Does LED flashlight emit EMI ? - Page 1
Forum section: electronics / beginners

author: BravoV
author_rank: Super Contributor
timestamp: December 16, 2020, 03:57:29 pm
content: As common ones use power switching inside to power the LED.Say the whole metal body is totally shielded, connected to battery negative as usual, and uses head twist mechanism to turn it on/off, so no rubber clicky switch at the rear as RF can pass thru it, also the reflector is metal that is electrically connected to the front of the body, and in contact/touching with the LED base heatsink, say a copper that also connected to the body, so no gap what so ever.

author: radiolistener
author_rank: Super Contributor
timestamp: December 16, 2020, 04:56:52 pm
content: Quote from: BravoV on December 16, 2020, 03:57:29 pmYes, if it uses switching power regulator, it emits EMI.Some cheap LED is so bad that they can be used as a jammer for a short wave and VHF radioQuote from: BravoV on December 16, 2020, 03:57:29 pmit may not help from a strong switching noise. Because low frequency needs very-very thick shielding (at least several centimeters).The most complicated issue is to protect from magnetic fields at short distance.

author: tszaboo
author_rank: Super Contributor
timestamp: December 16, 2020, 05:01:25 pm
content: Everything emits EMI, even you. The question is if the EMI is within the expected limits.

author: Vovk_Z
author_rank: Super Contributor
timestamp: December 17, 2020, 10:59:48 am
content: Quote from: BravoV on December 16, 2020, 03:57:29 pmThe EMI (from switching) will be definitely highly shielded but I guess some amount I'm sure still will go out. (With an inductive or a capacitive coupling I don't know). The frequency range of a single pulse is extremely wide, so it doesn't need a very wide hole in a case to be emitted.When it is switched on and just lights (without switching off and on) - then of cause it doesn't have to emit EMI.

author: BravoV
author_rank: Super Contributor
timestamp: December 18, 2020, 10:14:27 am
content: Thanks for the replies, appreciate it.I guess its a no..no, as I had plan but where its sensitive to EMI, back to incand flaslight.Quote from: radiolistener on December 16, 2020, 04:56:52 pmJust for curiosity, say the circuit is switching at 1Mhz, how thick the shield needed ?

author: Siwastaja
author_rank: Super Contributor
timestamp: December 18, 2020, 03:50:54 pm
content: Switching frequency is, directly speaking, almost meaningless.This is because switching waveform is square wave like, containing frequencies only limited by rise/fall times. For high efficiency, rise/fall times are minimized, maximizing EMI.Indirectly, slower f_sw allows the designer to slow down edges in similar ratio while keeping switching losses unaffected, so slower f _swmaymean less EMI, as well.

author: tkamiya
author_rank: Super Contributor
timestamp: December 25, 2020, 05:03:23 am
content: I have never seen a flash light that has EMI problems. They are usually series connected batteries wired to a lamp of some kind via a switch. Indeed, 120v lamps do have inverters, those flash light bulbs do not.One can say anything above absolute zero temperature will emit something and yes, light itself is an emission - just very high frequency. But realistically speaking, the EMI we speak of is a harmful and recognizable interference emission we encounter for the purpose we intend. Those, there is none from flash lights LED or incandescent. You can have a very high gain op amp, turn on a flash light, and recognize nothing. You might hear a "zip" as the moment LED turns on or off. But not once they are in steady state. Those will be worse in incandescent as current consumption is more.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 25, 2020, 07:46:42 am
content: Quote from: tkamiya on December 25, 2020, 05:03:23 amLED flashlights that work with a switch-mode converter arereallycommon today, have been for a decade.Two different reasons:1) Flashlights that work from one 1.5V cell. They have a boost converter for the white LED requiring at least Vf=3.0V. Using one cell is convenient for the end-user as well as for the mechanical design.2) Higher-end models provide regulated current for the LED despite varying battery voltage (and varying Vf ~ temperature) so they run a DC/DC converter for better efficiency than limiting current with series resistors.Additionally, some flashlights use PWM dimming even if the LED itself runs with a standard series resistor. This is a possible EMI source as well, even though the frequency is small but no one knows what the edge rate is.The question whether they have EMI problems would boil down to the question, does it matter, has anyone measured? I could imagine a physically tiny (some millimeters) boost converter just centimeters away from the power source and the load simply won't produce enough EMI to cause real interference (enough to make someone investigate), even if the designer did lousy job.

author: BravoV
author_rank: Super Contributor
timestamp: December 26, 2020, 12:56:39 pm
content: Quote from: tkamiya on December 25, 2020, 05:03:23 amI guess you have been out of the loop for quite some time now, probably you are still thinking that LED flashlight like in early days that people used direct drive straight from stack of batteries without any circuit at all like incand ones, or minimalist linear based CC circuit.Now days, I think almost all LED based "flash light" use sort of switching circuit to power the LED.

author: hfleming
author_rank: Regular Contributor
timestamp: December 26, 2020, 05:26:10 pm
content: Well, even if everything is shielded, there is still capacitive coupling between your body and the flashlight, so your body will act as antenna....

author: BravoV
author_rank: Super Contributor
timestamp: December 26, 2020, 05:41:13 pm
content: Yep, thanks, that what I've recently learned, I guess the old incandescent flash light like the old Mag that has no circuit, just batteries, switch and bulb is the quite flashlight, right ?

author: madires
author_rank: Super Contributor
timestamp: December 26, 2020, 05:59:35 pm
content: They all create EMI in the 400 – 790 THz range.

author: Nusa
author_rank: Super Contributor
timestamp: December 26, 2020, 07:03:24 pm
content: You can buy LED bulbs for old mag lights.Another option, if you want to eliminate electricity entirely, is chem lights. Available in a wide variety of colors, brightness, and duration.Still another option, for outdoors, would be road flares. Or gas lanterns.I've got an old candle lantern around here as well. Used to be part of my camping gear back when one couldn't carry enough batteries for long-term light.

author: tkamiya
author_rank: Super Contributor
timestamp: December 26, 2020, 07:04:57 pm
content: Quote from: Siwastaja on December 25, 2020, 07:46:42 amPlease suggest one cheap one on Amazon. I'll buy it, test it, and tear it apart.

author: BravoV
author_rank: Super Contributor
timestamp: December 26, 2020, 07:42:22 pm
content: Quote from: tkamiya on December 26, 2020, 07:04:57 pmAny white LED based, especially can be powered by single cell alkaline, 1000% must use a boost converter in it. Just pick the cheapest one.