LED Dimming - Page 1
Forum section: electronics / beginners

author: Mad Mart
author_rank: Contributor
timestamp: August 25, 2014, 10:43:46 am
content: As my computer desk has now gotten very cluttered with electronic componenets, multimeters etc. I decided to locate a spare desk in another corner of the room specifically for working on my projects. There is shelving already there so I thought I would put a strip of LEDs under the bottom shelf to light the new 'workbench'. I then thought "Why not make them dimmable as well?"So I had a look on Youtube and came across Dave Jones's video where he hacks his Mantis Elite microscope. So I put his DaveCAD schematic in to Circuit Wizard and all was well. But when I transfered it to the breadboard it didn't work. I only had a 2N2907 transistor and 1N4001 diodes so used those in place of the components that Dave specified. This isn't really my query. My question is, why do I need to use a 555 timer to create PWM for dimming the LEDs? why can't I just control the dimming with a pot?

author: tiltit
author_rank: Regular Contributor
timestamp: August 25, 2014, 11:52:32 am
content: I don't know how much power you need for your led strip but a 2N2907 looks a little wimpy to me for that application. I usually use a mosfet like the irl540 to drive led strips. What kind of LED strip do you have?My understanding is that you can't use a pot because it would get really hot and waste a lot of power.

author: Mad Mart
author_rank: Contributor
timestamp: August 25, 2014, 12:41:38 pm
content: I don't have the LED strip yet but I was thinking about getting a 12V strip like this:-LED Strip

author: Yago
author_rank: Frequent Contributor
timestamp: August 25, 2014, 12:50:04 pm
content: It is PWM (pulse Width Modulation).It varies the "ON" time of the LED at a fixed voltage by switching the current on and off, more on time the brighter.Rather than varying a DC voltage/current, as what I suspect you may be thinking of.The 555 is used to generate the frequency that the switching takes place at.

author: con-f-use
author_rank: Unknown
timestamp: August 25, 2014, 12:51:02 pm
content: Quote from: tiltit on August 25, 2014, 11:52:32 amIf you use a pot for dimming, all the current will pass through the active side of the pot. The pot than has to dissipate a power P = I*I*R. Your average small pot can dissipate about 200mW tops. So for a couple of small LEDs it might work, for a larger strip, it will burn through the isolation, shorting the wires and maybe melting the pot.Another big problem is that you waste energy by dissipating power in a resistor. Warming up the resistor is wasting energy (warming up anything is unless you're feeling cold). For small LEDs that's not a problem. You waste more energy by reading this sentence. For larger LED with a series resistor, you might as well use a light blub.If you switch the power on and off quickly as done by PWM, no current flows and thus no power gets wasted. Theoretically that is! Practically your switch will have a small resistance and will waste power, when cycling on and off. Thus it will get warm, too. But much less compared to a resistor. By changing your PWM with output voltage (feedback), you can even limit the current. With capacitors and/or inductors, you can generate higher or lower voltages. The 555 is a dirty but cheap and functioning hack. There is a multitude of specialized chips for driving LEDs that all do much better than the 555 and have feedback and protection build in.As for your led strip: No current is specified on the page, no light intensity, no power consumption - just the operating voltage. It is severely lacking information.

author: RenGate
author_rank: Contributor
timestamp: August 25, 2014, 01:10:34 pm
content: You can use switch-mode DC-DC converter with variable output voltage. In such way you can decrease Vout to lower LED`s brightness without significant power losses. There is some good info about driving high power LEDs using various DC-DC converters topologies in Freescale`s ANhttp://cache.freescale.com/files/microcontrollers/doc/app_note/AN3321.pdfalong with description of some lighting protocols.

author: klr5205
author_rank: Regular Contributor
timestamp: August 25, 2014, 01:25:38 pm
content: LED brightness is a function of current. There is more than one way to skin a cat, but PWM is pretty much the accepted way of controlling the average current through the LED, and therefore its brightness. As was already mentioned, the using a pot would be expensive and inefficient, and wouldn't give you the same degree of control you'd have by using PWM.

author: Mad Mart
author_rank: Contributor
timestamp: August 25, 2014, 02:01:49 pm
content: Thanks for the replies guys. I now understand why I need to use PWM.

author: Yago
author_rank: Frequent Contributor
timestamp: August 25, 2014, 02:11:36 pm
content: Without being cheeky and hijacking the thread, I would ask a related question.I seem to recall that LEDs can driven harder if they are pulsed on and off (a cooling period).Is this is correct, and if aiming for such a high output, 100% duty cycle would negate the cooling and thus be unsuitable for the application?Sorry if OT and hijack.

author: Ribster
author_rank: Frequent Contributor
timestamp: August 25, 2014, 03:12:28 pm
content: Quote from: Yago on August 25, 2014, 02:11:36 pmWhat is the desired effect in mind? To get more lumens out of the LED?The proper way of driving a led, is with constant current. You make the current through the led be constant and turn the voltage on/off.When do you turn the voltage off? When you see that the current is too high. And switch it back on when the current is too low.What you are proposing is pulsing a constant voltage to the LED. Because of the way leds work (look at the figure), the LED draws more current when it gets warmer.When the led then draws more current, it wil get hotter and draws even more current. It then just drifts away.When you're talking about simple low-power applications such as a small LED ring, this is not a problem.You just supply the led with a PWM voltage.Note: If you buy led strips from ebay for example, they will have 3 leds and a series resistor in series.This series resistor burns the power like explained above. You can use PWM voltage on this without any problem.I tried to explain something about leds, though i did not understand the question fully.Does this answers your question ?

author: Yago
author_rank: Frequent Contributor
timestamp: August 25, 2014, 07:46:14 pm
content: Cheers Ribster.My question was not very clear TBH, but your reply had more than enough information to answer it.Thanks

author: Ribster
author_rank: Frequent Contributor
timestamp: August 26, 2014, 12:03:08 am
content: Quote from: Yago on August 25, 2014, 07:46:14 pmEverybody has to start somewhere.2 years ago i didn't knew this much either, but doing high power led projects helps a lot.A small buck design for a LED driver can be found below. Small appnote, lots of graphs/schematics to do the explaining.Be aware: this is an inverting buck topology. Normal buck sources the current/voltage. Inverting buck sinks the current.I'm maybe overdoing my explanation, but i find this useful information for beginners.http://www.st.com/st-web-ui/static/active/en/resource/technical/document/application_note/CD00222928.pdfhttp://ww1.microchip.com/downloads/en/AppNotes/00874C.pdfFYI: A buck converter takes a higher voltage and steps it down to a lower voltage.Because there is a coil in the circuit acting as an energy reservoir, the efficiency of a 'switched mode supply', as they are called, is much much higher.