# Problem with using SCR to control power for an electric grill

I'm doing a power controller for an electric grill, where through PWM I allow to pass x number of cycles from the 124V @ 60 Hz source of my lab.

Here you have the diagram:

simulate this circuit – Schematic created using CircuitLab

It consists on a triangle signal that joins the LM311 comparator on one pin, and in the other one the input is the LM35 signal multiplied x 7. The system has been designed to get thinner pulses from the LM311 as the temperature on the sensor gets closer to 60 °C, where pulses must disappear. The PWM signal goes directly to a MOC3011, whose job is to excite the Gate pin from the SCR TIC116. I've checked everything so far with and scope. So, so far, so good.

As you may see, the frequency of the square signal that generates the triangle one, is 0.6 Hz. This means that in a whole pulse, 100 cycles from the 60 Hz might be involved. So, when the Gate is on, the grill would be working the number of cycles that are "inside" the gate pulse according to the temperature on the sensor.

Here comes the problem: the grill keeps working even when there is not any signal at the comparator output. I haven't seen the voltage signal at the grill because I have no way to isolate the scope from ground (it is not mine so I don't want to break it). But, anyway, something is happening in between the moc and the scr.

I can't use a TRIAC, my professor banned it.

EDIT: Do you think that replacing the moc3011 with a moc3041, the one with zero-cross circuit inside, may help to solve this? What would you be so kind to suggest?

Regards.

• In your circuit you show a MOC3011 ...in the description a MOC3041, these are very different .....which one is it? Nov 24 '17 at 22:46
• @JackCreasey I just editted it to make it clear, thanks. Nov 24 '17 at 22:52
• Your schematic shows +12 V feeding the MOC3011 triac, out to the SCR gate but no path back to the 12 V GND. Can you add it in to your schematic? You have a GND on the opto-LED but no positive supply. Is the left side (control) completely isolated from the mains? Nov 24 '17 at 23:10
• The MOC3011 is an optotriac so it doesn't turn off until the current through it goes to zero. You have it hooked up to 12v (coming from where) which makes no sense.
– τεκ
Nov 25 '17 at 0:38

The main reason to include opto-isolated drivers is to ensure there is no connection between the mains AC side and any other supplies, especially those you might get to touch. I'd suggest the following:

1. Get rid of the 12 V supply

2. Stay with the MOC3011 you have, they work quite well

3. If you are stuck using SCR's then you need two in antiparallel to switch both half cycles of the AC mains. The other way to do it is to use a single SCR and a Diode bridge, though the thermal losses are higher in this configuration.
Note: Your current circuit provides only a half wave supply to your heating element, since you only have a single SCR.

4. The TRIAC output structure in the MOC3011 is not great at switching close to the zero crossing (it may have an on voltage drop of close to 3 V), so it's best when using them to wait until the voltage is high enough to ensure good turn on current for the power SCR(s). If you get an interrupt from zero crossings you can wait a fixed time (or sync your saw tooth generator) to ensure you have enough voltage to provide adequate gate current pulse amplitude.

Here's the same information in schematic form:

simulate this circuit – Schematic created using CircuitLab

As for the element remaining powered all the time, you have a fault that cannot be ascertained from your current information since it's incomplete. Providing a full schematic may help.

• I am thankful with your explanation, however I just found that the error was more simple than I thought. I was using a 330 ohm resistor connected to +12V on the TRIAC of the MOC; this makes the gate being active all the time since Vcc never makes zero. I just changed the resistor for a 150 ohm @ 10W one connected between anode and TRIAC and it worked very well, now I can see how the pulses control the on/off states. Nov 25 '17 at 15:24
• However, you still have the problem that you are only using half cycles to power the grill. Nov 25 '17 at 16:19
• That's ok, because I'm using a half-wave rectifier. Nov 27 '17 at 21:23