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:[email protected]] On Behalf Of Ron > D'Eau Claire > Sent: Friday, January 06, 2017 8:17 PM > To: [email protected] > 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:[email protected]] On Behalf Of Bill > Frantz > Sent: Friday, January 6, 2017 6:07 PM > To: [email protected] > 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, [email protected] (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 > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html Message > delivered to [email protected] > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html Message > delivered to [email protected] > > > --- > This email has been checked for viruses by Avast antivirus software. > https://www.avast.com/antivirus > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [email protected] > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
