The title 😀
I would understand it for LED strips and so on, but if both bulbs are made for 230V (or whatever in US), why do they need different dimmers? Is there a difference in some dim range or something?
Thanks.
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LED bulbs do not run at 230V. They have a specific voltage they run at which is created from the original 230v by a driver circuit.
This driver circuit is usually not designed to dim the LED as a response to a voltage decrease or a cut-in-half wave.
To dim incandescent bulb, all you have to do is send less power to it, and it’ll glow less brightly. An old-style incandescent dimmer just adds some resistance so that the bulb gets less bright.
LEDs don’t work that way, unfortunately – if you don’t supply enough power to an LED, it just won’t light at all. In order to “dim” an LED, what you actually have to do is blink it on and off really fast – faster than a human eye can tell – and it’ll look less-bright overall. A dimmer that can dim LEDs is called a PWM dimmer – pulse width modulation. As you adjust the brightness up or down, a PWM dimmer adjusts how long the “on” pulses are compared to the “off” pulses, which adjusts how bright the light looks to us.
To grossly over simplify, very old dimmers adjusted the voltage/current running through the light. Lower voltage = lower current and thus less bright.
Newer dimmers keep the voltage the same but muck with the wave of the alternating current – they actually keep the circuit to the bulb off for part of the A/C wave and switch it on part way through. When/where in that cycle is up to the dimmer, but the end result is your light isn’t dimming because its getting less current run through it, its actually flickering on/off, its just doing this so fast that we can’t see it. Well, most of us, turn a light dimmer down really low and catch the light out of the corner of your eye you might see it flicker.
Anyways, there are various types of the new dimmers and how they adjust this cycle (switch the light on/off – there are leading edge and trailing edge if you want to dig in more) and some of them work with some types of LEDs and others don’t.
LEDs are ultra low power so some of the cheats for dimming an incandescent don’t work
Dimmers for LEDs need to consume less power themselves. Normally they leak some power through the light to keep themselves functional even when its off. On an incandescent this works fine because a few milliwatts won’t heat the filament and make it glow. On a 3 watt LED bulb this leakage power can charge the bulb and make it flash occasionally or glow dimly
The lower current of the LED is also a problem to drive. Most dimmers these days use a triac to chop up the line voltage and only send some of it to the bulb. The triac sits off, voltage hits the right point in the line cycle, triac turns on allowing current to flow, current drops to 0 at the end of the cycle and turns the triac off, then it sits and waits for a bit
Well LED bulbs use low enough current that they won’t hold some triacs on so instead of turning on halfway through the cycle and staying on until the end, the triac will turn on, stay on for a bit, then turn off well before the end of the cycle and cause cause all sorts of flickering in the bulb as it comes on and off.
Controlling a toaster is easy. Controlling electronics is finicky
LED light are not made for 230V, each element run on closer to 2.3V. So an LED bulb needs some way to reduce the voltage for the elements. But the main difference is that while traditional light bulbs have a very linear response to the voltage LED elements are almost purely on or off. The difference between fully on and fully off might be as low as 0.1V.
Traditional dimmers do not actually limit the voltage to the bulbs but rather turn the power off for parts of the cycle. If you dim the light to half strength the voltage is still 230V but the light bulb only get power 50% of the time. On very low settings you might notice a tiny flicker as the fillament cools a tiny bit between each cycle but not much. An LED on the other hand use so little power that even just the stray capacitance and inductance is enough to keep it powered for some time even after the dimmer cuts the power. So you need to turn the dimmer all the way down for it to have an effect at all. And then the LED will not just glow a bit darker but it will turn fully off and fully on for each cycle turning it into a strobe.
There are multiple ways of fixing this. Some LED light have electronic regulators in them and these can detect the precense of a traditional dimmer and regulate the light output to match. This is why you see some LED bulbs labeled as dimmable. The dimmers you see labeled as working with LED lights do not reduce the power by turning the power off for parts of the cycle, or even directly regulate the voltage down. Rather it limits the power in the circuit thorugh a variable capacitor. This allows it to regulate the power down to exactly the voltage where the LED turns off and will keep the voltage right in that narrow band. These do also work on traditional lights although the voltage band will be significantly greater.
An incandescent bulb is a resistor, it can tolerate just about any form of dimming. If you cut the voltage in half it will use about 1/4 the power.
Lets say you have an LED with a forward voltage of 3V, if you put 1.5V accross it it won’t turn on at all, if you put 4V across it it will be destroyed. They need some mechanism to regulate current. The LED needs almost the same amount of voltage for full brightness and 1% brightness, just with different amounts of current, so a dimming compatible bulb must be able to figure out what brightness is desired, and also run on a much wider and noisier range of input voltage, and then regulate the current appropriately.
LED’s run on much lower voltages than the 110 Volts that services your home. Most of the lightbulb’s base is a transformer/rectifier that converts alternating power from the light socket to a much lower DC voltage.
LEDs have only one brightness. To “dim” the bulb, a small microcontroller pulses the LEDs on and off at a frequency far higher than the eye can discern. By varying the proportion of on time during each on/off cycle, you control the perceived brightness.
It’s called “duty cycle”. The LED is brightest when it is on 100 percent of the time. At a 50 percent duty cycle, it is about half as bright, etc.
The dimmer sends a signal to the microcontroller telling it what brighten thr user desires, and the microcontroller responds by shortening or lengthenjng the duty cycle.
Whatever you do to the AC power to dim it is completely different to what you do to the LED to dim it.
Take a leading edge dimmer for example – AC power cycles through from max +ve voltage to max -ve voltage and back several times per second (usually 50-60 times depending on country). A leading edge dimmer waits for the wave to reach zero, then keeps the power off until it gets part way into the cycle then turns it on. to get half power, it only turns on the power from the top of the curve to the zero point (There’s some pretty graphs of what the looks link in my link)
There’s also trailing edge dimmers that do exactly the same, except they start from the top instead of the zero. Basic old style dimmers often just reduce the overall voltage like a resistor – this works because incandescent bulbs glow brighter the more voltage you give them until they get too hot and burn out. It doesn’t work for electronics with a power conversion to DC because they can either take in a reasonably wide range of voltage and convert it to much lower voltage DC or they can’t work at all outside a defined range.
To dim an LED, you usually just turn it on and off really fast – generally between 500 and a few thousand times per second (compared to 50-60 for AC power). To get different brightness’s, you change how much of the cycle is “on” vs “off” (this is called the duty cycle) – 50% brightness means it’s on 50% of the time and off the other 50%. 25% brightness means 25% on and 75% off (Some controllers might have 100% at less than 100% on but the proportions of max are still the same)
To make an LED dim by acting on the AC input, the controller needs to be recognise the type of dimming signal it is receiving (leading / trailing edge), continue to supply the full voltage needed for the controller and LED and translate the dimming signal coming in to the right duty cycle to make the LEDs do what you want. Reducing the overall voltage is a problem for this because the voltage can vary anyway for a number of reasons and you need a minimum level to make the circuit work.
All this circuitry is very complicated compared to a basic AC powered light – the simplest LED driver from AC can be as simple as chaining a few LEDs together in sets – that add up to a higher voltage, say 9x 5v modules to get 45v overall then put a resistor in to consume the remaining voltage from AC and 4 diodes at the start to convert the negative voltage half of the AC cycle into positive (called a rectifier). This will give you an LED light that turns on and off 100-120 times per second (twice the AC frequency) and doesn’t need so much as a single transistor, let alone any programmable controllers.
The reason you can use a normal dimmer is that normal globes brightness is based on the voltage so you add some resistance and the brightness lowers. LEDs are a semi conductor they will glow at a voltage based on the construction of the led and the brightness is affected by the current through it.
There are some factors like AC and high voltage but they could be easily accounted for in the design if not for the current dependant brightness.
Pulse width modulation (turning on and off fast) is more because it is cheaper and easier than building a current controlled supply.
I taught electricity design
To add to some of the existing comments on here-
A LED and a normal bulb are inherently different.
To supply some background- LED stands for light emitting diode. Diodes are constructed by creating a PN (positive/negative) junction, with a depletion layer in the middle. This means that one material (called P-type) has an abundance of positive charge carriers (holes) and one material (N-type) has an abundance of electrons. The depletion layer can be considered neutral territory. When voltage is applied, the ‘holes’ are pulled across the depletion layer.
It is only when a sufficient number of holes are pulled that we consider the gap “bridged” to complete the circuit and, in the case of an LED, turn on the light (this is called the “band gap”). If this voltage is not met, the circuit is not completed and the light is off- there is no dimming effect. If it is, we can think of the diode as a regular old wire (for this application).
An incandescent bulb does not operate on this idea of bridging a gap to complete a circuit. The gap is always bridged, through a very small bit of wire. As electricity flows through this tiny wire, it creates heat. Enough heat will make light- similar to how a wook poker glows when you leave it in the coals too long.
By increasing the amount of current, an incandescent bulb will glow brighter, but an LED will stay approximately the same brightness. By decreasing the amount of current an incandescent bulb will become dimmer, while an LED will stay the same or turn off.
So we know to change the brightness of an incandescent bulb, we adjust the flow of current (usually done by increasing resistance in the dimmer switch). For an LED, we know that the voltage doesn’t really matter as long as we exceed the band gap voltage. Once that is met, the current flows like a regular wire. So to dim it, as others have mentioned, the light is turned on and off very fast. Faster than the human eye can see. Our eyes effectively take an ‘average’ of what it sees, so this rapid on/off of the light appears dimmer!
This rapid on/off is called pulse width modulation, or PWM for short. By this, people are talking about changing the percentage of time the LED is on (a pulse of voltage).
Because PWM is a bit more complicated than adjusting current, a different dimmer switch is required.
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