Triac softstart, switching point

[Sebastiaan]

Dear,

I am working on a soft-start system by micro-controller. In my design decision I don't want to go for thermistors or power resistors with relays bypass.

A Triac is a perfect device to use as switch and as soft-start, especially when used with a micro-controller.

Not only in my own designs I need a good soft-start system but I think in the DIY market there is also a demand for a good and clever soft-start system, and we are will offer soft-start boards for the DIY community as well once we really finalize it.

But to get there there are some unclear things for me.

The switching point. We will use a zero crossing detector. It is a bit unclear though where to switch the transformer on. In my understanding a coil (and therefor a transformer) has the current 90 degrees out of Phase with the voltage. So one would think we need to switch in on the peak of the mains voltage.

Also confirmed by this paper: http://relays.tycoelectronics.com/ap...fs/13c3206.pdf

However other people said that you should switch on the zero crossing point instead, because there is no current yet at switch on.

We also discovered that the switch off point is also important. The way a transformer is switched off defines the flux magnetization in the transformer. A magnetized transformer will have bigger inrush current with next time start up.

Of course the answer is easy to find by just building the circuit and measure myself, and I will, but before we have all the stuff it is nice to hear some input.

If there are any other suggestion for the soft-start with a Triac and a micro-controller, they are more then welcome.

The idea is to use the Triac as dimmer, and slowly rise the voltage as part of the soft-start.

With kind regards,
Bas

[Zero Cool]

Many companies have used Triacs for soft start. peavey has done it for years. Also Biamp and Carver is the king of using triacs as a method for controlling a transformer. there early magnetic field amps use a method of Voltage regulation that is basically a big light dimmer!

You can also look at the Soundcraftsmen PCR series of amps for an alternate system using SCR's on the secondary side for what they call phase control regulation. same basic principals and lots to learn from.

[Sebastiaan]

Originally Posted by Zero Cool

Many companies have used Triacs for soft start. peavey has done it for years. Also Biamp and Carver is the king of using triacs as a method for controlling a transformer. there early magnetic field amps use a method of Voltage regulation that is basically a big light dimmer!

You can also look at the Soundcraftsmen PCR series of amps for an alternate system using SCR's on the secondary side for what they call phase control regulation. same basic principals and lots to learn from.

Thank you Zero cool. I will google them. I am very intrigued by this.

With kind regards,
Bas

[Sebastiaan]

No one else?

I realize I place this topic in the wrong forum (chipamps) Is there any way I can place it in the right forum?

With kind regards,
Bas

[janneman]

Bas,

I have done some work on this and posted scope curves on the forum. I *think* it was in the now-closed thread about audible differences between cables, maybe it was the John Curl Blowtorch II thread...
Sorry don't exactly remember.

BTW A loaded transformer is not really an inductance (except for the stray inductance) but at the primary you see the (transformed) secondary load. Can be pure R.

jd

[Sebastiaan]

Originally Posted by janneman

Bas,

I have done some work on this and posted scope curves on the forum. I *think* it was in the now-closed thread about audible differences between cables, maybe it was the John Curl Blowtorch II thread...
Sorry don't exactly remember.

BTW A loaded transformer is not really an inductance (except for the stray inductance) but at the primary you see the (transformed) secondary load. Can be pure R.

jd

Thank you Jan for commenting.

I really try the search function in all possible ways on this forum, but wasn't able to find it. Sad that such a knowledge base is lost.

My main concern is, the first moment when you switch the transformer in, if the current is 90 degrees out of phase with the voltage, and where to switch in. The zero or the peak.

With kind regards,
Bas

[darkfenriz]

peak

[janneman]

Originally Posted by Sebastiaan

Thank you Jan for commenting.

I really try the search function in all possible ways on this forum, but wasn't able to find it. Sad that such a knowledge base is lost.

My main concern is, the first moment when you switch the transformer in, if the current is 90 degrees out of phase with the voltage, and where to switch in. The zero or the peak.

With kind regards,
Bas

Actually, it doesn't matter for a supply that is rectifier/capacitor loaded.
I'll try to find my results.

Edit: Try this:

John Curl's Blowtorch preamplifier part II


jd

[Sebastiaan]

Thank you Janneman for the Effort. In that case the inrush current limit doesn't get reached by the switching point, but by the dimmer function of the triac, to slowy rise the voltage.

In the meanwhile. I have some good stuff to read as well for anyone who is interested in this: http://www.st.com/stonline/books/pdf/docs/1863.pdf

With kind regards,
Bas

[AndrewT]

Hi,
My view on this may not be right.
I have said this before and I don't recall anyone saying I was wrong.
Here goes.
When a iron cored inductor is sitting idle with no current flowing (OFF) there is no flux in the iron core and effectively the dormant inductor is an air cored inductor.

At the moment of start up the voltage is zero and the current is zero.
We have inductor (primary) resistance and Zero volts = Zero current.
As voltage rises, current flows through the primary resistance and starts to increase. This increase in current is resisted by the air cored inductor.
For the first few milliseconds after switch on the mains electrical flow is determined by the mains voltage, the mains resistance, the air cored inductor and the primary resistance of that air cored inductor.
This is the time during which the soft start is required to limit the peak current that will flow before the iron cored inductance has built up to react against the mains voltage.

Has anyone got ideas on the ratio of iron cored inductance of the primary of a transformer compared to that same coil with no iron core in it?

As the mains cycle continues, the flux in the iron core builds up and starts to follow the mains cycle. The transformer has now started. It now behaves as Janneman is telling you.

It's that very short period before the flux builds up that results in the high start up current.

Does this explanation hold water?