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

Sat Jan 7 11:08:08 EST 2017

A brute force solution would be to get four LED lighting strips that run off 12V and
use four switches. Turn on as much light as you want.

wunder
K6WRU
Walter Underwood
CM87wj
http://observer.wunderwood.org/ (my blog)

> On Jan 7, 2017, at 6:32 AM, Chip Stratton <lightdazzled at gmail.com> wrote:
> 
> What John says is true. Over time the forward voltage drop of an LED will
> decrease somewhat as it ages. It isn't great, and it isn't fast, though it
> happens more quickly initially.
> 
> The forward voltage drop decreases more dramatically in the short term with
> temperature - the higher the die temperature, the lower the forward voltage
> drop. This can result in thermal runaway and destruction of the LED when
> using a constant voltage supply and not a constant current supply. This can
> be mitigated with a current limiting resistor in series, but it does
> decrease overall efficiency of the system.
> 
> Chip
> AE5KA
> 
> On Sat, Jan 7, 2017 at 8:45 AM, John Pitz <crustacean at brig-elec.com> wrote:
> 
>> 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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>>> delivered to ron at elecraft.com
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