[Elecraft] LED bulbs

[Fred Townsend]

It's not that the switching PSUs are so noisy, they are, but that these LED strips are such good antennas! The regulating authorities, particularly the EU, are very sensitive about conducted powerline emissions but since the load side of the PSU is unspecified, there are no filtering requirements. Simply the PSUs radiate out the DC side. Adding a conducted filter line at the PSU output can reduce a lot of EMI. Whether that is enough is as you mention depends on how close the LEDs are to your antennas so no two will be alike.

There is another issue with many of the cheaper LED strips. The strips are rated for 12V while the LEDs are 3 volts (round numbers). This means that 9 volts must be dropped across a resistor (typically a 1/2 watt carbon film). This is throwing away 3/4 of the energy on the input side without figuring any LED efficiency. The better way of driving a LED is from a current source. Many LEDs can be driven from a single current source but this is a great deal more difficult (and consequently more expensive). This is a bit like the old series string Christmas tree lights which have a bad reputation for reliability. Because of cost and perceived reliability series string strips are the exception rather than the rule. 

Conclusion: LED light strips with switchers can be very efficient and quiet but take some searching to identify. Investigate the technology before you buy and make sure anything you buy is returnable and test before you throw the package away.

73, Fred, AE6QL

-----Original Message-----
>From: Jim Brown <jim at audiosystemsgroup.com>
>Sent: May 12, 2015 9:26 AM
>To: elecraft at mailman.qth.net
>Subject: Re: [Elecraft] LED bulbs
>
>On Tue,5/12/2015 6:18 AM, Marc Veeneman wrote:
>> owever, the only ones I can't live with are the undercounter LED strips powered by WAC 24 volt power packs.  Not only are those things, the power packs, unusually expensive, I am surprised that the FCC allows their sale in the U.S.  They obliterate the 160 and 80 meter bands on my KX3 and K3.
>
>You should not be surprised about the FCC's inaction -- their 
>enforcement division was gutted in the name of small government, and 
>what was left of it is going away in the name of a balanced budget.
>
>You've correctly  zeroed in on the noisy component -- it's the PSU. You 
>should return them and the lights that use them as defective and demand 
>your money back. If you can't or don't want to do that, throw those 
>noisy PSUs in the trash and replace them with a linear supply.
>
>Check out the LED lighting from this vendor. I'm using their strips to 
>light my hamshack, running five strips in series to light my ham shack. 
>The combination draws 1.2A from my 12V system. Their strip lights have 
>built-in rectifiers ahd regulators that allow them to run from any AC or 
>DC source between 10 and 24 V. They have been exhibiting at Pacificon 
>and the Visalia DX Convention for several years.
>
>https://www.wiredco.com/LED_Flashing_Lighting_s/1847.htm
>
>There are several good ways to power these lights. I've trashed all the 
>new switching power supplies that come with new electronics and replaced 
>them with old linear wall warts that came with much older stuff. And I 
>buy more of them at hamfests and second hand stores, typically for a 
>buck apiece. Another trick is to use a smaller linear supply to trickle 
>charge a battery of the voltage that the equipment needs. I'm doing that 
>for stuff around my living room entertainment system, and for the cable 
>modem and WiFi router. In addition to killing the noise, it functions as 
>an inexpensive UPS.
>
>73, Jim K9YC
>
>
>
>
>
>
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