Why Would an LED "Bulb" Be Non-Dimmable? - Page 1
Forum section: electronics / projects

author: xrunner
author_rank: Super Contributor
timestamp: May 25, 2013, 06:55:13 pm
content: The pic of the package is attached. I don't have them on a dimmer, but I had been wondering why they would say it was non-dimmable? I know that LEDs can be PWMed and also be run at as low of a current as you want. So why do you think they say that on the package? What would happen if you tried it on a dimmer?

author: Arp
author_rank: Unknown
timestamp: May 25, 2013, 07:01:21 pm
content: Reckon if you run the leds directly from DC it will work with PWM but through the 220/110v AC/DC converter inside the lamp there are capacitors and stuff that prevents it

author: c4757p
author_rank: Super Contributor
timestamp: May 25, 2013, 07:21:04 pm
content: Yep. It's not an LED, it's LEDs hooked up to a switch mode power supply/controller. That controller is going to try to give the brightness it's designed to give regardless of the dimmer TRIAC mucking about with its AC input, and will likely end up destroying itself in the process. Standard dimmers are only for resistive loads.

author: mariush
author_rank: Super Contributor
timestamp: May 25, 2013, 07:23:00 pm
content: The electronics inside probably use something equivalent to Active PFC, boosting the input voltage to about 350v-400v, or it simply needs at least 90v or so to work right. Then a high frequency transformer is used to get whatever voltage is needed to drive those 9 leds, with a suitable controller to limit the current going through them.Dimming is done by adjusting the ac voltage going to the lamp, so the lamp would have to work with wide range of voltages and also vary the current going through the leds to give the luminosity the person expects (led brightness doesn't always decrease linearly like a regular lamp's brightness decreases).It just adds a lot of complexity to the design.

author: Rufus
author_rank: Super Contributor
timestamp: May 25, 2013, 07:26:53 pm
content: Quote from: xrunner on May 25, 2013, 06:55:13 pmMains incandescent lamp dimmers apply a varying proportion of each half mains cycle to the lamp and rely on the thermal capacity of the filament to provide a constant light output.Mains LED lamps try feed a constant current to the LEDs over the whole of the mains cycle and only need energy stored in a capacitor to cover the small gap around zero crossing.Dimmable mains LED lamps need much more stored energy and circuitry to detect what proportion of the mains cycle they are being fed with to control the current through the LEDS.If you run a non-dimmable lamp on a dimmer I imaging flicker will be very noticeable and effectively exposing them to turn on transients 100 or 120 times a second will damage them, possibly within minutes.

author: muvideo
author_rank: Frequent Contributor
timestamp: May 25, 2013, 08:17:09 pm
content: Dimmable led drivers I've seen are high pf flyback,the control IC usually can digest (or be adapted towork with) "sliced" voltages.Apart topology and driver, the other concern iskeeping the triac in conduction once fired, sincethe holding current of the triac can be higher thanworking current of the lamp, this results in triacturning off too early and generates flicker and instability.Another problem is damping the voltage spikegenerated by the triac at turn-on, generated bymains wiring inductance, this also can turn off thetriac soon after turn on.A bleeder and damper can be simple capacitors andresistors but dissipate some power, or be morecomplex (and costly) designs that minimize impacton efficiency.