How can you create a solution for a LED constant current driver that can handle TRIAC dimming? Can you convert the waveform into a PWM and is there a simple solution?
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Triacs are typically used in mains powered devices because they rely on the zero-crossing of the mains current to switch off after each half cycle. Driving the triac with a PWM signal doesn't work. The pulse will switch the triac on, but the zero current condition to switch off is never met, so it will stay on forever. Controlling brightness of a constant current driven LED with PWM, for instance using a FET, is also a bad idea: current sources should never be unloaded, as they will build up a high output voltage in an attempt to keep the current flowing. Maybe not a problem for linear devices, as their maximum voltage is the input voltage, but a switcher's inductor can create voltages high enough to destroy the FET. |
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@stevenvh I agree about what you said, that is the triacs relay on zero crossing and if it get the trigger at any point it will be on for next zero crossing, but it is not that we cannot use the PWM signal. At first I too thought like the same, but after doing some experiments and little circuit designing I came to know that we can use a simple pwm signal by changing the optocoupler section of the circuit, with this we can control the voltages of the ac loads instead of using the phase control firing which is the traditional one, though both are effective and simple |
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There are several LED driver ICs designed to be TRIAC dimmable from TI, Fairchild and others - the circuitry is quite complex. Google Triac DImmable LED driver for some datasheets and appnotes describing the details. |
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