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EPC programmable controllers. Data Reviewer. Select Country Please select your country by clicking a flag below. For more details please read our Privacy Policy. Have a question? Request Information. Start a live chat. Support Center. Find a Distributor. Figure 3 shows one way of incorporating the above circuit in a complete electric power switching system. The LED control circuit can take any of a variety of forms; some simple examples are shown in Figures 4 to 7.


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The bistable latches into the 'output low' state when S2 is briefly closed, thereby killing the DC power feeds to the red LED and the triac. Figure 5 shows a simple circuit that also provides the option of automatic control via an adjustable thermostat switch that is normally closed but opens when its temperature exceeds a selected value. Here, the red LED and the electric heater are off when SW1 is in the 'off' position or in the 'auto' position when the thermostat is open, but are on when SW1 is in the 'on' position or in the 'auto' position when the thermostat is closed.

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Figures 6 and 7 show high-precision versions of the basic Figure 5 circuit, with the thermostat replaced by a temperature-sensitive electronic switching circuit. The Figure 6 circuit uses an ordinary silicon diode D1 as a thermal sensing element. Here, zener diode ZD1 is wired in series with R1 so that a constant 5.

To initially set up the circuit, set RV1 to mid-scale, adjust the temperature of D1 to the desired mid-scale trip value, then trim RV2 so that the red LED is on, but goes off again if the D1 temperature is increased slightly by briefly applying finger heat to D1. In practice, the circuit has a typical switching sensitivity of about 0. Here, potential divider RV1-TH1 applies a temperature-sensitive voltage to pin 3 of the op-amp, and potential divider R1-R2-RV2-R3 applies a preset reference voltage to pin 2 of the op-amp.

The two potential dividers are actually wired in the form of a Wheatstone bridge, and the op-amp is used as a high-gain bridge balance detector; the bridge balance point is unaffected by variations on supply voltage. Capacitors C1 and C2 help to ensure circuit stability. RV2 is a linear rotary pot that is used to manually adjust the heater system's operating temperature over a limited range, and RV1 is a multiturn preset that is used to set the circuit's nominal with RV2 at mid-scale setting trip temperature.


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  • To initially set up the circuit, set RV2 to mid-scale, raise the temperature of TH1 to the desired mid-scale trip value, then trim RV1 so that the red LED is on, but goes off if the TH1 temperature is increased slightly. Note that the Figure 7 circuit has a typical switching sensitivity similar to that of the Figure 6 design about 0.

    Also note in Figure 7 that the thermal 'span' range of RV2 can be increased or reduced by increasing or reducing the value of resistor R2.

    Triac Principles and Circuits — Part 2 | Nuts & Volts Magazine

    Finally, note that — in all cases where an 'automatic' heater-control circuit is used to regulate the temperature of a room — the actual thermal sensor device thermostat, thermistor, or sensing diode must be sited roughly one meter above floor level, in a position where it can directly and safely sense the temperature of normally-circulating air; this position must be free of drafts or direct radiation from the heater, and must not be obstructed by furniture, etc. There are three basic ways of controlling the AC power feed to resistive loads such as filament lamps or electric heaters via a triac.

    One of these is the variable phase-delay-switching system, which gives fully-variable power control and is often used in lamp dimmers, but generates substantial RFI and is thus unsuitable for driving high-power greater than about W loads. The third method of AC power control is the burst-fire integral-cycle system shown in Figure 8 , in which bursts of complete half-cycles are fed to the load at regular line-frequency-related intervals. The burst-fire system thus gives variable power control and generates minimal RFI, and is often used to control the thermal output of electric heaters.

    Note that the burst-fire integral-cycle control system operates on the synchronous 'zero-voltage' triac switching principle, and practical circuits of this type can thus be made by using suitable control circuitry in conjunction with the basic power switching system of Figure 3. Two suitable circuits are shown in the next section of this article. The optocoupled synchronous circuits shown in Figures 2 to 7 all — when powering a heater load — give a simple form of control in which the heater is either fully off or is operating at maximum power.

    Figures 9 and 10 show circuits that drive the heater in the synchronous burst-fire mode, thus enabling the heater's thermal output to be varied over a wide range. The Figure 9 circuit enables the heater's thermal output to be varied manually, via RV1. The Figure 10 circuit varies the heater's output automatically, to maintain a room's temperature at a precise pre-set value. The operation of the Figure 9 circuit is fairly simple.

    When SW1 is switched to the Man manual position, this output is fed to the input of the Figure 3 optocoupled synchronous electric heater control system, where it enables the mean power input to the heater to be varied via RV1 from zero to maximum in 68 discrete 'half-cycle' steps in a 50Hz system or 82 steps in a 60Hz system.

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    Finally, to complete this look at burst-fire heater control circuits, Figure 10 shows a self-regulating synchronous burst-fire heater controller that automatically varies the heater's input power to maintain a room's temperature at a precise pre-set value. Here, the circuit to the right of R3 is almost the same as the Figure 7 thermistor-controlled automatic circuit, but the IC1 circuit to the left of R3 is taken directly from the Figure 9 circuit and superimposes a mS ramp waveform with a peak-to-peak amplitude of about 40mV on the RV1-TH1 junction and pin 3 of IC2.

    The circuit thus automatically adjusts the heater's thermal output level to meet the room's heating needs; when the temperature reaches the precise pre-set value, the heater does not switch fully off, but generates just enough output power to exactly match the thermal losses of the room.

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    To initially set up the Figure 10 circuit, set RV2 to mid-scale, raise the TH1 temperature to the desired mid-scale trip value, then trim RV1 so that the red LED flashes on and off at roughly a 1. When experimenting with this circuit, note that the thermal 'span' range of RV2 is determined by the R5 value, and the burst-fire thermal operating span is determined by the R3 value. Finally, note — when using burst-fire systems to control domestic electric heaters with built-in lamps — that the control system must be fed to the heater elements only, and must not be applied to the lamps.

    Triacs can be used to make very efficient lamp dimmers by using the 'phase-delayed switching' technique in which — in each power half-cycle — the triac is gated on at some controlled phase-delayed time after the start of each AC half-cycle, thus controlling the mean power fed to the lamp. All such circuits require the use of a simple L-C filter in the lamp feed line, to minimize RFI problems. The two most popular ways of obtaining variable phase-delay triac triggering are to use either a diac plus C-R phase delay network, or to use a special-purpose IC as the triac trigger.

    Figure 11 shows a practical diac-triggered lamp dimmer, in which R1-RV1-C1 provide the variable phase-delay. This circuit is really a simple variant of the basic lamp dimmer circuit shown in last month's Figure 6 , with the addition of the L1-C2 RFI suppressor and with RV1 and SW1 ganged together to easily enable the lamp to be turned fully off. A weakness of the simple Figure 11 design is that it has considerable control hysteresis or backlash, e. This backlash is caused by the diac partially discharging C1 each time the triac fires.

    Backlash can be greatly reduced by using the 'gate slaving' technique in Figure 12 , in which the diac is triggered from C2, which 'follows' the C1 phase-delay voltage but protects C1 from discharging when the diac fires. If desired, the backlash can be reduced to virtually zero by wiring a current-limiting resistor in series with the diac, to reduce the magnitude of the C2 discharge voltage, as shown in Figure A 'smart' lamp dimmer IC Many modern lamp dimmers have their triac driven via a dedicated 'smart' IC that can turn the lamp on or off or control its brilliance, the IC taking its action commands via a touch-sensitive pad or push-button input switch.

    For many years, Siemens was the leading producer of this type of IC, first with the IC known as the SB, and then starting in with the SLB, which remained in full production until but was still widely available in early The HTB is an eight-pin DIL IC with the outline and pin notations shown in Figure 14 , and provides four levels of brightness plus switch-off control, all sequentially selectable via a simple metal 'touch' pad or plate that is AC coupled to pin 4 of the IC.

    In use, the first 'touch' turns the lamp on at its lowest brighness level; the next three touches bring the brightness up to maximum level in successive stages; the fifth touch turns the lamp full off, and so on. Figure 15 shows the basic application circuit of the HTB without fused overload protection or RFI-suppression circuitry , with alternative component values and pin connections shown for use with 50Hz or 60Hz AC supplies with nominal values of V or V. Note that R1 is a 2W type and needs a value of 22K on V AC supplies or 47K on V AC supples, that pin 6 must be tied to pin 7 on 50Hz supplies or pin 3 on 60Hz supplies, and that the ratings of triac Q1 must be chosen to suit the lamp power and supply-voltage rating of the individual system.

    Triac Principles and Circuits — Part 2

    In use, triacs must always have an RMS current rating greater than that of the load that they are driving and must always be protected against catastrophic damage from current surges or malfunctions in their loads. Adequate protection can usually be obtained via a suitably-rated quick-blow fuse that is effectively connected either directly or via a supply-connection plug in series with the load and the triac's main terminals, but in a few special applications, additional protection may also be needed.

    Note that the fuse value must always be chosen with great care, and should be of the minimum practicable rating; a fuse with too high a rating provides no useful protection. When a triac is used in an electric-heater driving circuit, a quick-blow fuse with a current rating greater than that of the heater but less than the maximum current rating of the triac provides adequate protection. When a triac is used in an electric-motor driving circuit, a quick-blow fuse with a current rating greater than the stalled current rating of the motor but less than the maximum current rating of the triac should be used.

    In most filament-lamp driving triac circuits, the triac needs a current rating at least three times greater than the normal running current of the lamp, and should be protected by a quick-blow fuse with a rating of mA 1A absolute maximum per W of lamp rating in V AC systems, or 1A 2A absolute maximum per W of lamp rating in V AC systems; in some special filament-lamp driving circuits, however, additional protection may also be needed, as described later in this article.

    To understand the principles of triac protection in filament-lamp driving circuits, it is necessary to understand certain characteristics of fuses, filament lamps, and triacs as follows. An ordinary 'quick-blow' fuse consists of a short length of wire, which burns out 'blows' if the current passing through it exceeds a limit determined by the wire's diameter.