Design Center » TriacOut Schematic Notes

These are notes describing the Triac Output circuit schematic. This discusses the triac output circuit in general, and the TriacOut4 specifically, since it was designed first.

Start with this page, then view these:
TriacOut8 Schematic Notes
TriacOut12 Schematic notes

This is meant to be a general discussion, but assumes some basic electrical knowledge. We assume no liability for any content here, or what you do with it; experiment at your own risk, and with low voltages. Please let me know if you have any corrections or suggestions.

Triac

A triac is a solid state semiconductor that acts as a switch for AC power. It has three pins that act similar to a light switch. Two pins are for the AC in and out (they are generally interchangeable, so they are named A1 and A2 in the schematic, rather than Ain and Aout), similar to the top and bottom terminals on a light switch.

The hot or live AC is connected to one pin (A2 in the schematic), and the switched hot AC is connected to the other (A1 in the schematic). Like a light switch, when the triac is off, these are disconnected, and no AC current flows; and when the triac is on, they are connected, and AC current flows through the triac.

The other pin on the triac is the gate, which does the switching. When voltage is applied to the gate, the triac turns on, and AC current can flow through the triac. The gate is usually controlled by a an optoisolator, a specific component for driving triacs. See the next section.

The triac on the Triac Output boards is a BTA08-400B, from ST Microelectronics. This is just a basic inexpensive triac, and many other equivalent ones would work fine. For this part number, the BTA is a popular series, the A means it has an isolated tab, the 08 means it can handle 8 amps, the 400 means it can handle up to 400 volts (you need headroom for the peak voltages, and this is really the lowest I could find anyway), and the B suffix means it is a standard sensitivity (which I think means you need an opto to drive it) and it is not a snubberless triac (meaning you need a snubber if you switch an inductive load).

The tab on the triac is the top of physical case, often metal with a hole in it, for connecting to a heat sink. Triacs can pass some current by themselves and just get warm. The more current, the more heat, and often (almost always…) it needs a heat sink to dissipate the heat. A heat sink is just a chunk of metal, usually with fins, that draws the heat away into the air.

The tab is usually connected to one of the AC pins (in this case A2). It is always thermally connected, so heat sinks will work. Some triacs electrically connect the tab to the AC, and some isolate from it. These -B triacs on the Triac Output boards are isolated. I just like the idea of the heat sink not being live, and this just lets you connect a big heat sink across all the triacs without shorting everything together. I believe the non-isolated triacs have better thermal transfer ratings, so they dissipate heat more efficiently, but the safety and ease of isolated triacs win me over.

When the triac is non-isolated, the tabs and the heat sink are LIVE. In the case of our Triac Output boards, I chose pin A2 for the hot side, somewhat randomly for board layout ease, and since the triacs were isolated. However, this means if you use non-isolated triacs on our boards, the tabs and the heat sink are ALWAYS LIVE, even when the outputs are off!

Optoisolator

The opto is an integrated circuit (IC, chip) that is specifically designed to connect low voltage DC controls to high voltage AC triacs. The opto in the Triac Output boards is a 6-pin chip, but only 4 pins are used: 2 for the DC in, and 2 for the AC out.

Internally, the opto has three major sections: an LED input, a zero-crossing detector, and a triac driver output. (Not all optos have a zero-crossing circuit, but these do.)

The LED input is a Light Emitting Diode, very much like the ones you see on circuit boards. Turn it on, and it shines light. Pin 1, the anode, is the high side, which is connected to a pull-up resistor, which is in turn connected to a DC supply voltage, in this case 5V. Pin 2, the cathode, is the low side, which acts as the DC control. Ground this pin, and the LED turns on. See the Logic input section below.

When the LED is on, it shines a light inside the opto to turn on the zero-crossing circuit. This provides the optical isolation (hence the name), since there is no electrical connection from the DC input to the AC output.

The second section in the opto is the the zero-crossing voltage detector circuit. This starts to work when it detects the LED light. It switches on and off only when the AC voltage is zero. When you turn on or off the opto, it will turn on or off the AC on the next half-cycle that crosses zero volts. This means the load (say, lights) will be turned on when there is no voltage, which is less spiky, less noisy, and easier on the load. (Ever hear a light dimmer switch buzzing, or the filaments of a bulb buzzing when dimmed?) This zero-crossing is very handy for flashing lights, but prevents using the triac as a dimmer. Since there is more control circuitry needed for dimming anyway, I thought these basic Triac Output boards would be good for on/off and flashing, and we'll develop a separate dimmer board.

The third section in the opto is the triac driver output. This is essentially a mini-triac that switches AC on and off, with a little power, enough to trigger a triac (but not enough to drive a load itself). When the zero-crossing circuit turns on, this triac driver switches the AC on to the gate of the triac, which in turn switches the AC on to the load.

There is a series gate resistor between the AC line input, and the pin 6 high side of the opto output, R2 as one example in the schematic. This resistor limits the peak current through the optoisolator. Its value is a balance between limiting peak current, and allowing enough gate current to turn on the triac. Typical values range from 100 to 180 ohms. See the Triac Series Gate Resistor application note for details.

The opto on the TriacOut4 board is a MOC3041M, from Fairchild Semiconductor. The MOC30xx series is very common, with different versions for different voltages and currents. The MOC3041M is a 400 V peak, so it will handle 120 and 240 AC, and has an input current of 15 mA. There are lower input currents available, which would affect the selection of the pull-up resistor. (See the TriacOut8.) This opto was chosen for least cost and most availability, but the others would be fine as well.

Note: the two unused pins, 3 and 5, should remain disconnected. They may be connected to something internally in the opto. Do not ground them or connect to any other line.

Picky note: The zero-crossing is indicated in the schematic symbol by the little box with the 1 in it. What the number 1 is for, I don't know - I actually just noticed it creating this zoomed image. I think this is a symbol for an edge trigger, which I guess is sort of like a zero-crossing. If someone knows what this is, or if I have the wrong symbol, let me know!

DC Logic input

The Triac Output board's logic input is made up of two connectors and a pull-up resistor. These connect to the opto's LED input.

The two connectors are a right-angle square-pin header (J1), and a screw terminal block (J2). They are just in parallel with each other, to be flexible with how to connect to the board. The header allows for board-to-board connections or cables or other plugs, and the screw terminal block allows you to connect individual wires directly.

Pin 1 of the connectors is the DC supply to the pull-up resistors, typically 5 V.

Pin 2 of the connectors is not connected to anything, but is available as a DC ground for other boards, or if needed for some modification.

Pin 3 is the control for channel 1, Pin 4 for channel 2, etc. These pin assignments provide consistency for the various Triac Output boards, as the number of output channels varies.

Connect the input pins to a controller's output pins, typically a transistor driver such as a UDN2803. Pull the input to ground to turn on the AC output. The device pulling the input down must be able to sink 15 mA.

AC input and Fuse

The AC input is just made up of a connector and fuse. The 2-pin screw terminal block, J3, connects the switched side (typically hot) to the fuse and on to the triacs, and connects the common side (typically neutral) to the common pins of all the output connectors.

Typically, you switch the hot to the load, and connect neutral directly as the common to the load. Some local electrical codes reverse this, switching the neutral. The circuit doesn't care, and used the terms Vac and Common on the input connector to avoid Hot and Neutral. Just keep it straight.

The fuse is used for all the channels, so they are not isolated from each other. If one channel blows the fuse, it will disconnect all the channels on the board. This was just a tradeoff for cost vs. convenience. There are boards, such as the ones from Aldor, that have a fuse per channel, paying more but gaining the convenience of keeping other channels running when one blows.

The fuse is sized at 6.3 A, which was the largest before the price jumped up. I figured a typical 1 A per channel (about 100W), so this works fine. At your own risk, if you are going to run more current through the board, you could replace the fuse with a higher value.

The AC input continues from the fuse to the triacs and optos, to be switched on and off to the AC output connectors.

AC output

The AC output is made up of connectors, and footprints on the board for MOVs.

The connectors are pairs of screw terminal blocks, with a switched output (hot) and a common (neutral).

Connect the load to the switched outputs, numbered on the board with the channel number.

The common pin on each output channel is just for convenience, simply run from the AC input common pin. If you have a hot and neutral wire for each load, connect the neutrals to each output's common pin, and avoid having to wire-nut them all together.

These start at J4 for outputs 1 and 2, J5 for outputs 3 and 4, etc. These connector assignments provide consistency for the various Triac Output boards, as the number of output channels varies. The 4-pin connectors are more cost-effective, so I used them for 2 channels each. At 4 pins and up, the cost is linear per pin. The 2-pin connectors, such as the AC input, are more expensive.

The MOV is a Metal Oxide Varistor, one of several devices that acts as a snubber. These are in the schematic in order to put footprints on the board for them, but are not supplied with the board. The footprint is just for convenience, to avoid having to wire components to the output connectors.

MOVs or other snubber circuits such as a resistor and capacitor, are for driving inductive loads. These are loads that have a coil, such as a motor or solenoid or neon transformer. These inductive loads can cause large spikes when turned on and off, and the snubber circuit helps squash (snub) the spikes.

Reference Designators

Just for completeness in these notes, here are some comments about the reference designator assignments (J1, R1, etc)

My goal was to assign the ref des numbers so they would be consistent from board to board, as the number of channels varied. This makes it easier to write common descriptions and instructions for all the boards. Also, I wanted to keep the general standard of having the numbers increase across the board, to locate them easily (even though these are small boards).

This lasted as long as one board – the TriacOut4 and TriacOut12 have the original numbers, but the TriacOut8 added resistors for the sink/source option. However, the pattern is similar between the boards.

The components common to all the Triac Output boards:
- Logic input connectors J1 and J2
- AC input connector J3
- Fuse F1

The components for each channel:
- Pull-up resistor R1 (R3, R5, R7, etc.)
- Opto U1 (U2, U3, U4, etc.)
- Opto output resistor R2 (R4, R6, R8, etc.)
- Triac T1 (T2, T3, T4, etc.)
- MOV1 (MOV2, MOV3, MOV4, etc.)
- Output connector J4 (J5, etc.)