Zero Crossing Triac Driver Circuit

Zero Crossing Triac Driver Circuit' title='Zero Crossing Triac Driver Circuit' />SCR Phase Control Speed ControlDimmer. This SCR phase control works much like the common TRIAC dimmer, but has numerous advantages including increased current capability, robustness and absence of minimum voltage snap on. A complementary, symmetrical trigger circuit consisting of two PUTs programmable unijunction transistors enables firing of two anti parallel THYRISTORs SCRs. The circuit makes up a two terminal power device that is simply inserted between the AC power source and load. Besides controlling the intensity of incandescent lighting, it is useful in controlling the speed of universal commutator brush type AC motors. Bill of Materials. Bill of Materials File. Buat Program Penjualan Di Excel on this page. The ideal triggering device. The DIAC is a 2. 8V bidirectional bilateral trigger device that is used on virtually all inexpensive phase controls. The trigger voltage is somewhat high for phase control of 1. VAC. Years ago, there was a similar low voltage 6 to 8. V trigger device called a Shockley diode. Unfortunately, these never caught on and today are EXTINCT. The 2. N6. 02. 7 Programmable Unijunction Transistor PUT can perform a similar function, but is polarity sensitive so it does not lend itself to TRIAC control. However, if two such PUT trigger circuits are employed for anti parallel SCRs, some interesting things are apparent. Most important is the ability to control both trigger circuits with a single potentiometer so that both half cycles are controlled identically. To obtain best balance, the zeners and capacitors must be matched. Both zeners and capacitors are specified for a tolerance of 5, but I selected mine for better than 1 with my DMM. Purchase a few additional components so that you may obtain a good match. PUT operation is simple. Its threshold voltage is programmable so that it can trigger at low voltages. In this circuit it is set via the 1. V zeners. When the PUT anode voltage exceeds the gate voltage by one junction drop, the PUT fires and dumps the timing capacitor into the SCR gate circuit. It resets when the AC line voltage reverses. I made a PUT relaxation oscillator flasher circuit that you may wish to experiment with http www. Snap on. The typical DIAC controlled TRIAC dimmer tends to snap on at the minimum voltage when the adjustment is slowly increased. After snap on, the voltage may be reduced if desired. The snap on phenomenon is caused by the lagging phase relationship of the AC voltage signal that appears across the timing capacitor before the DIAC threshold is reached. After it initially fires, the timing starts at line voltage zero crossing. The timing circuit in this control completely avoids this problem by resetting the capacitor voltage each half cycle via reverse diodes across the two timing capacitors. Advantages of SCRs over TRIACs. Thermally, there are numerous advantages. Splitting the output current in half reduces the device current significantly and provides a much lower thermal resistance to the ambient. Also, SCRs are rated for a maximum Tj of 1. TRIACs may be limited to only 1. C. Then, of course, SCRs are available in current ratings extending to hundreds of amps well over ten times that of the largest TRIACs. SCRs are simply more rugged. Its only disadvantage is obvious it simply takes more hardware and circuitry. The snubber. R1 C3 form a snubber that is connected across the power devices. It performs two important functions as follows First it provides a circuit to absorb reactive energy when either SCR recovers stops conducting current. As you may know, a rectifier conducts current in the reverse direction for a short period of time e. S when it becomes reversed biased. The flow of this current stops abruptly and any series circuit inductance then causes the generation of a transient voltage spike. The snubber is a place for this current to go so it cannot develop a high voltage. Second, it provides a resistive load for high frequency line noise and transients. This FM transmitter circuit is a quite fun project for electronics beginners, so heres a circuit with the 2SC9018 transistor. It uses the 2SC9018 high frequency. Fairchilds Knowledge is Power Community offers an online forum for technical discussions and support, including a growing knowledgebase of frequentlyasked questions. Zero Crossing Triac Driver Circuit' title='Zero Crossing Triac Driver Circuit' />Such noise or transients can potentially cause an SCR to turn on for a half cycle. One common source of transients is simply the power switch when it closes. Film resistors are not robust in handling voltage transients so I specified a ceramic composition resistor. Polyester capacitors have poor reliability at 2. VAC, so I specified a polypropylene capacitor that is AC rated. In my circuit, I used a Quencharc device that contains both resistor and capacitor it is AC rated for 1. Zero Crossing Triac Driver Circuit' title='Zero Crossing Triac Driver Circuit' />VAC. Oscillographs. The load voltage is easy to see here as it increases in phase. Soul Reaver Torrent Pc Italia. The base line is rather indistinct due to the low power load used 7. W lamp the snubber reactance causes significant voltage drop across the lamp. The gate current indicates a peak current of about 2. A and a time constant of about 3. S essentially 4. The leading edge spike is caused by the inductance of the 1 shunt resistor used. This can easily drive a much larger SCR the first one I made used a 9. A thyristor doubler. The minimum gate current pulse width rule of thumb is 5S in which the gate current exceeds Igt gate current threshold this allows sufficient time for the load current to ramp up to the SCR holding current. The gate voltage has a different appearance than the output of the programmable unijunction for two reasons First, the gate is a non linear resistance that tends to act like 2 or 3 series diodes. Secondly, when the SCR is conducting, junction voltage appears on this terminal as well. SCR Phase Control Project Photos. Note the absence of the isolation transformer it is located under the bench. Semiconductor relays explained with example circuits to build your own. Skip to Bottom. OA91A NTE109 Glass Germanium Diode OA95 NTE109 Glass Germanium Diode OA95 NTE177 General Purpose Silicon Rectifier. In this post we learn through two examples how to build a simple light dimmer switch circuit for controlling light intensity with pot, using the principle of triac. Zero Crossing Triac Driver Circuit' title='Zero Crossing Triac Driver Circuit' />For the future. AC Switch Circuit turn this into an AC switch via a photomod.