[Elecraft] Dimmable LED desk lamp for operating position recommendation needed

[Dave Fifield]

An LED is basically a fancy diode, so a linear voltage regulator to control
its brightness will not work well.

Starting from around 2V output (say) the LED/s will be OFF. As you increase
the regulator's output voltage, somewhere around 2.5V (depends on the exact
type of LED you're using), the LED will begin to conduct forward current and
start to emit light. At this point, increasing the regulator's output
voltage just a tiny bit will increase the brightness of the LED a lot (it's
basically an exponential curve). So at just a couple hundred mV above the
voltage where the LED began to emit light, you will reach full brightness.
Any further increase in the drive voltage will merely result in the series
current-limiting resistor dissipating more heat.

What is really needed here is a linearly-variable constant-current generator
circuit. This is a fairly simple circuit to design, usually consisting of a
voltage reference, a comparator, and a drive transistor, with some feedback
from a current sensing resistor. It will need to be designed so that it can
generate enough voltage to overcome the LED's forward voltage (Vf), and with
an output current that's variable from 0mA to around 20mA (or whatever the
LED's maximum forward current (If max) is specified at). 

Hope this helps.

Cheers es 73,
Dave
AD6A

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Ron
D'Eau Claire
Sent: Friday, January 06, 2017 8:17 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] Dimmable LED desk lamp for operating position
recommendation needed

A linear voltage regulator is very quiet. That's just a bipolar transistor
passing the current with adjustable base voltage. I use them on my HB power
supplies for the same reason. No square switching, no RFI. 

73, Ron AC7AC

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Bill
Frantz
Sent: Friday, January 6, 2017 6:07 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] Dimmable LED desk lamp for operating position
recommendation needed

To avoid color shifts with dimming, most LED dimmers use Pulse Width
Modulation (PWM). PWM has the potential to generate RFI, so be careful. The
LEDs themselves should be very quiet. Using LEDs with resistors to limit the
current should also be quiet. A system that switches LEDs for brightness
control would be a safe solution, but I don't know of any commercial systems
which do this.

On the other hand, I have not noticed a problem with the LED dimmers in my
house, so perhaps most of the problem is in the wall-wart power supply.

73 Bill AE6JV

On 1/6/17 at 3:50 AM, elecraft at mailman.qth.net (Marc Veeneman via Elecraft)
wrote:

>I mounted (dual side adhesive tape) an LED strip to the underside of an 
>equipment shelf.  The strip came with a 12v switcher that I ignored.  I 
>use my 12 volt supply.  The strip was, I think, 24 inches long and has 
>a dimmer that can be inserted in the power lead.  No RFI.  Plenty 
>bright.  Mine came from Amazon but there are many to choose from these 
>days; you can even select color temperature when you order.
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(408)356-8506      | over lies and hate.          | 16345 
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www.pwpconsult.com |               - Vaclav Havel | Los Gatos, 
CA 95032

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