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.
In: 14
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.
Latest Answers