before activating the Opto coupler to fire the triac. For inductive voltage control you trigger in reverse starting from the back end of the sine wave and moving forward in order to get a solid and consistent adjustable voltage.
Long time delay gives small voltage short time delay gives you more voltage.
 

MrAl

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Most Helpful Member
Hi again,

Well, what i was thinking was maybe using a diode as rectifier from the mains
(half wave rectification) with common being neutral and cap to filter. A
transistor output opto would then be able to perfectly control the triac
without any worry about the varying voltage.
I was assuming that he already had the PIC synced to the line too, which
i think he does, so that takes care of that part.
Maybe throw in a zener to keep the dc voltage suitable for the
opto.
 
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hhhsssmmm

New Member
Hello people.

Thank you all so much for your kindest consideration. Both of you are making good sense and after googling for many days I have just discovered the perfect solution in getting this challange out of the way. Please see the enclosed PDF attachment. It is a TOUCH CONTROL HALOGEN LAMP DIMMER from LSI Computer Systems. This is a US company and im in UK. Unfortunately their distributor in the UK does not hav the respective chip and nor does farnell or RS etc.....regardless though i must instead use my PIC controller.

This application example demonstartes a typical wire wound transformer fed halogen lamp dimmer controlled from a MCU powered off from the mains...no isolation transfromer here!

On the last page 4....a Typical Halogen Lamp Dimmer powering a wire wound transformer is shown with all the necessary components to go with the 230V AC supply. You will be surprised to know that i will not be using any of the input methods as described in the schematic such as touch or electronic input etc to vary the dimming and instead have a wireless receiver feeding manchester encoded signals to the MCU. This is currently how I have the project successfully running dimming either an electonic transformer or an incandescent light bulb....basically a wireless dimmer!

From the shcematic i analysed that the TRIG pin 8 is being latched HIGH/LOW based on a dimming variable in the MCU software that is adjusted by the various input control methods as shown in the shcematic. All i have to do is to replace this MCU with my PIC and do the same thing what im doing now with the wireless input.

Essentially, the only difference this circuit diagram has with my current project is that the PIC will now be powered from mains supply - ZENER 5V direct...and no step down transformer with 7805 regulator. This will solve the TRIAC and PIC triggering syncronization problem as duely highlighted by "MrAl"...since now there will be no opto in between.

Please post your comments and suggestions on how to proceed from here and if this is the right approach and should I go ahead and start building the circuit as per the new PDF schematic? Also should I be aware of any hazords or dangers etc or of any false indications in the results i achieve?....you might wish to kindly highlight them please.

Thank you
hhhsssmmm
 

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MrAl

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Most Helpful Member
Hi again,

The only part of that circuit in figure 5 that i would question is the part where
they develop the low voltage dc to drive the chip. These are parts:
R1,C2,D1,Z,C4.
This is almost like i pictured it when i replied earlier, except that with the way
they are doing it i have to wonder if enough voltage would be available when
the conduction angle is near or equal to 180 degrees (fully 'on').
It looks like (although a measurement would be in order here) it could be possible
that the voltage goes too low to operate the circuit correctly.

A simple solution would be to resize R1 and C2 and run R1 directly to the 'N'
external terminal (Neutral). That would ensure that there will always be about
5vdc for correct operation.

Please let us know how this works out once you get it tested...thanks.
 

hhhsssmmm

New Member
Hi

Just wanted to discuss another method before i go on with hardware mods....

Currently my PIC MCU gets interrupted @ every 10ms (zero crossing) with normal non-inductive load. However when I connect the transoformer inductive load then I do not get interrupts at every 10ms since the zero crossing is now occuring at every 5ms due to the V & I not being in phase....(by 90 degree I lags V)

Please see the enclosed attachment.

So inorder to make the firing of the TRIAC symmetric during both halves of the AC wave.....i have come up with a software fix for my PIC MCU which I will now describe as below.

As per the enclosed attachment, lets suppose the first time the PIC is interrupted at zero crossing at -ve 180 degree. I then shut off the zero crossing interrupt. I fire the TRIAC as normal and let the TRIAC conduction continue to my desired level. Then at point 'A' which is approximately at 6ms I then turn back on the zero crossing interrupt. Hence the next zero crossing will be occuring at 0 degrees which is exactly after 10ms. The same logic then repeats over. By this method I predict that the TRIAC voltage wave form will be symmetical as shown in my diagram. This is how i intend to over come my problem of V & I not being in phase in an inductive load.

Please comment/suggest or modify or correct my logic if im wrong.

Thank you
hhssmm
 

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tcmtech

Banned
Most Helpful Member
As I stated before you go back wards with triac or SCR switch's on AC line loads.
You turn on the triac starting at the end of the wave not the beginning.

For low voltage output you only use a small part of the trailing end of the wave. For more voltage you move the triggering point further forward on the wave.
When you trigger the triac at the very front of the wave you will get the full voltage.

Its how its done on many welders and works very well. Even with higher voltages and tens of KW's of inductive power.

Flip your picture around from front to back. (mirror image) Thats what wave form your looking to get.
 

hhhsssmmm

New Member
hello

thanks for your reply.

However please excuse me in the sense that im a bit confused at your thought of starting from the front or back of the AC wave form:confused:

I mean my PIC will simply be INT zero cross at every 10ms regardless which direction the waveform is flowing. Then how can I even know what and when is front and when is back in the AC waveform?

Usually in the time domain axis (x-axis), the AC wave is flowing from -ve infinity to +ve infinity. Therefore it can be at any of these instances and I can never know which side to follow...all I can detect are the equally spaced 10ms interval zero crossing interrupts.

I horizontally flipped my jpeg image of the AC waveform for inductive load. Of course then it appears that the current leads the voltage or V lags I. However due to the above reasoning, i still could not understand you.

Please can you explain your concept further with some example.

Thank you

HHssmm
 

tcmtech

Banned
Most Helpful Member
Say you had a half wave that was 100 counts long.
When your sine wave crosses the zero point(count 100) you reset your counter back to zero. The next time the wave crosses the zero point(count 100 again) for the other half of the cycle you reset your counter back to zero again.

If you trigger your triac to wards the end of the count cycle, around 90 counts, you will only get a fraction of the wave form energy then (10% duty cycle). A triac has to have the voltage reverse in order to turn off. That reversal is the next zero point crossing, or the count 100 reset point.

By triggering at the end of the wave form there is only a short burst of energy transfered. As you trigger closer the front of the wave form (less off time, sooner trigger point) you will get more energy transfered. (greater duty cycle)

Think of it as crude PWM using a sine wave. But instead of triggering at the front of the timing cycle(count 1) you count going from the back end and move forward to make a longer pulse width. turning on at count 20 is a longer pulse cycle(80% duty cycle) than turning on at count 50 (50% duty cycle).

Your triac turn off point is fixed by the next zero crossing point. So to get longer pulses you need to move further forward on the wave when triggering the triac.(smaller count numbers)
Hence the moving from back to front for longer pulse widths.
 

mneary

New Member
You should find a lower current triac. You need one with a holding current which is less than 10 mA in all 4 quadrants; which probably means a 4A triac. Where are you located? Where do you shop?

I have successfully used ballast lamps as little as 7W to bring V & I into acceptable range! If this could fit into your design, it could be worth a try. Remember, the ballast is needed principally when the transformer is unloaded.

A second method that I would try is a snubber across the load. Put 220 ohm flameproof resistor in series with a 0.1 uF (100 nF) mains-rated capacitor across the load.

Either of these should help provide starting and holding current. But this is fruitless if the triac needs too much holding current such as the 16A the schematic calls for. If you're actually using this one, it has a holding current of 50 mA, which means that it will release very early in the cycle when the transformer is lightly loaded. This triac or similar ones will always work like this.
 
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hhhsssmmm

New Member
Hello tcmtech

thanks for your reply.

Your making much more sense now...thank you;)

But there is just one more thing. What im about to ask has nothing to do with programming concept being used or how to increment a variable in the software etc......that I know......the question now being asked is from the software algorithm point of view.

Thus my question is....from first zero cross INT and until the next zero cross INT, which is 5ms delay (half wave @ 50Hz),....how will i count to 100 or any number when im only getting zero cross INT? At the moment I can not understand how will the count be incremented when i hav no external tick between each zero cross INT.....do you understand my point? I mean i can start/reset a counter at zero cross INT and then keep incrementing it my self after a certain interval.....or I can use a timer being set to INT between 1 to 5ms. The desired timer INT setting will then produce the desired dimming intensity since i will fire the TRIAC only after the timer INT. This timer will be start/reset in the zero cross Interrupt Service Routine. Is this how you mean?

Currently with the perfectly working hardware and software configuration that i have (for non inductive load).....im starting/reseting a timer always set to a fixed value of 100us at every zero cross INT. So the TRIAC is firing just after (or 100us later) from zero cross INT.

Do you think with all this discussion going on with you, that my solution to inductive load dimming is clearly related to a software tweak? I have seen many inductive load hardware design dimming concepts which are exctly how I have described it in the begining of this thread (except for the light bulb ballast). My boss does not agree with the light bulb, even at 5W/230V:(. He says its a cheap fix and will only heat up the enclosure box too much.

Please advice/suggest/recommend. I need your help.:)

Thank you so much.

hhssmm
 
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