Voltage Regulators and questions

Discussion in 'The Projects Forum' started by 496 BB, Jan 5, 2011.

Thread Status:
Not open for further replies.
  1. 496 BB

    Thread Starter New Member

    Nov 30, 2010
    5
    0
    I am working with LEDs as Ive become somewhat of an LED freak over the past 2 or 3 years. Ive always loved lighting but these things are cool.

    First and foremost let me explain my situation. I am running these LEDs on boards Ive designed out of plexiglass. Ive used PCBs before but these fit the situation better....the clear color. I am still a learner on some things but have the general idea of these to say the least as far as the circuitry and resistors and such. I am in need of a voltage regulator. I am running these LEDs with a power source of roughly 14v. Just for ease I would like a 12v regulator so I dont have to worry about voltage drops as the power source could be varying. Is there an inline voltage regulator I could install on the positive wire into the array? It cant drop ANY volts like in the searches I found here. With it only dissipating ~2v will it produce alot of heat and need a heat sink? Any recommendations on where to buy?

    My other question is more of a personal preference. I buy my LEDs from Super Bright LEDs.com. Are their LEDs decent quality and do you know where they get them from? I have used them quite a bit and not had any problems but wanted an expert opinion. I want to do something once and thats it (of course when the learning curve is over...lol).

    Last question is about PCBs. I have searched high and low for a PCB manufacturer and have seen many but not sure how to judge a good one form a lousy one. Really all I want is a place where I can send them a drawing of the array and have them make the PCB and install resistors for me already. That way all I have to do is install the actual LED and be done. In addition to that is there such a thing as different color PCBs? Like clear but with obviously with traditional tracer wire colors? Just wondering.

    I do appreciate your time and any info you may have.


    Thanks,

    Chris



    PS - I have searched the Wall Wart (wort??..lol) thread and had a few questions I didnt see or understand. I just want a simple cheap (if the two goto together) adjustable (by the .10v) power source that I can plug into the wall and have DC output that is regulated. Really only need the 12-15v range but would be nice to have more or less but not required. I have tried to use the Wall Worts and cant find a stable output one for testing. I dont need much amperage as its mauinly for LEDs anyways. If I could find a wall wort that is adjustable from 12 to 15 and stable that would suffice also I guess. Sorry for long PS post ;)
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,086
    3,024
    I think you've probably learned of the 7812 fixed 12v voltage regulator. Is there a reason you don't like it? You'd be able to get up to ~0.5A from that using a 14v supply. A small heat sink would be useful if you're near that current level. I use a 7805 screwed to the inside of an Altoids tin, for powering USB stuff in the car, and it gets hot but has lasted for years now.

    But more generally you should try to abandon your fixation on voltage and learn about regulating current - that's what LED control is all about.
     
  3. 496 BB

    Thread Starter New Member

    Nov 30, 2010
    5
    0
    Thanks for the reply. What do you mean when you say get away from voltage fixation and learn regulating?

    I was looking at the regulator that was posted in many threads here (cant remember numbers but was posted in Mr Marsden thread) and it was stated it could drop as much as 3v. So from 12 that would be down to 9v and would be too dim. I need something that is stable at 12v and stays only at 12v +-.5v. All of my LEDs in the array are rated at 20mA and by ohms law they are within that spec with the resistors Im using.

    Please do explain as thats why Im here ;)
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    You can get "Low Dropout" voltage regulators, which can regulate within 0.5V (500mV) of the power supply, compared to the 1.5V+ dropout of the 78xx series.

    LEDs have a forward voltage drop, around 2-3V depending on color and brightness. Above their turn on voltage, LEDs quickly "look like" a short circuit electrically, so the current needs to be limited so the LED doesn't turn into a blown fuse. Most LEDs cannot handle over 20mA for any long lifetime.

    If you limit the current to 20mA or less, the exact input voltage doesn't matter, as long as it is enough to reach the turn on voltage of all the LEDs. The typical current limiting device is the resistor, which simply drops more voltage as current is increased (dissipating heat in the process). A fixed current source is a "cleaner" way of running LEDs.

    A LM317 Adjustable Voltage regulator can be set up as a current source with a resistor, but has nearly a 1.5V dropout voltage as well.

    Another option is a switching regulator, such as a "BuckPuck" that provides a constant current source from a 12V supply, in a ready to use package.

    Switching regulators aren't as easy to use as the 78xx regulators, the ICs usually require at least an inductor or capacitor, and some need external transistors as well. This increases their cost, so switching regulators are typically used with 1W or higher LED modules, leaving the <1W typically in the linear regulator realm. A notable exception to this rule of thumb are the "Joule Thief" circuits to run one or two LEDs from a AA battery.

    To give more precise information, how many LEDs were you looking at lighting in the circuit? Do you need dimming ability, flashing, etc? The more details given, the better the solution that will result, though starting out with very little information is quite common as well.
     
  5. 496 BB

    Thread Starter New Member

    Nov 30, 2010
    5
    0
    OK heres my setup and please bear with my non formal electrical drawings and talk.

    My current setup (which works fine) is this: 7x3 array

    X X X X X X X
    | | | | | | | |
    X X X X X X X
    | | | | | | | |
    X X X X X X X
    R R R R R R R

    X = Yellow 30* 2.2 typ Vf with max 2.4Vf if = 20mA

    R = Resistor value of 390 ohms and 1/2W each

    I have these ran in groups of 3 in columns with resistor on negative side of led on bottom. I ran top led - to middle + and middle - to bottom + with bottom - to resistor. I ran all top anodes across and tie into a single wire out. All end of resistors are tied across to a single wire out.

    My power source varies from 12-14.6v. Thats why I wanted to use something stable at 12v since it will be at the end of the spectrum of volts. If it goes up I dont have to worry about frying the array. Now if I ran these in groups of 5 that will put me at 11v and I could use a regulator at that with no resistors (right?) but seeing how the layout is I dont think that would be easy to do.

    I use a resistor in line of power wire currently to dim (20 ohm). I then have T'ed wire above that resistor to act as a full power wire with a diode so power is not drawn back into it when dimmer wire is hot. I hope someone understands this as it makes more sense in my head.


    I thought I had this under control until I seen this place :D
     
    Last edited: Jan 5, 2011
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    As long as you calculate the resistors for the max voltage, running at a lower voltage isn't usually a problem, unless you have 4 3V LEDs in series, and the supply drops below 12V, then none will reach their turn on point.

    The way you've been wiring them is fine, and quick schematics can be auto-calculated for you here: LED series parallel array wizard

    For "better" control of the LEDs, dimming should be done with Pulse Width Modulation (PWM), which is essentially turning them on and off a few hundred to thousand times a second. The perceived brightness is determined by how long the "off" time is compared to the "on" time (Duty Cycle). The LED is either driven at 20mA or Off in PWM. The average of duty cycle will be roughly 8-10mA at "half dim" though the peak current is still at 20mA.

    PWM is preferred for dimming as the exact color of a white LED will change when the current is 10mA vs 20mA. The second reason is efficiency. When dimmed by limiting current to 10mA with the extra resistor, more power is dissipated by the extra resistor, wasted as heat. The efficiency of PWM results in longer battery life and LED life. PWM can generated in several ways, from a simple oscillator and comparator (555 timer) to microcontrollers.

    PWM also allows a greater range of "dimness" from barely perceptible to extremely bright. When using non-white LEDs, dimming by current limiting doesn't effect the color as much. The "flickering" of the on/off cycles isn't perceptible if the frequency is over 100Hz minimum, though usually in the kHz range by practice. Many people will notice a "flicker" even at 80Hz, and a few will see flicker at 100Hz, especially when in the peripheral vision area/edge of vision.
     
  7. spinnaker

    AAC Fanatic!

    Oct 29, 2009
    4,866
    988
    I have the BuckPuck from Luxdrive. They are really easy to use. Some models even have a regulated 5vdc out to power things like micro-controllers. It really heled me in my project.
     
  8. 496 BB

    Thread Starter New Member

    Nov 30, 2010
    5
    0
    Problem is they wont be receiving the full power across the diode if I calculate for 14 and only receive 12v. It would be running at less than optimal and light output would suffer. Correct me if Im wrong.

    I have looked at those PWMs but dont really understand the whole idea. These LEDs are in a closed tight housing and space is somewhat of an issue and there is no PCB as I need clear color. Really all I need is a place where I can find a regulator that can accept up to 15v input but is firmly stable on outputs with it only being 12v. It has to be able to be wired inline of power wire (18-22ga stranded wire) into board or somewhere thereabouts. Maybe they dont make anything like this I dont know. The biggest thing here is running each LED at its Typ voltage so the output is as bright as it can be while safely doing so.

    I appreciate your help and keep it coming ;)
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Please do not recommend that "wizard", as it often makes poor recommendations, and does not take into account many variables.

    496 BB:
    This thread sounds like you are attempting an automotive application, most likely safety lighting.

    We don't support such discussions here, as very few (if any) of us are automotive engineers, and furthermore cannot hope to ensure that any such projects would be in compliance with applicable regulations.

    Automotive transient voltage spikes can reach 60v or more during "load dumps" such as A/C compressors cycling, headlamps being turned off, etc. This eliminates simple current limiting resistors from consideration.

    Please abandon your attempts.
     
  10. 496 BB

    Thread Starter New Member

    Nov 30, 2010
    5
    0
    Ok but this vehicle is from from being an on road vehicle. Its a truggy I am building and is for NOTHING but trail riding and off road. NEVER will it be on public highways. So with this being said Im not sure as to why you got an attitude with me. I know the rules. I read them. I did not see anything stating an off road rig and thought it would be ok since it was specifically for OFF ROAD. Where no applicable laws are required per se. Also I never mentioned ANYTHING about a vehicle so no mis guided young kids would come in here and "kill" themselves trying to put this together. Id hate to mention on here how to do the correct thing instead of having a mis guided kid just do it his way anyways. Not like there is mis guided info ALL over the internet anyways.

    But I understand rules are rules so if you feel the need to shut yet another thread down please do so. I will find the answer locally the old fashioned way....by talking to a simple human being at an specialty electronics store.

    BTW - What is the reason behind auto electronics here specifically lights? I can understand PCMs and such but lighting? Seriously I understand the effects of bad lighting but if they found this place then obviously they care enough to do a good job. I got 21 5mm LEDs in these housings that are rated at ~4000mcd each. Its a forward firing only housing and flat surface only. These are 30* angle LEDs. They fire better than the incandescent and are WAY brighter. They work perfectly. This to me is a safety upgrade. Im not sure I quite see the point on your part. Im sure I dont have too though as Im a guest here and not an owner.

    Good day.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    We'll need the Moderators to look at this before proceeding.

    That's funny, I don't see where I had any kind of "attitude" other than a desire to be in compliance with the forum rules/guidelines. I did mention that simple current limiting resistors are not adequate for automotive-type lighting due to the high peak voltages that can and do occur.

    You made no mention of the application being for off-road use prior to this reply.

    I don't shut threads down; I'm not a Moderator - just another forum member.

    Please be aware that there are ready-made LED lighting assemblies available at many auto supply stores that will likely be much more reliable than something that you could make at home. You should consider what might happen if your home-made lights happened to fail while driving at high speed on an off-road course at night.

    I'm not an owner either.
    While electricity/electronics are the main topics of discussion on these Forums, safety is paramount.

    As I've already mentioned, I'd like for a Moderator to weigh in on this before continuing.
     
  12. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    WE have all sorts of lighting mods requested. Some are clearly not acceptable, as the lighting is to flash brightly in time to music. Some is for spiffy effects under the car. Some are for progressive turn signals that are distracting.

    In no case can we see the utility of projects like these, as there is no safety enhancement intended. They are just to make the owner look cool, and safety is not a concern.

    You might find that reading through the entire set of posts this topic accumulated would be interesting - http://forum.allaboutcircuits.com/showthread.php?t=40361

    Our problem is that it's never just the OP that gets to read the advice. We have decided to limit some automotive topics, as there is never any guarantee that some other hobbyist will use proper techniques/wiring/connectors. We are very reluctant to give advice on purely homemade lighting, simply because another reader might think that those LED's are way cool and want to kludge some onto his Honda, with bad wiring and no concern for legal output or aim.

    We can't satisfy the requirement of 50 states and any number of countries.
     
Loading...
Thread Status:
Not open for further replies.