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Triac based Lamp Dimmer power controlThis is a phase angle control of a Triac which is a
evolution from an SCR. The 50 Hz or 60 Hz Sine wave of mains can
be turned on at any point after the Zero Crossing The Voltage ramps up in the sine wave which gives a near "Linear" slope which can be used to trigger the ON at a time delay after zero crossing when the voltage is zero. This is like a PWM but works on low frequency only. Some of the early SMPS(HV for TV and Mains Inverter) by Siemens were built around SCRs. Turning off a SCR is a difficult job for a designer, MOSFETS and IGBT are now used for PWM drive stages. You can use it with a bulb to vary brightness of bulb,
this is a live circuit it can give a shock, enclose in plastic box, the
pot should have a plastic knob, use a fuse, you can also use it for
temperature control of soldering irons. See an Application Phase
control dimmer Ultralite Micro The Source file in CadSoft EAGLE format is here del00004.zip 2N2646 based Voltage controlled SCR PulserA Center tap 50Hz Step Down Transformer with two diodes is used to get a train of Positive Sine Pulses at 100Hz which is applied to Q6 base via R51. 2N2646 is a unijunction transistor(UJT) in a TO-18 metal package. The control voltage or voltage proportional to error is fed to R56-Q3, The UJT drives a pulse transformer which provides isolation from the Load which the SCR bridge is controlling. This phase angle control gives a near Linear closed loop control for a SCR bridge which may be used in Electroplating or a Preregulator of a big power supply.  |
