TRIAC or RELAYS? :)

[ulipollo]

Hello electro-tech users.
I'm new to this forum that looks good.

I'm developing a DIY SMT toaster and I'm wondering about the control circuit for the heater lamps..

Now i've chosen to use a microcontroller that switches on-off two relays in order to control dynamically the temperature but i think that i will hear relays saying "tic tac tic tac" every few seconds.. So i'm thinking about interfacing a dimmer used for lamps to the controller.. do you think that it can be a good choice? Have I to use a DAC for driving the TRIAC gate? Or have I to monitor the phase of the AC network?
Lamps/heaters are 110V/300W each.

I could use PWM but does it work at such power? i just used PWM for low-charge applications with small mosfet driver (<1A DC motors and so on)..

thanks for replies and regards.
Ulisse.

 

[Reloadron]

If the system is to be uC based I would think about using SSRs depending on the power requirements of the heaters. They can be pulsed or whatever you choose for control.

Ron

 

[KeepItSimpleStupid]

This might help you: http://www.cpustick.com/examples.htm

 

[4pyros]

You can PWM it with SSRs.

 

[Boncuk]

Hi ulipollo,

welcome to the forum.

Since you are going to control temperature I suggest to use phase control for the heater(s).

Here is a phase control circuit which works (tested) with 2.5KW inductive loads without the power triac misfiring.

Check out the parts of the circuit framed blue. The left hand part contains a zero-crossing unit with Schmitt-Trigger that generates very short positive going pulses upon zero crossing detection. The right hand part contains an opto coupler with triac which in turn fires the power triac.

Using a BTA41 has two advantages: a) it's fully isolated, b) it can switch 40A at full sine wave. Package is TOP3 which offers strong PCB traces for Terminals1 and 2.

Omit the snubber network (C2 and R17) or reduce to smallest possible value for resistive loads.

If the attached screenshot is unreadable don't hesitate to drop me a PM (Personal Message) leaving your email address. I'll send you the original Eagle files.

Regards

Boncuk

P.S. I just saw the part values are unreadable. Will add a pdf file in a moment.

 

[ulipollo]

Thankyou all very much for replies.
So there are 2 approaches: the first uses Solid States Relays with PWM control and the second one posted by Boncuk uses a zero cross detector in order to control the sine-wave phase.
Sorry for the question but where the SV2-7 pin is connected? Is the MOC3021 driven ON/OFF directly by the microcontroller? and last question: Can I use a 4N25 instead of the MOC3021? thanks and regards.
Ulisse.

 

[Boncuk]

Thankyou all very much for replies.
So there are 2 approaches: the first uses Solid States Relays with PWM control and the second one posted by Boncuk uses a zero cross detector in order to control the sine-wave phase.
Sorry for the question but where the SV2-7 pin is connected?

SV2-7 connects to SV1-7 which connects to PB3 of the ATTiny2313. The two connectors connect two boards (sandwich mounted) being housed in a standard USA wall outlet box.

Is the MOC3021 driven ON/OFF directly by the microcontroller?

Yes, the cathode of the light emitting diode connects directly to PB3. The anode is tied to +5V via a 150Ω resistor (R7).

and last question: Can I use a 4N25 instead of the MOC3021? thanks and regards.

No, you cannnot use the 4N25 instead of the MOC3021. The 4N25 is an optocoupler with transistor output while the MOC3021 is an optocoupler with triac output. You can't generate a firing pulse of relatively high voltage using an optocoupler with transistor output.

You might as well PWM the power triac, but this applies to pure resistive loads only. With any inductive part it is wiser to use synchronized zero-crossing control to avoid misfiring the power triac. The load (motor) will react very erratic and reverse direction suddenly if that happens. As a result the triac overheats and gets destroyed.

My circuit uses the zero-crossing detector with Schmitt-Trigger for precise timing. Schmitt-Trigger output pulse is 80µs. The circuit synchronizes automatically within the main frequency range of 46 to 54Hz.

Ulisse.

 

[ulipollo]

Thanks Boncuk, now everything is clear! I haven't understood that the board was sandwich-mounted and that SVX-K is connected to SVY-K!
So instead of pulse width modulating the IR Heater (that i think it is a bit inductive too) the best solution is to detect the AC Wave with the zero-crossdetector and drive the TRIAC for a period Tk starting from that point.

I've ordered the MOC3021 online, while i'm waiting for the MOC3021 I do a try with the standard (not solid state) relays solution because i had already designed the board and firmware before this post.

I'll let you know if it works or if relays sing too much while reflowing

Thanks again and regards.

 

[Boncuk]

I'll let you know if it works or if relays sing too much while reflowing

Thanks again and regards.

Click - clack --- click - clack.