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Author Topic: Light fixtures - 24v LED bulbs  (Read 4601 times)
Chaz
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« on: May 09, 2008, 07:50:48 AM »

I just got my 24v led rope light in......... cool!

Now, I want to put in 24v LED light fixtures for task lighting and specific purposes. You guys have any suggestions?? Where to get them? What to get?
 
So far, the best I can come up with, is getting fixtures that will accept the 1156 automotive bulbs and buying 24v LED bulbs for them. Is there any other/better way to do it?

Do any of you have 24v lighting?

I did also find some adapters, but would rather just do it the "correct way" (if you will) from the start.

Illuminating my world,
    Chaz

p.s. Hey Cliff, I also found some "CUTE" little LED rechargable flashlights that plug right into great little outlets you sent me.  Grin Grin   I may have to get one for each outlet!!  Grin Seriously!!  Thanx again buddy.
« Last Edit: May 09, 2008, 08:00:44 AM by Chaz » Logged

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« Reply #1 on: May 09, 2008, 08:09:16 AM »

Chaz,
You may want to check here http://www.superbrightleds.com/
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« Reply #2 on: May 09, 2008, 08:20:17 AM »

Chaz,
I was going to recommend the same site, or similar ones that sell on the eplace.

Look closely at the MR16 bulbs, there are many MR16 fixtures that sell at the large home places for very reasonable prices. I would suggest that you get the wide angle flood light bulbs though, otherwise the light is a very narrow spot.

Jim
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« Reply #3 on: May 09, 2008, 08:33:10 AM »

Chaz,

All our incandescent lighting is 24v.  Incandescent bulbs are readily available in that voltage (although much pricier than 12v equivalents).  However, you asked about LED's.

Unfortunately, 24v LED fixtures are not commonly available, with the exception of exterior models such as clearance lamps, stop and turn signals, etc. (and even these are usually expensive compared to 12v models).

What we did for our LED interior lighting was to purchase 12v models and wire them pair-wise in series.  As long as you use identical fixtures, this will work fine.  That will open up a huge selection of fixtures to you that are only available in 12v, and will probably save you money even on ones that are also available in 24v.

-Sean
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« Reply #4 on: May 09, 2008, 10:43:49 AM »

Hey Sean,
  I do remember you mentioning that is what you did. (brain fart!) That does sound like a viable option, but the MR16 sockets that Jim mentioned sounds like that might be easier. I even have a few of those in my house. I also seen where they sell just the sockets in case I would want to get "creative"!  Roll Eyes Who me!!??!!??  Grin I might order a couple bulbs to just check them out. But it seems like they are pretty cool.

  Next thing, Linda wants a "Clapper".  (OMG!!)

Shine on me,
   Chaz
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« Reply #5 on: May 09, 2008, 12:00:42 PM »

Chaz - how far do you want to go?

If I recall correctly, you're kind of a "hands-on" type...  You may want to try doing a custom LED based light fixture.  I have one PCB design that is pretty cheap and puts out a decent ammount of light for its size (2.5" diameter, 1/4" thick).  The below pictured layout is for qty 4 Luxeon Rebel LEDs (any color) - and will run on any voltage from 12.5V up to 29V (DC).  Based on your needs and budget I might be able to work up something that would suit your specific needs and/or budget. This fixture is designed to put out about 280 Lumens (about 75% of what a 30Watt incandescent brake light bulb would put out) of cool white light (6500K - or around daylight color), and would fit within a standard "puck" halogen fixture (using bent header pins it will plug in to a standard bi-pin halogen bulb base).  Only draws about 5Watts (including the on-board voltage converter).

Cheers!

-Tim
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« Reply #6 on: May 09, 2008, 12:29:46 PM »

Tim,
  Wow, those bad boys are SMALL!!!  Shocked  But even after reading about them, I'm still in the dark (no pun intended Roll Eyes) I'm definitely "hands on" but I'm not sure what you are saying.

In the dark,  (yet again)
   Chaz
 
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« Reply #7 on: May 09, 2008, 08:47:16 PM »

Chaz,
here is a link to an old thread on the led's.

http://www.busconversions.com/bbs/index.php?topic=5407.msg50485#msg50485

Jim
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« Reply #8 on: May 10, 2008, 06:47:22 AM »

Thank you, thank you, thank you Jim!
Last night, after spending WAAAAAY to much time on the net looking for fixtures I decided to look at building my own. The only ones I could really find were Track Lights or Outdoor Lights. I would have to modify those also. Your post here is perfect timing!!
 I seen where I had even responded to that old thread!  Roll Eyes  Unfortunately, I had slept since then and forgot about it.  Grin
 The way you did your lights is a stroke of genius! I actually ordered a couple bulbs and sockets, a couple days ago, on the outside chance I might have to build what I want. ("might"...HA!! Who am I kidding!?!?  Grin lol) I wish there were more fixture choices for an MR16 bulb, but I'm not finding them. And MR16's seem to be about the only 24v LED bulb.
Oh well, I'll just keep making my bus more "mine".  Smiley

  Now if I could just figure ont this back cabinet.............  Wink
     Chaz
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« Reply #9 on: May 10, 2008, 09:28:20 AM »

Chaz... here is some interesting stuff on setting up your own LED's:

http://www.nabble.com/OT:-How-to-install-LED-bulb-in-Yaesu-G-800DXA-td15654479.html

http://www.uashem.com/pageid-164.html

http://forum.allaboutcircuits.com/showthread.php?t=10978
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« Reply #10 on: May 10, 2008, 10:31:39 AM »

Oh s**t...................... Dang!!!! I thought I could just hook them up and run with it. Thanx for the heads up Dallas. I guess I am going to have to go back to school to hook up my lights.  Angry *&^%$#@$%#&*^!!!!!!!!!!!!!! I hate school.

Chaz
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« Reply #11 on: May 10, 2008, 02:14:17 PM »

So Chaz,

   One of the things I started with before getting into the light fixtures themselves is coming up with a number of how much light I needed for a specific space.  So allow me a few questions:

  • What type of space is this being used for?
  • What type of activity is ocuring in this (these) space(s)?
  • What time of day are these activities being done in this space? (this speaks to the ammount of available natural light)
  • If there is a work surface (i.e. counter/sink), how big is it?

These types of questions can be used to find a general light quantity required for a space (in lumens/ sq.ft.).  Specific values can be retrieved from one of many books on the subject (mostly written for architecture students and architects) of which I have a few (though I’m not and architect or student…).

Once you know how much light you’ll need, it’s a good idea to take a look at the space around the area you want to light to see where you can put light fixtures (and what shape they can be) – without getting in the way of the light’s path to the lit surface (so you won’t cast a shadow on your kitchen counter Smiley).  Once you know how much light you need and where you can fit fixtures, you’ll want to look at two budgets: Financial, Power.  Based on this you can decide what you can afford in cost which will determine what type of fixture you can put in (and what each option will do to your battery run-time).

You seem to be leaning towards LED light sources, which suggests two data-points: 1) you have a pretty good budget for task lighting, and/or 2) you have a limited power budget, so you need the most efficient light fixtures you can manage.  By “hands on” I’m talking about complete assembly – not just integration of a module into a fixture.  So how good are you at SMT soldering? Grin

I can spec out a board (Bill Of Materials and performance characteristics) for you and draw up the PCB file, if you want to have the boards built, put on (SMT Solder) all of the parts and do the integration as well...  The good news is that once built (if built well) these fixtures will normally run 50K hours before they really start to lose light output.

National Semi has a new-ish part (LM3404HV) that would allow you to drive several high-power LEDs from a 8-75VDC source (based on the voltage of the LEDs + the switch loss).  The efficiency is pretty high with this part – better than 85%, so not much heat other than what the LEDs put out needs to be handled.

Keep in mind – the cost of a single high power LED is about $4, the cost of an LED driver IC is about $5, and the cost of a PCB in low-volume is about $12.  For a design like the one I pictured above, the cost is around $30 per fixture in volumes of 12.  If you can find a fixture that will put out the right amount of light for less – it’s probably going to be less headache for you…

Cheers!

-Tim

P.S. Given the cost of gas and every other point in energy, I should recommend that you look at posibly putting in some solar capability to off-load your lighting needs to a solar system (as oposed to running a generator or shore-power).  Even a few watts of solar like 30 (would be about 6 sq.ft. using cheap Northern Tools panels), would likely keep you lit for quite a while (of course - based on your lighting needs). -T
« Last Edit: May 10, 2008, 02:35:37 PM by Tim Strommen » Logged

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« Reply #12 on: May 10, 2008, 04:50:31 PM »

Chaz,
I may be wrong, but the sites that Dallas linked to are for making the entire light fixtures from individual led bulbs, or a string of them. When you buy the MR16 LED bulbs listed on the websites cited in the previous post, they will already have the resisters and diodes that are needed built into them so they are pretty much plug and play!

At least that is what i found, they are already set up to just ad the correct voltage or power source, a switch, and light them up, therefore all you have to do is find or build an appropriate fixture for them.

Don't give up, its easier than it seems, and the power savings with them is worth the effort. I have about $7.00 per light invested in mine, and I feel that is a great investment when I can run about 12 of them and light up the whole front of my bus, using only about 18 watts of energy, PLUS they run straight off the house battery, not through the inverter.
 
Jim
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« Reply #13 on: May 11, 2008, 03:44:58 AM »

Chaz,
I may be wrong, but the sites that Dallas linked to are for making the entire light fixtures from individual led bulbs, or a string of them. When you buy the MR16 LED bulbs listed on the websites cited in the previous post, they will already have the resisters and diodes that are needed built into them so they are pretty much plug and play!

At least that is what i found, they are already set up to just ad the correct voltage or power source, a switch, and light them up, therefore all you have to do is find or build an appropriate fixture for them.

Don't give up, its easier than it seems, and the power savings with them is worth the effort. I have about $7.00 per light invested in mine, and I feel that is a great investment when I can run about 12 of them and light up the whole front of my bus, using only about 18 watts of energy, PLUS they run straight off the house battery, not through the inverter.
 
Jim

Yup, those links are for DIY. Being that Chaz is a of hands on kind o' guy, I thought he might like to try some on his own.
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« Reply #14 on: May 11, 2008, 06:19:20 AM »

Holy Krapp, youve invented the FLUX Capacitor  Shocked



http://www.busconversions.com/bbs/index.php?action=dlattach;topic=8233.0;attach=8220
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« Reply #15 on: May 11, 2008, 09:17:07 AM »

Unless you are using your bus now, I would wait to by any LED fixture for lighting in the bus.  LED's are being heavily worked on since it is the next best lighting system.  In the past LED's are doubling their brightness every year.  If you just want exterior lighting, for stop/turn/running lights, plenty are available.  But for interior lighting, I would wait another year at least.  Good Luck, TomC
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« Reply #16 on: May 12, 2008, 02:06:28 PM »

Holy Krapp, you’ve invented the FLUX Capacitor  Shocked


LOL! Only my improved version doesn't require 1.21Gigawatts, nor does it need to be driven to 88MPH to light up Grin (the side effect of course is no time travel Tongue).


...When you buy the MR16 LED bulbs listed on the websites cited in the previous post, they will already have the resisters and diodes that are needed built into them so they are pretty much plug and play...

...At least that is what I found, they are already set up to just ad the correct voltage or power source, a switch, and light them up, therefore all you have to do is find or build an appropriate fixture for them...



I believe that I brought this up with a previous thread about a dash board LED, but in this case it’s even more important:  A series resistor with high-power LEDs is a very inefficient way of doing things.  When you place a resistor in-line with an LED to consume the extra voltage from the supply, it burns the power not used for lighting the LED as heat – this gets worse the higher your supply voltage relative to the operating voltage of the LED…


For example:  Let’s say you have a Luxeon K2 TFFC cool-white LED (3.65volts @ 1Amp – for about 160 lumens).


If you were to drive that LED from a 5 volt supply, you would need a 1.35Ohm resistor rated at 2Watts or higher to consume the power above 3.65 volts.  That would mean that you are burning 5Watts of power (3.65Watts for the LED + 1.35Watts for the resistor = 5Watts). This is about 73% efficient.


If you were to drive that same LED from a 12 volt supply, you would need a 8.35Ohm resistor rated at 10Watts or higher to consume the power above 3.65 volts.  That would mean that you are burning 12Watts of power (3.65Watts for the LED + 8.35Watts for the resistor = 12Watts).  This is about 30% efficient.


If you were to drive that same LED from a 24 volt supply, you would need a 20.35Ohm resistor rated at 25Watts or higher to consume the power above 3.65 volts.  That would mean that you are burning 24Watts of power (3.65Watts for the LED + 20.35Watts for the resistor = 24Watts).  This is about 15% efficient.



Granted – you can offset some of the in-efficiency by putting more LEDs in the series string, for instance putting in 6 LEDs in a 24Volt system.  This would raise the diode voltage loss to 21.9 volts, so a resistor would need to be 2.1Ohms and only 5watts or more.  This would also raise the efficiency of the string to about 91% (21.9Watts for the LEDs + 2.1Watts for the resistor = 24 watts, much less being burned by the resistor).  This does however come with a catch – if your voltage drops below 24 volts, your LEDs will dim or go out (since the resistor value does not change with the supply voltage).


        This is why the most common way to regulate “Power” LEDs from a higher voltage supply is something called a “buck” regulator.  This is a solid-state device that can regulate the current of an output (using a very low ohm value “sensing” resistor), and will switch on and off at very high speeds a MOSFET or Transistor to charge something like an inductor (think of that like the coil in a relay – only without the contacts…).  The inductor resists having voltage come in at a very high speed, so when the driver turns on the “switch” the voltage/current at the output rises slowly (this is a relative term – since this happens in microseconds or less).  When the voltage at the output is high enough, the driver turns off the switch and the voltage/current begins to fall very slowly (again relative term).  When the voltage/current gets low enough, or a time-out occurs – the driver again turns on the switch and the cycle repeats.  Because the driver is either on or off, the resistance of the switch is very low so only a little heat is generated.  Also, because only the power required to sustain the output is drawn – the efficiency is very high.  This type of regulator is know as a “switched supply”.

        This has some benefits as well – most LED regulators have a dim input (either analog or PWM) so dimming is very easy (relative to finding a 5Watt 200Ohm rheostat…), they are also very small (since they don’t put out a lot of heat – they can be smaller – making the whole design much smaller/cooler), and they allow for other advanced features, such as temperature compensated current regulation (where the driver will reduce the current output as the LED’s temperature rises – so that the LED won’t burn out prematurely).  These regulators also adapt to the supply they are given – so if you only need a few LEDs, you can drive them from a supply with a wide range of voltages (instead of designing it for just one voltage…).

        This is the basis behind my design (shown on the first page of this thread) – it allows a wide operating range so I don’t need to replace it if I change its supply from 12volts to 24 (a change I had planned to do), and it offers temperature compensation so that if the LEDs get hot, they will not be driven as hard and will cool down (a self stabilizing loop).  If you have a whole rig power management scheme (like the one I’m drawing up), you can de-rate (turn down) lights so they don’t draw as much power, but still put out light as your battery wears down (so they don’t have to just go out when the load gets dumped).  Using the LED driver method just opens up a whole lot of possibilities that a resistor does not allow for.

Cheers!

-Tim
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« Reply #17 on: May 16, 2008, 12:43:27 PM »

Wow, talk about over my head!  Roll Eyes  Grin
I think I will just use the 24v. LED bulbs I can find with my 24v. system. I did get the two I ordered, along with the sockets, in today. I think I can make things happen.  Wink

Thanx for all the info guys!
   Chaz
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« Reply #18 on: May 16, 2008, 10:21:41 PM »

Very nice reply, Tim.

I've never used these buck regulators. I can see that it's something I will have to get used to.

Thanks.

Tom Caffrey
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« Reply #19 on: May 17, 2008, 12:15:28 PM »

Tim,

Is a buck resistor a speed control with a coil in line?  I thought the speed control/dimmer was the way to go.  Is there any reason they can't be used and what is the advantage of the buck resistor over them.

The temp and voltage sensing features are just so interesting to me.  Very much want to see your design schematic.

You have to go a ways to impress PVCCES....congrads!

John
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« Reply #20 on: May 17, 2008, 12:54:10 PM »

I used a 6 volt rated zener diode, low wattage, to drop the 12 volts to six volts on my restored 46 Ford. I converted the Ford to 12 volts and wanted to still use the 6 volt instruments. It worked great and never got hot at all.

I really see no reason why a person could not do the same to drop 24 volts to 12 volts. Cheap and easy by using a 12 volt zener.

Richard
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« Reply #21 on: May 18, 2008, 12:28:17 PM »

The only problem with a MOSFET switching voltage regulator, or dimmer switch, is I can see the light flicker and it bugs me.  At least this is on a halogen light with dimmer switch when on the lower lighting setting.  I assume that it would in fact be worse with an LED since LED's react so quickly to on or off (no lingering light after it is turned off).  I would think the best regulation for your output is just to get as many LED's necessary to run at that voltage without regulation (like series wired Christmas lights).  But-then I'm not an expert on LED's either.  Good Luck, TomC
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« Reply #22 on: May 18, 2008, 05:14:06 PM »

Below is a simplified schematic of the described switch supply.


Is a buck resistor a speed control with a coil in line? I thought the speed control/dimmer was the way to go.

Hi John,

That's the basic theory behind it - but it's a little bit different (and please allow me a slight terminology correction - it's buck regulator).  A PWM speed controller is an device that simply cuts-off and turns on the power in rapid sequence - based on the user's set-point, not based on any circuit conditions.  Since the setting of the dimmer is external to the system, and does not get affected by the conditions of the system - it is defined as an "open loop system".

The "coil" (an inductor), is not a "consumer" of energy but rather a "resistor of energy chage" - much like a capacitor.  Where a capacitor acts like a "short" for high freqencies (i.e. it allows changes at a high rate) and blocks lower frequncies or DC (resists change by dumping its stored energy, or storing surplus energy), an inductor acts like a "short" for low frequencies (i.e. it allows changes at a slow rate) and blocks higher frequencies (resists change by dumping its stored energy, or storing surplus energy).  With the buck regulator, there is also current-sense resistor in the circuit (this IS a "consumer").  This resistor works on the same principal as every other resistor - only the circuit is designed to cash in on the fact that for a fixed resistance, if either the voltage or current changes - the other will change in same direction.

 E
---
IxR

Where:

E = voltage
I = current (in amps)
R = resistance

If you use a fixed resistance - like say 5-Ohms, now multiply a current value like 700mA (0.7Amps) to get the voltage across that resistor (5 x 0.7 = 3.5volts).  If the current changes to say 900mA (0.9Amps), the voltage will change to (5 x 0.9 = 4.5volts).

In order to measure across a resistor, the driver IC needs two points.  Typically with high-side switch drivers, this current resistor is between the load and ground, so if the driver IC has a ground, that's one point.  A current sense pin that hooks up to the side of the resistor between the resistor and the load is the other point.  In the schematic the second pin is marked as "CS" on the driver part.

This lets the driver IC "watch" the current of the load by watching the voltage across the current sense resistor (if the current goes up, the detected voltage goes up, and if the current goes down, the detected voltage goes down).  This is compared to an internal voltage reference (since this is a very low current reference a very accurate linear regulator can be used without generating a lot of heat, or burning a lot of power).  When the voltage is above the reference the switch turned off, and when the voltage falls below the reference it turns the output back on.  This makes this type of control, a "closed loop system" where the system reacts to conditions within the same system.

Based on the size of the coil (measured in "Henrys", and marked by "h"), the speed of the switching action can be very fast or very slow (but with faster frequencies, the overall efficiency goes down).  A common frequency range is between 30kHz an 100kHz (30 to 100 thousand switch cycles per second).  A low-Henry inductor causes the switching frequency to go up, where a high-Henry inductor causes the switching frequency to go down.  Higher Henry inductors for a given current rating require more wire to be wrapped - so as a result they end up being physically larger.  There are some small inductor designs built for very compact system (at the expense of a few percent of efficiency), which have very small Henry inductors - and they can have switching speed in the megahertz (MHz) range (I've seen a few designs at 3MHz and higher... that's 3 million on-off switch cycles per second).



The only problem with a MOSFET switching voltage regulator, or dimmer switch, is I can see the light flicker and it bugs me...
...I assume that it would in fact be worse with an LED since LED's react so quickly to on or off (no lingering light after it is turned off)...

Hi TomC,

With PWM dimming, this can be an issue depending on the implementation - and based on the dimmer topology used, the switching frequency CAN be very low.  I too have noticed that with low frequency PWM (like that which is used on newer cars for their brake lights), I can detect a flicker as the car passes into my peripheral vision.  This is because the PWM is not a true high frequency PWM (above 30kHz), but instead is a modified "pseudo PWM" at a very much lower frequency (sub-100Hz, so it uses differing "chunks" of on-off time - this is probably what you are seeing).  I've discussed this with some people in the industry and it seems to be a trade-off in dedicated hardware vs. the use software on slightly more expensive in-vehicle networks.  Basically, more and more systems in a car are being pulled into a computer as it helps with diagnostics and management (something the car makers will kill for).  There just aren't enough processor cycles to run PWM at high frequencies (the change of the current sense line would be too fast for the software to react to), and there isn't enough incentive to use dedicate hardware drivers in every fixture (yet - prices are coming down, and this will soon be a "past" problem).

I have never met anyone who could see the flicker in a true PWM dimmer on LEDs at a frequency over 1kHz (and I'm in the video industry - where everyone's eyes are about as good as it gets Wink).

One other point of interest - because the current sense voltage does not have to fall all the way to 0 volts (which means 0 current across the resistor - and through the load), the brigtness doesn't change as aggressively as with PWM dimming.  It may fluxuate about 5-10%, and you will not be able to see that (people need at least a 30% change over a long period of time to notice a difference).

I would think the best regulation for your output is just to get as many LED's necessary to run at that voltage without regulation (like series wired Christmas lights).

I did mention this before as an option to help with efficiency - but I mentioned the primary draw-back of this approach:  It's designed to work at or slightly above the design voltage.  If the voltage changes, one could over saturate the LEDs (causing them to over-heat and have a very short life), or they could just turn off from under saturation (no electrons can "jump" the gap in the LED die, so no photons are generated).  This is how the lower cost "single voltage" LED fixtures are designed.  As anyone in the LED industry will tell you, "The #1 killer of LEDs is HEAT".  Over-heating the LEDs will cost you longevity and reliability.



I used a 6 volt rated zener diode, low wattage, to drop the 12 volts to six volts on my restored 46 Ford. I converted the Ford to 12 volts and wanted to still use the 6 volt instruments. It worked great and never got hot at all.  I really see no reason why a person could not do the same to drop 24 volts to 12 volts. Cheap and easy by using a 12 volt zener...

Hi Richard,

First, I don't imagine that the instruments draw a lot of power.  This is a big help with zener regulation.  With these LEDs, we are frequently talking in the 500mA and above range (the highest power single-die LED I've seen draws 1.5Amps at 3.7volts - that's 5.5 Watts!). To run three of those from a 12 volt supply (the max you could do, because of the voltage drop of each LED [11.1 volts]), you'd need an 11 volt zener at 2Watts or more (this is achievable).  This would also cost you about 40-50 cents.  But, if your supply voltage dropped below 11 volts (a lead-acid battery is considered "dead" below 12.01v) due to some high transient load like a starter or something else, the LEDs would shut off.

Now if you throw in an alternator to that, where the charge voltage is now 13.65 volts or up to 14.4 volts - the zener solution is a bit more risky.  You can't just add another LED to raise the regulation point up to 14.8 because the LEDs would never get enough voltage to turn on - also, because the voltage is now 2.65 volts above the regulation point of the zener, it now has to dissipate 4Watts of power to keep the LEDs safe.  This means your efficiency has gone down from ~92.5% (with the 12volt supply) to only ~80.4% with the 13.65 charging supply.  That's a big penalty, and a lot more heat that has to be dealt with.  If your supply does actually go up to 14.4volts, your efficiency takes another dive to ~76.4% [almost a quarter of the power consumed is lost to heat!] and you need a Zener that can absorb more than 5Watts (these power ratings start to get hard to come by in small quantities...).

Buck regulators are more constant with their efficiencies, with the National Semiconductor part I mentioned staying above 94% no matter what voltage you give it - up to 60 volts!  One other problem is that power LEDs tend to change their equivalent series ressitance (ESR) in the negative as they get warmer.  This means that they can draw more power as they get warmer, shortening their life even further.  A current regulation supply like a buck which watches a voltage based on the load - has a better chance of regulating a dynamic load like a power LED.  More advanced LED driver ICs even integrate a negative temperature coefficient resistor that changes with temperature variations - to create another reference voltage to compare to (and if that voltage changes - it can again be used to either add or subtract drive current based on the circuit design).


Again to each their own, and there are many "owns" that will work.  One person's solution may not be another's - or it might be.

Cheers!

-Tim
« Last Edit: May 18, 2008, 08:12:14 PM by Tim Strommen » Logged

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« Reply #23 on: May 18, 2008, 08:40:30 PM »

TomC,

  I just re-read your post - and I think I discovered the problem.  It sounds like the PWM device you are using is a fixed on-time, variable off time PWM modulator (aka a variable frequency PWM).  This is not a good device to use on lights, rather a better method is with a fixed frequency, variable DUTY CYCLE.  This does not change the start time interval of the "on" signal, but shortens the length of "on" portion of the total period.

As illustrated in the below picture, I've shown a 50% wave-form (what the voltage would look like on a scope) for both the fixed frequency and variable frequency PWM generators.  Note that they both have four "on" (rising) events from left to right.  Now look at what happens when you only want 25% of the period to be on - with the variable frequency PWM, it adds time to the "off" portion of the signal, whereas with the fixed frequency variable duty cycle PWM - it subtracts the same ammount of time from the "on" portion as it adds to the "off" portion.  This retains the four time periods for the same frequency (for both 50% and 25% outputs), but the variable frequency PWM has dropped to only 3 time periods - this causes the load to have more "coast down" time - and for a halogen bulb this would let it cool off and put out less light until the next "on-time".

With a fixed frequency PWM, there is a consistent period until the next on-time, which will not in all likelyhood allow the load too much coast down time to cool off - so there should not be flicker as the duty cycle is dropped off lower and lower (the output should dim smoothly).  If we were to assume that the below wave-forms were looking at a second in time, this would show that the 50% wave-forms had a 4Hz frequency.  For the 25% wave-forms, the constant frequncy PWM kept the 4Hz frequency, while the variable frequency PWM dropped to 3Hz.

Normally, a fixed frequency PWM would be above 100Hz (more like 28kHz or higher), to keep loads from "whining" (a motor will emit a tone at the frequency of the PWM - and if it's in the human audible hearing range, it will sound like the motor is squeeling... because you can actually hear the current "banging" through the windings).  TV systems are designed with a 60Hz or 50Hz refresh rate - while movies (Film) have a 24Hz refresh rate.  With these low rates often a flicker or "judder" is noticable in the picture.  At PC rates over 85Hz, most of these issues go away.


At any rate (punn) - if you have PWM that does variable duty cycles, your result will be better.

Cheers!

-Tim
« Last Edit: May 18, 2008, 08:45:12 PM by Tim Strommen » Logged

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