SimpleIO - Application Note - Triac Output Override

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Design Center » TriacOut Override Application Note

This Application Note discusses various methods to override the outputs of the TriacOut SSR output board, switching them on or off regardless of the incoming control. There are overrides demonstrated on the logic DC input side, and on the AC output side.

This came about due to a Computer Christmas forum post.

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.

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Output control

To set the stage, this note uses the SimpleIO TriacOut4 board, which has 4 output channels consisting of optoisolator inputs driving triac outputs. For a detailed discussion of this board, see the schematic notes.

This also assumes a typical controller driving the board, with transistor outputs such as the ULN2803.

DC Input override

The goal of overriding the DC inputs is to set the outputs on or off, independent of the controller settings. This still uses the triac output circuit to drive the outputs. This might be useful when changing the controller or the software, while keeping the loads in a known state.

DC all off

To turn all the outputs off, it is simplest to remove the DC supply to the optoisolators. Switch S1 connects or disconnects the +5V to the optos.

DC all on

To turn all the outputs on, a switch can pull the opto inputs low, through a diode for each channel. Switch S3 turns on all the optos when the switch is closed, and lets the controller operate normally when the switch is open. The diodes avoid shorting the channels together.

To turn individual channels on, use individual switches in place of S3, each with a diode to its channel.

DC on or off

To turn individual channels on or off, use individual switches to interrupt the signal from the controller to the opto. Switch S2 is a 3-position switch, such as a rotary switch. Position 1 grounds the opto to turn it on. Position 2 connects the controller to the opto for normal operation. Position 3 disconnects everything, to turn the channel off. With individual switches, the extra diode is not needed, since the channels are separate. (The position choices here are just to be consistent with the AC output override switch below.)

AC Output override

The goal of overriding the AC outputs is to set the outputs on or off directly, independent of the controller settings, and also independent of the triac output circuits. This might be useful when replacing boards, while keeping the loads in a known state.

AC on, single throw

To turn all the outputs on at the AC output, one approach is a single throw switch, with as many poles as channels (e.g., 4PST). The switch simply shorts the AC input to the load, shorting across the triac on the board. Switches S4 and S5 are shown as a DPST switch, with the double-pole controlling channels 1 and 2.

AC on, double throw

To remove the triac board completely, rather than short it, a double-throw switch shorts the AC input to the load while disconnecting from the triac output. Switch S6 shows one position shorting the output on, and the other position connecting the output normally. Use individual switches to control each channel separately, as shown here, or have an n-pole double-throw switch to control all the channels together.

AC on or off

A 3-position switch, such as a rotary switch, would allow one output to have on/normal/off overrides, for complete flexibility. The three positions connect the load to the AC input, the triac output, or nothing, respectively.

The top pole of switch S7 shows position 1 connecting the AC input to the load, turning it on. Position 2 connects the triac output to the load for normal operation. Position 3 opens the connection to the load, turning it off.

As a side note, it may be important to select the on/off/normal positions carefully, depending on the type of load, such as a motor. For example, the sequence off/normal/on goes directly from normal operation to the desired override, without going "through" another setting (as normal/on/off would glitch through the on position going to off).

Sensing override

To complete the override discussion, it may be useful for the controller to know when an output has been overridden.

DC override sensing

On the DC input side, this can be done by running the optoisolator input back to a digital input channel on the controller. The controller sets the output, then reads the input to see if it matches. This is not shown on the schematic.

To be picky, this only detects if the override is different than the controller setting. For example, it won't know if the controller is set to on and the override is on. To detect all override conditions, even if they match, then the switches need a second pole, similar to the AC output sensing discussed below.

(If the opto is running at more than 5V, this will require some input buffer circuit. In general, it is typical to buffer this sort of input anyway, at least through a small series resistor.)

AC override sensing

On the AC output side, sensing the override requires an additional pole on the switches. One pole does the work of connecting and disconnecting the AC, the other switches a controller input high and low for detecting the position.

AC override sense

Here, the bottom pole of switch S7 shows the full sensing. Position 1, on, connects the controller input to +5V. Position 2, normal, connects it to the resistors that are splitting the signal to +2.5V. Position 3, off, connects to ground.

This is just using three convenient voltages as separate signals; they can be adjusted or mixed as needed. The controller input must be an analog input, to read the 2.5V.

To avoid the analog input, if it's sufficient to know if there is an override or not (and not care if the override is on or off), then only two signals are needed. Connect positions 1 and 3 to one voltage (say +5V), and position 2 to another (say ground). A digital input could read this as normal or override.