Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Triac Circuit

Status
Not open for further replies.

ibwev

Member
I am trying to use an opto-isolator (MOC3023S-TA1) as a switch for a 1 amp circuit. I have a lot of questions but would like to start with the 3 on the image.

Triac.png
 
the moc3020 can be driven directly by a microcontroller. 30mA max doesn't mean you need to drive it with 30mA, it means don't drive it with more than 30mA.

there is no + and - sides... it's AC... there may be a HOT and Neutral... the capacitor needs to be nonpolar AC type.

I use a MOC3032 which lists max current at 60mA, and I drive it at 7.2mA.
Attached are two drive schematics directly from the MOC3010-3020 data sheet:

moc3010-3020.JPG
 
1) It depends which micro you use. Check the datasheet to see what the max allowable current is.
2) and 3) You are driving this with 24V AC so the polarity is constantly reversing.
 
yes, that is a NAND gate, but all you need to know is to hook pin 2 to your uC and drive it low to turn on the circuit. To set the pin 1 resistor, take the 5v source minus the on LED voltage (1.5V) minus the output low voltage (assume .7V), divided by the current.

Note the design specs are listed at 10mA, so, designing for 10mA, (5-1.5-.7) / .01 = 2.8V / .01A = 280 ohm = 270 ohm or 300 ohm.
{2.8V / 270 ohm = 10.37mA or 2.8V / 300 ohm = 9.33mA Note that the uC output low and/or LED Vf (voltage, forward drop) can be a little higher or lower, as well as the power supply, so that is why we design with 5% resistors when it doesn't matter if we use 10mA, +/- 10% (9-11mA).

As the note in the schematic states, the resistor/capacitor values are only representative of a specific design, so depending on what your load is will depend on whether or not you even need a snubber. Basically, you calculate the RC value needed for the frequency response you are looking at. Then, since capacitors come in 10% values (or larger), you pick a capacitor, then base the resistor value on that. The values given are a good place to start, and I'm am not an expert in snubber design, but I can research it. Unless someone else wants to weigh in here???

note that you need an extra snubber (rc network) when driving it high vs low, that's why driving the low side is more popular.
 
it mainly depends on your load type: is it resistive, inductive, capacitive? A mix? what is the 24vac source? is it noisy, clean?
 
Load type would be inductive (I think). It would control a dipole 40 amp contactor
(**broken link removed**).

I do not know how to tell if the 24 VAC source is noisy or clean. The source would be from a 24 V transformer like this one **broken link removed** .
 
if you're using an optoisolated triac output triac driver, you drive the triac in the proper two quadrants, so you can use any triac. What this means is that the polarity of the gate pulse is in the same direction as the polarity of the power going through the triac.. gate positive when T2 is positive, and gate negative when T1 is negative.

Most triacs, unless it's a 4 quadrant friendly triac, doesn't like to be driven in the fourth quandrant, which I believe is the gate polarity postive and the T1 voltage negative... I believe...

One way to improve on your design is to use the MOC3032 instead of the MOC3020... as the MOC3032 is also a zero crossing reference, so it turns the triac on (first time) at the zero crossings...
 
Thanks again for the response Mike Odom.

Since the Moc3032 is better to use than the TRIAC 3-QUADRANT SOT-223, how do I calculate RC values for snubbing? The circuit would control a dipole 40 amp contactor (**broken link removed**) and be powered by a 24 VAC transformer (**broken link removed**).
 
WOW!!! I was afraid it was going to be complicated. I just did not realize how complicated. Thank you for the answer. It gives me something to strive for.
 
yeah, I know, that's why I'm no expert either... however, as suggested earlier, just start with the values they use and if you don't get any false triggers or latchup, then you're ok! If you do, then you have to adjust something, cap or resistor...
 
now, after all that has been said, here's another tidbit to chew on...

I build a commercial waffle cooker board. It controls a waffle cooker to make Belgian waffles (holds the temperature at 392°F and cooks for 2 minutes to make perfect waffles). My board uses the MOC3032 to turn on a Q6025A (600V 25A) triac, which in turn swtiches 120VAC to two calrod heaters rated at 750W each at 120V (12.5A). I have approx 20,000 units in the field. Neither the triac nor the driver have a snubber on them. I haven't seen any switching related problems with these units even though they are located in commercial kitchens and hotel cafes. We've been shipping these units for about 16 years (last year and this next year we are shipping 2500/year).
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top