Using TRIAC to switch on a mains transformer

[sarah90]

There are moderators on the forum for that job, and there is a report button in case you felt inconvenient with a post/member, moderators then can decide what to do.

Yeah, I know but it felt kind of bad to do things behind peoples back. Will use the button next time.

[Toxible]

@Gallymimu: You are correct! Thanks! I've not considered the load... big stupid error... sorry

@sarah90: I'm working hard as possible to give my contribution to the forum. I do not care to have a good score writing posts at random! But there are some topics that interest me, and some material that i can't download if i don't contribute (and it is correct!)
So I'm doing possible to help someone...

@metal: When vin=0 D5 are off. so the only path for the carge on C1 is R3 + BE junction.
If you don't belive, you can also simulatethe circuit. You can find a simple, useful, free simulator here: http://www.falstad.com/circuit/
If you don't want to spend time, i've done the simulation for you. (file attached).
In the image we can see the circuit and the o-scope plot of C1 voltage:
Sorry but i've placed D5 near a net so it seem that there isn't catode... it's an overlayer problem of sim interface....
1-your circuit (note: C1 is always <4V). the red circle is the LED
2- your circuit without load (LED). note C1 now reach 9.73V because the led don't discharge it. but if you see there is some ripple on signal! is the discharge due of current on Q1.
3- i've turned off Vin  (you can short but it's the same in this case). you can see that C1 discharge (throught Q1 base).

"There are moderators on the forum for that job..." Well said

[metal]

As long as you have simulated already, can you find out the amount of current passing through the BE junction when it discharges the capacitor? I am interested to see the actual ripple measurement in (fig 3) caused by this discharging as well as the current.

[metal]

Okay, I this will solve that issue you mentioned earlier. I moved R4 between the emitter and C1.

[Toxible]

Ok! Im here again Current throught BE is little (obvious) and it reaches 108uA..
So, I've attached the txt containing the schematic of java circuit simulator. if you want copy the text and use file>import to open the circuit.
in the graphics you can see Ibe and Vc1.
the file circuit 2 contain the modified schematic you've posted.
Ibe reaches 53uA and Vc1 3.38V
This solution is the same thing as before... and it work good as before
The only advantage is that you can drive more led in series with the same bjt (R1 on emitter fix the Ic current at about Vc1max / R1)

[metal]

Then, I don't have to change the  circuit, I can use it as is, when I moved the resistor down there, I noticed that the ZC pulse got worse, 108uA is not a big problem after all, I measured it yesterday and I thought a change is not required, this is why I asked you about the base current :- )

Great help Toxible.

BTW, the snubber is a must in such inductive circuits?

[Toxible]

Hi all folks!
@metal: lets talk about the snubber network...
If I understand correctly you use the triac to bypass the resistor. now there are some way.
In my opinion, a small snubber connected to triac is required to prevent damange when triac goes off during system turn off.
but to i want understand how you want use the resistor. I see 2 alternatives.
1- charge slowly the cap after the transformer
2- avoid the connection if the voltage is not zero.
this problem are similar but they are not the same thing. If you have a very big cap to charge, is not enought to switch on the triac on zero crossing. infact in this case the derivative of the voltage is high enough to turn the magnetothermic breaker off.
So we need to get a value for the capacitance "seen before primary" and then find the best solution.
If this value is quiet small, we can use triac only to solve problem 2 and the resistor is not essential.
If the value is high the resistor it's a method to solve problem 1 (and 2) but we need to calculate the power dissipated by resistor.

[metal]

Here is what I got until now. Simulation is truly nice for a lamp, there is a transformer in the component list, try it (without lamp) and see how things turn bells for the Christmas.. I don't like what I see with the transformer and TRIAC, may be I did not know how to setup transformer parameters, regardless of that fact, things started to become more and more worse so quickly. May be it is a bad idea to use a TRIAC? I believe it is really difficult to control inductive loads with a transformer, but how others use a TRIAC then?

I started with 4 pulses and the 5th continues connecting the mains. I tried many ways tbh, I don;t know, the more I dig in this topic the more I get lost :- ) damn power electronics...

@Toxible
Look at the schematic, I don't understand your last post.

[Toxible]

... there is a problem.. i have not proteus intalled.... can you post a screenshot?
In next days i will try to install proteus

[zab]

Hi,
The simulation can not give you a realistic picture. So my suggestion is try it practically.
The simulation is ok. the difference on inductive load is due to back emf of transformer . so donot worry about it as long as it is with in working range.
The difference after full conduction is because triac can not conduct at less voltage near after zero crossing.

The 5th pulse need to be delayed so that switching  can be at zero crossing. Short time triggering pulse create problems near zero crossing.
I use keep pulse on up next zero crossing for smooth fuctioning.
I am attaching pics and proteus file for ref,

[metal]

The 5th pulse need to be delayed so that switching  can be at zero crossing. Short time triggering pulse create problems near zero crossing.
I use keep pulse on up next zero crossing for smooth fuctioning.

I don't understand, can you please give more explanation.

[zab]

Just see the simulation, you will see that 5th pulse trigger the triac before voltage across triac reaches at zerocrossing.The first 4 pulses were triggering traic in positive half. while the 5th pulse trigger the triac in negative half pulse.


The width of trigger pulse is very important. Ii was my personal experience that when the trigger pulse is of short duration it some times does not
trigger the triac at all. the phenomena normally occurs when applied voltages on triac are not sufficient to hold it on in conduction mode.To overcome this problem I use to sense both positive and negative zero crossing.and trigger pulse remain on up to next coming zero crossing to ensure that triac conducts in all situations.
I think it would be clear now what I intend to tell.

I have been looking for some isolated way of measuring current with oscilloscope,What type of method you are using to monitor the current ,As the clamp meter does not show the peaks.

[metal]

This means I will have to keep the pulse even after zero-crossing, but how long after zero-crossing?

That's funny indeed, I was going to ask the same question because I am about to buy an oscilloscope because of this project, clamp meters can't catch this exact halfwave current, which is the current I am seeking.. I am thinking about tektronix tds2002c but I find its record length to be really bad.

[alexxx]

I am thinking about tektronix tds2002c but I find its record length to be really bad.

If you are to pay a respectable amount of money to buy a decent tool like tektronix, then in my humble opinion it should have 4 channels. Otherwise search for a cheaper oscilloscope. You will regret for paying lots of money on it when time comes and more than two channels are needed...

[sarah90]

I am thinking about tektronix tds2002c but I find its record length to be really bad.

I paid less than a third for my (now hacked to 200mhz) hantek scope and it has similar features to the tds2002c. If I would buy a scope today I would go for the newest rigols, or if money is no object the new agilents. Of course a second hand agilent/tek scope is an option too. I would not go for a recent tek, as they seem to lag behind.

[Gallymimu]

For high end brand I would only go Agilent at this point.  I have found the Tek scopes to be a bit behind in user interface quality and responsiveness.

For budget I use Rigol.  Their DS1000D series scopes are the same hardware and mostly the same firmware as the low end Agilent scopes (Rigol makes them for Agilent)

[sarah90]

For high end brand I would only go Agilent at this point.  I have found the Tek scopes to be a bit behind in user interface quality and responsiveness.

For budget I use Rigol.  Their DS1000D series scopes are the same hardware and mostly the same firmware as the low end Agilent scopes (Rigol makes them for Agilent)

Is that another way to say "I second that!"?

[Gallymimu]

Is that another way to say "I second that!"?

YES!

[metal]

if you want, I can split the posts where oscilloscope discussion starts and we can discuss it better over there, what do you think guys?

[Gallymimu]

if you want, I can split the posts where oscilloscope discussion starts and we can discuss it better over there, what do you think guys?

I don't know if there is a lot more to discuss but it does seem to be a useful topic if scopes haven't been covered so I'd split it as you suggest.  Maybe others will pipe in with opinions and we can have some good discussion!

[sarah90]

I agree. Oscilloscopes should not be part of this discussion. But I would like to see a topic where people can tell what equipment they have, their experiences and may be some tips for people looking to buy.

[zab]

This means I will have to keep the pulse even after zero-crossing, but how long after zero-crossing?

That's funny indeed, I was going to ask the same question because I am about to buy an oscilloscope because of this project, clamp meters can't catch this exact halfwave current, which is the current I am seeking.. I am thinking about tektronix tds2002c but I find its record length to be really bad.

As you are keeping the trigger pulse on permanently afterward so you need not for any wait for delay, you can switch on the trigger pulse as you get zero crossing pulse. At supply volt reaching to conduction level the triac will automatically on.

About the second point DSO will work. The simple solution is to use a resistor in series of load better at neutral point for measurement of current. But please insure that your scope should be supply thorough isolated transformer (not connected to any ground point).for safety reasons.As the probe share ground point. The other way is get some isolated current probe. But personally I too looking for some thing simple. and easily available.  I saw a topic where someone suggested to make it with hall sensor.But It performance is questionable.especially frequency response,
An other way is to use allegro ics. with some operational amplifiers.I saw any article in silicon chip about this way.but have not checked it.

[metal]

But what I really need is to switch triac on at specific timing for the first 5 pulses, you can see this in simulation, and then I switch the triac on and relay on then turn the triac off so that relay takes it from there. I don't think I will need to switch the triac at earlier time for the first 5 pulses.

Concerning the current, I will start with fast fuses to see if they burn first, there are many other cases to handle too, let's see, I don't have an oscilloscope at the moment, but will start testing to see if the concept works or not at least.