Future Concerns

[DanG]

Due to the high costs, both financial and environmental, the incandescent light bulb fell out of favor and was banned.  I disagreed with the banning for several reasons, but that is neither here nor there.  It was(is being) replaced by compact flourescents, which are now coming under fire because of still more environmental concerns.  Now, the talk is that we will turn to LEDs for our future lighting needs, which brings me to the point of this post.  I have heard absolutely no criticism of LEDs except for the initial expense, which should resolve itself in time.  Well, just the other evening, I noticed something that mildly disturbs me.  I have this really good LED flashlight, you see, and it lives right beside my recliner.  It is a Black&Decker rechargeable model with a whole bunch of candlepowers, or "Lumens" as they like to call them these days.  Let me digress for a moment to tell you that we recieve our TV signal via an old "knuckledragger" antenna.  It ain't very good, but it is free. ;D 8)  Anyhoo, a few evenings ago, my Darlin' Bride borrowed my flashlight to look for something that may(or may not) have been shoved under the furniture by the cats.  When she turned on the light, the sound went off on the TV.  I didn't immediately associate it with the flashlight, but it became obvious after it happened several times in the space of a minute or two.  She didn't find whatever it was, so she went out to look in the truck and it still happened when the flashlight was outside.  This brief, unintentional experiment tells me conclusively that these Light Emmitting Diodes are emmitting something besides light.  If a simple 12 volt flashlight can disrupt a television signal when it is 40 or 50 feet from the antenna, what is a whole house full of LEDs going to do to your cellphone or wifi signals, or worse yet, to your brain or skin?  I just wonder if any testing at all will be done before the mandate to replace all of the CFLs with LEDs comes down upon us.

[Jeff]

I'd noticed the same thing here at thecabin, and looked it up.  Its electromagnetic interference caused by a cheep flashlight. Better flashlights (more expensive) have suppression components built in. Seems that this is an unregulated type thing, so, what you are getting is the absence of some agency making sure that the technology is used safely.

This link might be a good read and help explain.

http://www.emcrules.com/2011/07/radio-interference-from-led-lighting.html

[Ianab]

The actual LEDs don't emit RF (interference), but a cheap switch-mode power supply certainly can.

You can run a simple LED torch simply by connecting the LED and a current limiting resistor to the battery. But it's not very efficient, and the light gets duller as the battery goes flat.

But if you run the battery through a relatively simple switch-mode power supply it can "regulate" the voltage to the lamp better, avoiding the loss of energy in the resistor, and keeping the lamp at full brightness until the battery is totally flat.

I use the term "regulate" loosely. The LED doesn't actually need steady DC current. It could be a square wave, with the correct average voltage and current and the lamp lights just fine. But you get all sorts of weird radio frequency noise as a side effect.

A torch is likely to be built light, compact and cheap. Not much in the way of filtering and radio noise suppression built in.

A house supply "should" be built to run the lamps on clean DC. Will still be switch-mode but with proper filtering and smooth DC to the LEDs. Like a computer power supply. Otherwise, yeah, all sorts of radio noise.

Ian

[DanG]

I'm glad to see that I'm not the only one to notice this.  I was feelin' like an oddball for a while there, not to say that I ain't an oddball anyway. : :D

I'll have to do a little more research to get up to speed.  A diode, light  emmitting or not, is a DC device which limits the flow of current to one direction only.  These really bright LEDs may require a higher voltage than a 12v battery puts out, necessitating a power supply.  I'll try to do some digging unless some of you already have the answers.

[Ianab]

Those high intensity LEDs run on about 3 volts. But they have a negative temperature coefficient. Means their resistance drops as they get hotter. If you just hook them to a good battery they just warm up, get really bright for a second, then the magic smoke gets out.

So it's common to drive them from an electronic circuit to regulate the current / voltage.

You could use 3 in series, needing 9 volts, then add a current limit resistor to drop the other 3 volts. But that's wasting 25% of your power, and as the battery gets flat, the voltage to the LEDs drops as well, so they get dull.

The electronic circuit keeps the correct voltage on the LED until the batter dies totally, and wastes less energy. Just a cheap one one is probably noisy.

You know the saying, cheap, good, fast.. pick any two?

Ian

[Brucer]

Here's some more information for you, DanG. I've been playing around with these things for quite a while now.

Like any diode, they will only conduct current in one direction. And like any diode, when you have them hooked up in the proper direction, there is a small voltage drop across them (forward voltage).

This forward voltage is about 1.5 volts for most LED's -- it varies depending on the colour of the light emitted. If your power supply doesn't reach this voltage, the LED won't conduct any current at all. If you exceed this voltage, the LED will offer almost no resistance to the current flow and you will burn it out in a very short time.

A typical DC circuit will use a resistor in series with the LED to limit the current through it. The more current that flows through it, the brighter the light. The problem with the current limiting resistor is that you'll waste some energy across the resistor; With a 3 volt power supply and a 1.5 volt LED, the resistor will consume as much energy as the LED. The higher the power supply voltage, the more energy gets wasted across the resistor.

This doesn't matter much when you use an LED as an indicator light. A very low current will still light it up enough to see clearly. But when you want a very bright light, you need a high current flow and that's when the energy loses become important.

You can put several LED's in series to get a higher forward voltage. For example, 3 LED's in series will take about 4.5 volts to light up. Power this with a 5 volt supply and a small current-limiting resistor, and about 90% of the energy will be used to drive the LED's, with only 10% being lost across the resistor. This is fine, but if you are using rechargeable batteries, when the battery output drops below 4.5 volts your light won't work.

Another way to get a very bright light without burning out the LED is to send a very, very short voltage pulse through it without any current limiting resistor. This will greatly overload the LED but the pulse is so short it won't have a chance to do any damage. Send these pulses repeatedly (with pauses in between) and you will get a series of bright flashes. Increase the frequency and your eye won't notice the individual flashes.

There's lots of ways to create these short high frequency energy pulses, but they will all cause EMI of some kind.

[Dodgy Loner]

Wow, this is incredibly "enlightening". I had a "light bulb" moment (ok, no more puns ;D) while reading the article that Jeff linked to. My car cell phone charger completely disrupts my radio at certain frequencies. I never understood why, but not I now believe the cause is the circuitry for the little red LED light that appears when I plug it in. I've never had that problem with a charger that doesn't have an LED light, and my charger is a cheap knockoff from Best Buy. Hopefully they will get this problem corrected before LED lighting comes into broader use.

[Ianab]

Again...it's not the actual LED, it's the switching circuitry that's dropping the car's 12v down to the 3 or 4 volts that the phone (and LED) uses.

It would make the RF noise even if there was no LED in the circuit.

As you say, it's a cheap knockoff. They have found that it still works without the additional components to filter out the high frequency noise, so they save a dollar by not including them.

Ian

[Brucer]

... I now believe the cause is the circuitry for the little red LED light that appears when I plug it in.

I doubt very much that it's because of the LED. When the LED is simply used as an indicator light you don't need a fancy circuit to lower the voltage. I milliamp of current will light an LED bright enough to use as an indicator. In a 12 volt automobile system that's about 12 milliwatts. So what if 90% of the energy is wasted -- a car batttery wouldn't even notice it.

As Ian says, it's the power supply for the phone charger that's the source of the problem.

[Al_Smith]

You'll probabley see more LED's in the future than you will CFL's .Although I think the powers that be had a brain fart when they decided to phase out old Edison based incandescents .JMO .

LED's are nothing new just an idea they came up with for small high intensity sources of illumination .Fact LEDs' and light emitting transisters have been used for decades as the internal couping devices for high speed PLC's used in industrial applications with no apparent problems with RF interference .

You get to talking TV tuners and cell phones you can get all kinds of RF floating around in addition to the harmonics of same .A tuner for example because it's a short range type of device and because of it's physical small size obviously uses a high frequency which doesn't take too much to screw it up .

[Dodgy Loner]

This discussion reminds me how completely and utterly uninterested I am in electronics :D. I'll leave this to the folks that know what they are talking about - and I am clearly not among them.

[woodhick]

Not exactly relevant to the original discusssion  but what I dont like about the LED flashlights is "distance"  Yeah the provide a lot of light at the spot but try to shine one out 50 or 100yards to light your way.  To me the light drops off greatly.  I have both so use the LED for up close and the ole lights for distance.