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

Sat Jan 7 08:45:03 EST 2017

Actually there is a much simpler way to do this using linear 3 pin
regulators.  Take a fictional regulator that has a fixed output voltage
of 1V.  Assuming you want 15mA through your LEDs, make sure the output
current of the voltage regulator is 15mA by placing a 67 Ohm resistor
from the output pin to ground.  The current out the VOUT of a 3 pin
regulator will be equal to the current at the VIN of the regulator.
Then put your string of LEDs from the "unregulated" voltage in and the
Vin pin of your regulator.  You will have to add up the expected voltage
drops of the LEDs. and subtract that value from the unregulated Voltage
supply.  then subtract the voltage regulators output voltage, in this
case 1V from whats left.  Now, make sure that the remaining voltage is
at least a little bit higher than the dropout voltage of the regulator.
I have done this countless times.  It should be pretty simple to use a
POT and a resistor to dim the LEDs within a preset range, or if your
regulator has a shutdown pin you could PWM it from your favorite
microcontroller.

You should avoid putting LEDs in series when using a simple dropping
resistor.  That scheme will seem to work well for a little while then
you may find your LEDs failing.  In my experience this doesn't work long
term and in this scheme the LEDs fail one by one shorted.  I don't think
the voltage drop on each LED is all that constant over time and
temperature.

73
KD8CIV

On Sat, 2017-01-07 at 02:14 -0800, Dave Fifield wrote:

> 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.
> -----------------------------------------------------------------------
> Bill Frantz        | Truth and love must prevail  | Periwinkle
> (408)356-8506      | over lies and hate.          | 16345 
> Englewood Ave
> www.pwpconsult.com |               - Vaclav Havel | Los Gatos, 
> CA 95032
> 
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