LED Dimming |

Dimming LEDs


An LED is a semiconductor device, just like that used in microchips and electronic equipment. It is manufactured in a totally different way to conventional lights. The colour of an LED is not an after-thought added in the final stage, or even after manufacture as it often is with other types of light. The colour of an LED is set by the particular chemicals used during the fundamental manufacture of the device. This also affects the manufacturing process and the voltage the device operates at. As a result the circuit in which it is to be used has to be designed for a specific type, size and colour of LED.

DIMMING THEORY
Lights are designed to work on a particular typical/maximum voltage, on which they draw a particular current.They therefore have a particular power and light output rating. If they present a resistive load (linear or non-linear) then if the voltage is reduced (usually a simple thing to achieve), the current reduces, hence the power and light output reduce and dimming is achieved. LEDs are like fluorescent lights in that they need a fixed voltage to operate, so if you employ a conventional voltage dimmer they will very quickly cut out.

LEDs
The V/I (resistance) curve of an LED is very steep, this means that for a very small change in voltage there is a large change in current, and therefore in light output. So the voltage across the LED essentially has to remain constant and the current has to be varied in order to dim. The latest high power LED light engines actually operate from constant current supplies. If this current is lowered then the LED will dim.

12/24Vdc SYSTEMS
The voltage across an LED is dependent on the manufacturing process, chip size and colour, but lies in the region of three to four volts. Constant current supplies are relatively expensive so conventional medium power LEDs are still operated from constant voltage supplies. In order to make the supply standard, and independent of the basic LED, a number of LEDs (3 for 12V, 6 for 24V) are connected in series with a balancing resistor. The value of the resistor is dependent on the particular LED and supply voltage. An LED lighting product will consist of a number of these individual, series connected, LED chains.
Because of the resistors in the chain of LEDs, the whole product can, to some extent, show the characteristics of a resistive load. Therefore some dimming can be achieved by lowering the supply voltage. There is a bottom limit where the voltage is no longer high enough to maintain the necessary forward voltage across the LEDs and dimming will quickly change into the off state.
There is no point in connecting your 12V or 24V power supply via a mains dimmer; this will only alter the input voltage not the output. Most dc supplies are rated to work down to 100Vac and so the dimmer will have no effect. If you drop the supply below the minimum then the unit will simply cut out.

12Vac SYSTEMS
Low voltage lighting systems operate from 12Vac and there are LED replacement bulbs that will also operate from this type of supply. A lighting transformer is used to derive the low voltage from the mains supply. Some of these transformers are electronic and state that they are dimmable. This means that when connected to a thyristor dimmer they transmit the ‘dimmed’ power cycle through to the load at the reduced voltage. This is designed to work with traditional lamps and may work with LED lights. The LED lights designed to retrofit into these low voltage lighting systems contain electronics to condition the 12Vac supply so that it is suitable to drive LEDs. This conditioning may prevent the bulbs from being dimmed using the supply voltage.

230Vac SYSTEMS
230V products containing many LEDs may have their LEDs arrange in long chains similar to low voltage dc products. They will also have rectifiers in line to alter the supply to half wave. Because of the large number of LEDs in the chain, the resistor forms a relatively minor part of the circuit and so a voltage dimmer will have limited success. These products will often respond quite well to the higher quality thyristor dimmers, which chop the supply.
Other 230V products, such as bulbs, may condition the supply internally in order to supply the low dc voltage that the LEDs require. As a result they tend not to respond kindly to the use of dimmers.

HOUSHOLD RESISTIVE DIMMERS
These devices are designed to dim incandescent and halogen bulbs by dropping the voltage supplied to the bulb. Because LEDs require a fixed low dc voltage, this type of dimmer cannot be used with LED products.

HIGH QUALITY THYRISTOR DIMMERS
This higher quality household dimmer uses an electronic circuit to chop the mains supply. This means that the power is applied 100 times a second, for a period between 0 and 1/100th of a second as set by the control. This kind of dimmer is compatible with ‘dimmable’ lighting transformers for low voltage ac systems. Because full power is applied to the lamp, for part of the time, these dimmers will work with some kinds of LED products. Those where the power is applied directly to the chain of LEDs will usually respond to this style of dimming. When the dimming gets below 50% the remaining ac cycle being applied to the lamp will not be sufficient to power it and it will quickly turn off. LED products where the power is further conditioned will not be affected by these dimmers; this is usually those with 230V and 12V bulbs.

PULSE WIDTH MODULATION DIMMERS
This is the professional way of dimming LEDS. The full dc voltage is applied to the product for a very short time, thousands of times per second. The LEDs effectively flash, too quickly for the eye to see, the resultant brightness depend on the time the power is applied. This kind of dimmer should successfully dim, all the way from full to off, most LED products designed to operate direct from dc voltages. Again, if there is further power conditioning included within the product then this is likely to negate the dimming effect. Pulse width modulation is inherently incompatible with ac products, although thyristor dimmers offer a similar solution.

CONSTANT CURRENT DIMMERS
The latest high power light engines are designed to operate from constant current sources. It is also possible to operate other dc products from this type of supply. Basically the supply will apply the voltage necessary to achieve the required current. If this current can be varied then the supply can be used as a dimmer.