Drew

Light Dimming Circuits

A Triac is what we want. Only problem with it is that when used on inductive loads, such as an electric fan, you need a snubbing circuit on it to prevent extreme voltage spikes from the inductor. Snubbing circuits are real simple though ( as simple as an RC circuit) so that shouldn’t be hard. Alternatively, we could use an SCR( similar to a Triac), but Triacs are the low cost alternative. Interestingly enough while researching this I found that SCR’s are used for battery chargers.

The parts should be cheap, less than $5. Might have to buy in bulk though.

In Detail,

  • Triac

From

A TRIAC, or TRIode for Alternating Current is an electronic component approximately equivalent to two silicon controlled rectifiers (SCRs/thyristors) joined in inverse-parallel (paralleled but with the polarity reversed) and with their gates connected together. This results in a bidirectional electronic switch which can conduct current in either direction when it is triggered (turned on). It can be triggered by either a positive or a negative voltage being applied to its gate electrode. Once triggered, the device continues to conduct until the current through it drops below a certain threshold value, such as at the end of a half-cycle of alternating current (AC) mains power. This makes the TRIAC a very convenient switch for AC circuits, allowing the control of very large power flows with milliampere-scale control currents. In addition, applying a trigger pulse at a controllable point in an AC cycle allows one to control the percentage of current that flows through the TRIAC to the load (so-called phase control).

Low power TRIACs are used in many applications such as lightdimmers, speed controls for electric fans and other electric motors, and in the modern computerized control circuits of many household small and major appliances. However, when used with inductive loads such as electric fans, care must be taken to assure that the TRIAC will turn off correctly at the end of each half-cycle of the ac power. A snubber circuit is often used to assist this turn off. Snubber circuits are also used to prevent premature triggering. For higher-powered, more-demanding loads, two SCRs in inverse-parallel may be used instead of one TRIAC. Because each SCR will have an entire half-cycle of reverse polarity voltage applied to it, turn-off of the SCRs is assured, no matter what the character of the load.

  • SCR

From

A silicon-controlled rectifier (or semiconductor-controlled rectifier) is a 4-layer solid state device that controls current flow. The name "silicon controlled rectifier" is a trade name for the type of thyristor commercialized at General Electric in 1957.

SCRs are made with voltage ratings of 50 volts to 2500 volts, and with current ratings up to 3000 amperes per device. SCRs are used in power switching, phase control, chopper, battery chargers, and inverter circuits. Industrially they are applied to produce variable DC voltages for motors (from a few to several thousand HP) from AC line voltage. They can also be used in some electric vehicles to modulate the working voltage in a Jacobson circuit. Another common application is phase control circuits used with inductive loads. SCRs can also be found in welding power supplies where they are used to maintain a constant output current or voltage. Large silicon-controlled rectifer assemblies with many individual devices connected in series are used in high-voltage DC converter stations.

Much smaller SCRs are also used as ESD (electrostatic discharge) protection circuits on modern CMOS integrated circuits (see literature).

A typical SCR based light dimmer which dims the light through phase angle control. This unit is wired in series with the load. Diodes (D2, D3, D4 and D5) for a bridge which generates DC with lots of ripple. R and C form a circuit with a time constant, as the voltage increases from zero (at the start of every halfwave) C will charge up, when C is able to make ZD conduct and inject current into the SCR the SCR will fire. When the SCR conducts then D1 will discharge C via the SCR. The SCR will shut off when the current falls to zero when the supply voltage drops at the end of the half cycle, ready for the circuit to start work on the next half cyc

From

This little circuit can be used to dim lights up to about 350 watts. It uses a simple, standard TRIAC circuit that, in my expirience, generates very little heat. Please note that this circuit cannot be used with fluorescent lights.

Part / Total Qty. / Description
R1 / 1 / 50K Pot
R2 / 1 / 15K 1/2W Resistor
C1, C2 / 2 / 0.068 250V Capacitor
L1 / 1 / Lamp To Be Controlled (up to 350 watts)
L2 / 1 / Neon Lamp
TR1 / 1 / 40502 TRIAC
MISC / 1 / Case, Knob, Heatsink For TR1, Wire, Socket For L1
  • Snubber

From

A snubber is a simple electrical circuit used to suppress ("snub") electrical transients. Snubbers are frequently used with an inductive load where the sudden interruption of current flow would lead to a sharp rise in voltage across the device creating the interruption. This sharp rise in voltage might lead to a transient or permanent failure of the controlling device.

In a similar fashion, a snubber can also be a mechanical device that limits pressure surges on measurement devices. These connect between the process and the measurement device allowing a relatively slow change in of pressure to limit damage to the hardware. Such devices are common in industrial environments.

RC snubbers

Frequently, a snubber can consist of just a small capacitor in series with a small resistor. This combination can be used to suppress the rapid rise in voltage across a thyristor, preventing the erroneous turn-on of the thyristor; it does this by limiting the rate of rise in voltage (dV/dT) across the thyristor to a value which will not trigger it. Snubbers are also often used to prevent arcing across the contacts of relays (and the subsequent welding/sticking of the contacts that can occur). An appropriately-designed RC snubber can be used with either dc or ac loads.