Trailing Edge Triac. Circuit to make current 0A mid sine wave????

Thread Starter

Daan1997

Joined Sep 13, 2017
16
Hello,

Iam an eletronic engineering student. Iam building a triac dimmer that can dim leading edge and trailing edge (or both).
Currently i have the leading edge working but does anyone know a good circuit for trailing edge? I know a triac turns off on the nullpoint but i need a circuit that can get the current to 0 for a short pulse to turn off the triac while its not on the null point of the sine wave.

Thank you!!
 

Attachments

Thread Starter

Daan1997

Joined Sep 13, 2017
16
Thank you. Is a GTO bipolar? Like do i need 2 to form a triac or is one GTO good enought to do the positive and negative side of the sine wave?

I made a Equivalent circuit of a GTO just with a resistor and 120V 5hz (so i can manually pulse with a switch).
I can pulse it in the positive side and negative but i don't know if this would work with a actual GTO.

thank you
 

Attachments

DickCappels

Joined Aug 21, 2008
7,297
It is an SCR specially designed to be turned off with a negative pulse on the gate, and to do so reliably.

There is a variety of configurations in which SCRs can be used to control full wave current. The easiest approach in my opinion is to combine the SCR with a full wave diode bridge.

Something like this:

upload_2017-9-13_21-6-51.png

If you can use more SCRs you can skip the diode bridge and reduce the voltage drop -its more efficient.

For other full wave configurations:
This is a link to a Googl Search for SCR Full Wave Images

You can also use one or more MOSFETs and turn them on or off even more easily.

upload_2017-9-13_21-16-20.png
 

Thread Starter

Daan1997

Joined Sep 13, 2017
16
I talked with my intern supervisor and he only wants me to use a Triac :/
Is it possible to do it with a triac?
 

AnalogKid

Joined Aug 1, 2013
9,189
Not unless you put something else in series with the TRIAC. Pretty much by definition, a TRIAC latches on until the voltage across it goes to zero.

ak
 

DickCappels

Joined Aug 21, 2008
7,297
Series shut-down approach, building on AnalogKid's solution post #6: Sense the polarity of the voltage across the triac. When you want to turn it off, put a ;voltage in series with the triac of the opposite polarity sufficiently higher in amplitude than the voltage across the triac at that instant -this can be done with a pulse transformer. It might be necessary for the pulse width to be several microseconds in order to give the triac time for the minority carriers to recombine (turn itself off). I don't know whether this was ever designed, but it should work in theory.

Parallel shut-down approach: Sense the polarity of the voltage across the triac. When you want to turn it off apply a pulse between the main terminals of the opposite polarity. The pulse must be of low enough impedance and high enough voltage to reverse the voltage across the triac. This can be done with a pulse transformer among other means. It might be necessary for the pulse width to be several microseconds in order to give the triac time for the minority carriers to recombine (turn itself off). Again, this is more of an idea than a worked-out solution.
 

MrAl

Joined Jun 17, 2014
8,156
I talked with my intern supervisor and he only wants me to use a Triac :/
Is it possible to do it with a triac?

Hi,

It depends on the circuit. Some circuits use inductors to create a commutation turn off. That has to be timed though and may not work for every application.

Another idea is to use a transistor across the triac. The transistor has to be able to steal most of the current from the triac when you want to shut it off mid cycle. That means it needs a low enough Vce Sat spec.
The advantage is that the transistor only has to conduct for a short time so the triac does most of the power handling.
This is a little tricky though because the triac used also has to shut off quick enough. This means the transistor has to stay on for at least probably 10us but maybe even longer like 100us in order to get the triac to turn off so the transistor can be turned off next. So the transistor gets an 'on' pulse of maybe 10 to 100us or so, and that keeps power dissipation in the transistor low.

If you cant use a small transistor in addition to the triac then you will have to state what you are allowed to use. Stating that you can only use a triac is not enough of a description because triacs can not be shut off without forcing the current to go to zero and that can not be done without extra components of some kind.

Are you allowed to create a worm hole and reverse time? :)
 

Thread Starter

Daan1997

Joined Sep 13, 2017
16
Thank you.
I think i got it working.
I put a capacitor in parallel with the Triac and load them + and - 24 Volts. With a mosfet i switch the capacitor charging circuit of and on and onto the triac and off it.

My schematic looks a little bit messy but iam going to tight it up when the whole design is complete.
The 4 x 12V sources are going to be 4 x 5V pinouts of a PSoC 5 controller and boosted up to 12V.
 

Attachments

Thread Starter

Daan1997

Joined Sep 13, 2017
16
Does anyone know an IC Triac driver who does leading edge and trailing edge? So i just have to pulse it and add some components to the IC?
 
Top