Triac driver with a resistor

[arvinfx]

I know that the best diver for Triac is optotriacs , But I wanna turn it on-off by a resistor.

Look:

For this purpose on upper schematic I have to connect AC to DC ground , but it looks incorrect for me
How can I do this?

 

[MrAl]

Hi,

A couple errors appear in your schematic.

First, the 240vac line rectified with a bridge rectifier produces 370v peak at high current like 20 or 30 amps or more. That means when the zener tries to shunt it down to 5v there will be a HUGE current which will blow out the zener and maybe the bridge rectifier. So you need a current limiting resistor at least.

Second, the bridge rectifier does not function well like that. Better to use a half wave rectifier and capacitor if you really want DC.

Third, most triacs can be triggered with AC so you dont need DC. Check your triac data sheet or provide the part number so we can look it up.

Fourth, the triac needs a gate shunting resistor. That is, a gate resistor to MT1 which keeps the triac from false triggering. Maybe 1k or 5k.



I'll try to get back here with more info a little later.

 

[arvinfx]

You right about zener. I forgot to add a C1 capacitor , If we add a capacitor Vin-Vz=voltage capacitor and no need any limiting resistor.

I'm not a profassional person on datasheets , The triac is **broken link removed** , I have to function the triac by a micro so I thought DC is my solution.

About the resistor between T1 and G I have tried 1k to 10k but when I add this resistor the triac is always on!

 

[ericgibbs]

hi,
Are you sure that the Neutral wire is NOT connected to the Earth wire in the electricity supply.???
E

Extract:
As the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding (earthing) of equipment and structures. Current carried on a grounding conductor can result in objectionable or dangerous voltages appearing on equipment enclosures, so the installation of grounding conductors and neutral conductors is carefully defined in electrical regulations. Where a neutral conductor is used also to connect equipment enclosures to earth, care must be taken that the neutral conductor never rises to a high voltage with respect to local ground.

 

[arvinfx]

hi,
Are you sure that the Neutral wire is NOT connected to the Earth wire in the electricity supply.???
E

Extract:
As the neutral point of an electrical supply system is often connected to earth ground, ground and neutral are closely related. Under certain conditions, a conductor used to connect to a system neutral is also used for grounding (earthing) of equipment and structures. Current carried on a grounding conductor can result in objectionable or dangerous voltages appearing on equipment enclosures, so the installation of grounding conductors and neutral conductors is carefully defined in electrical regulations. Where a neutral conductor is used also to connect equipment enclosures to earth, care must be taken that the neutral conductor never rises to a high voltage with respect to local ground.

You right, But in this circuit we do not have any part between hot line and earth.

 

[ericgibbs]

We do not have any


You right, But in this circuit we do not have any part between hot line and earth.

So where is the bottom end of the zener connected too.?

You cannot have the Gate connected in that way.?
E

BTW: With a 5R load the triac current will be 240/5 =48Amps. RMS

 

[MrAl]

Hi,

Still a number of problems as Eric has also nicely pointed out. You have a 1 amp triac so it can not handle more than 1 amp.

The triac MT1 (T1) must be common to the gate voltage common. That means the negative terminal of the bridge rectifier has to connect to the triac gate. Unfortunately once you do that you get half wave rectification again. So you also need a smoothing capacitor.

What else might work is to put the series gate resistor in series with the negative terminal of the bridge rectifier. You still should have a gate resistor to ground also, and once you get the circuit right it will work just fine to help keep the triac off when it is supposed to be off. Still need a smoothing capacitor because the rectifier output will be pulsing DC and even though it has AC input from the line the pulsing could be out of phase with the line. This might not work though because the zener has to connect from the MT1 to the positive supply (+5v).

But since you want to drive this with a microcontroller, why not just use the microcontroller, or is it that you need to make a power supply for the microcontroller too?
The microcontroller pin most likely can support up to 20ma, and your triac can work in all modes if the gate current is 7ma.

Another option is to use an optocoupler.

A series resistor is also used with the series cap. The additional series resistor limits inrush current when the circuit is first plugged in if the line happens to be at or near the peak.

Does this make sense yet?

 

[crutschow]

You don't want the micro connected to the mains power or mains neutral, that's very unsafe. Use an optocoupler to trigger the triac and isolate the micro from the mains voltage.

 

[arvinfx]

So where is the bottom end of the zener connected too.?

You cannot have the Gate connected in that way.?
E

That means end of the zener is connected to anodes of D1-d3, not the earth .

BTW: With a 5R load the triac current will be 240/5 =48Amps. RMS

You are right again , But I put a random load . , real load sink 110ma.

 

[arvinfx]

Hi,

But since you want to drive this with a microcontroller, why not just use the microcontroller, or is it that you need to make a power supply for the microcontroller too?

Hi and yes I have to supply the micro too.

A series resistor is also used with the series cap. The additional series resistor limits inrush current when the circuit is first plugged in if the line happens to be at or near the peak.

Thus small random blank of light (my load is a compact fluorescent lamp) is for this reason!!?, but I have tried 1k-1000k resistor between T1 and G none of them solve this problem! What should I do?

Does this make sense yet?

I understand your English and also your meaning

 

[MrAl]

Hi,

The first thing you have to do is think about safety as Carl pointed out. If the micro controller is to be used by a human then you either have to use an isolated supply or you have to ensure that nothing conductive can ever come in contact with the human and any node in the circuit.

Second thing you have to do is modify the circuit to use a HALF WAVE RECTIFIER not a full wave. This will simplify things greatly as well as actually get it working.
To create a have wave rectifier you use a single diode. The simplest form is a diode in series with a resistor that goes to a filter cap, then another resistor and then the top of the zener. The bottom of the zener connects to MT1. The shunt gate resistor connects between gate and MT1. The top of the zener goes to the switch, the switch to the gate series resistor.

So what we basically are doing here is creating a filtered DC supply voltage, using a series resistor with a zener as the power supply, and making the common of this power supply connect to MT1 so that the gate can be driven properly.

The circuit would look like this:

AC  o-----R1----Diode-----+------R2------+-----R3------+------o Gate
                          C1            ZD1            R4
AC  o---------------------+--------------+-------------+------o MT1

"Diode" is the rectifier diode, cathode to the right,
C1 is the cap, positive terminal up,
ZD1 is the zener,
R3 is the gate series resistor,
R4 is the gate shunt resistor,
the switch is not shown,
the load connects to the top "AC" input, and the other side of the load connects to MT2.

 

[arvinfx]

I tried your circuit , Result:
I have to select a bigger capacitor (c1 in my schematic )about 2 time stronger 2uf because of half-wave style of rectifier, and also low voltage cap ( C1 in your schematic), So I've changed my mind and I'll use a optotriac driver instead.

It works and there is no problem except [ When I turn this circuit on , a short blank happens even the gate isn't trigger !, I have tried 360 ohms up to 1 mega ohm resistor between G and T1 or T2. but resistor couldn't resolve this problem! ] any suggestion ?

 

[MrAl]

Hi,

What do you mean by a short 'blank', not sure what that means.

Adding a diode helps too as shown here:

          +-----LOAD-----o MT2
          |
AC  o-----+----C----R1----+----Diode1-----+------R2------+-----R3------+------o Gate

                        Diode2            C1            ZD1            R4

AC  o---------------------+---------------+--------------+-------------+------o MT1

The cathode of the new Diode2 is up, the cathode of the old Diode1 is still to the right.
Also shown now is the LOAD and MT2 and the series capacitor C.
The voltage across C1 should be about 10 to 15 volts with the triac 'on'. The ripple bottom should be at least 2 volts above the zener voltage, maybe more.

 

[arvinfx]

Blank = I meant Flash...

 

[MrAl]

Hello,

Yeah, so what does "Flash" mean? Give a better description so i know what you are trying to say here.

 

[arvinfx]

umm, sorry, when I turn my circuit on by putting the plug into socket the load which is a compact florescent lamp once tune on for about 20-100ms and then goes off until gate of triac is triggered by me. just like professional camera flashes. It happens randomly ,It couldn't occur sometimes.

 

[MrAl]

Hi,

The gate shunt resistor never goes to T2, only to T1. That's the only way.

Try a low value again that way and if it does not work then it is the M2-M1 dv/dt turning on the triac. To remedy that you have to use a cap and resistor in series right across M1 and M2 but that requires a higher voltage non polarized cap and somewhat low value resistor like 500 ohms or maybe lower. Sometimes the data sheet shows that.
But if this only happens when you first plug it in that may not be a problem.