How modern light dimmers work
How modern light dimmers work ?
Solid-state light dimmers work by varying the "duty cycle" (on/off time) of the full AC voltage that is applied to the lights being controlled. For example, if the voltage is applied for only half of each AC cycle, the light bulb will appear to be much less bright than when it get the full AC voltage, because it get's less power to heat the filament. Solid-state dimmers use the brightness knob setting to determine at what point in each voltage cycle to switch the light on and off.
Typical light dimmers are built using thyristors and the exact time when the thyristor is triggered relative to the zero crossings of the AC power is used to determine the power level. When the the thyristor is triggered it keeps conducting until the current passing though it goes to zero (exactly at the next zero crossing if the load is purely resistive, like light bulb). By changing the phase at which you trigger the triac you change the duty cycle and therefore the brightness of the light.
The advantage of thyristors over simple variable resistors is that they (ideally) dissipate very little power as they are either fully on or fully off. Typically thyristor causes voltage drop of 1-1.5 V when it passes the load current.
What are thyristors and triacs
A Silicon Controlled Rectifier is one type of thyrister used where the power to be controlled is unidirectional. The Triac is a thyrister used where AC power is to be controlled.
Both types are normally off but may be triggered on by a low current pulse to an input called the gate. Once triggered on, they remain on until the current flowing through the main terminals of the device goes to zero.
Both SCRs and Triacs are 4 layer PNPN structures. The usual way an SCR is described is with an analogy to a pair of cross connected transistors - one is NPN and the other is PNP. +------+
If we connect the positive terminal of a supply to say, a light bulb, and then to the emitter of the PNP transistor and its return to the emitter of the NPN transistor, no current will flow as long as the breakdown voltage ratings of the transistor are not exceeded because there is no base current to either.
However, if we provide some current to the base of the NPN (IG(+)) transistor, it will turn on and provide current to the base of the PNP transistor which will turn on providing more current to the NPN transistor. The entire structure is now in the on state and will stay that way even when the input to the NPN's base is removed until the power supply goes to 0 and the load current goes to 0.
The same scenario is true if we reverse the power supply and use the IG(-) input for the trigger.
A Triac works basically in a similar manner but the polarity of the Gate can be either + or - during either half cycle of an AC source. Typically the trigger signals used for triggering triacs are short pulses.
Solid-state light dimmers work by varying the "duty cycle" (on/off time) of the full AC voltage that is applied to the lights being controlled. For example, if the voltage is applied for only half of each AC cycle, the light bulb will appear to be much less bright than when it get the full AC voltage, because it get's less power to heat the filament. Solid-state dimmers use the brightness knob setting to determine at what point in each voltage cycle to switch the light on and off.
Typical light dimmers are built using thyristors and the exact time when the thyristor is triggered relative to the zero crossings of the AC power is used to determine the power level. When the the thyristor is triggered it keeps conducting until the current passing though it goes to zero (exactly at the next zero crossing if the load is purely resistive, like light bulb). By changing the phase at which you trigger the triac you change the duty cycle and therefore the brightness of the light.
The advantage of thyristors over simple variable resistors is that they (ideally) dissipate very little power as they are either fully on or fully off. Typically thyristor causes voltage drop of 1-1.5 V when it passes the load current.
What are thyristors and triacs
A Silicon Controlled Rectifier is one type of thyrister used where the power to be controlled is unidirectional. The Triac is a thyrister used where AC power is to be controlled.
Both types are normally off but may be triggered on by a low current pulse to an input called the gate. Once triggered on, they remain on until the current flowing through the main terminals of the device goes to zero.
Both SCRs and Triacs are 4 layer PNPN structures. The usual way an SCR is described is with an analogy to a pair of cross connected transistors - one is NPN and the other is PNP. +------+
If we connect the positive terminal of a supply to say, a light bulb, and then to the emitter of the PNP transistor and its return to the emitter of the NPN transistor, no current will flow as long as the breakdown voltage ratings of the transistor are not exceeded because there is no base current to either.
However, if we provide some current to the base of the NPN (IG(+)) transistor, it will turn on and provide current to the base of the PNP transistor which will turn on providing more current to the NPN transistor. The entire structure is now in the on state and will stay that way even when the input to the NPN's base is removed until the power supply goes to 0 and the load current goes to 0.
The same scenario is true if we reverse the power supply and use the IG(-) input for the trigger.
A Triac works basically in a similar manner but the polarity of the Gate can be either + or - during either half cycle of an AC source. Typically the trigger signals used for triggering triacs are short pulses.
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