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12/24/2009, 02:42 PM | #51 |
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What you guys may be missing on these cap Life Numbers is the operating temperature. You may have noticed that your TVs, ovens, and car stereos last longer than say 2000hrs, (1/4 of a year!!!)
This is because that Life Number is only for the cap running at that label temperature. Think about it.. Do you think your caps are running at a temperature above boiling water? (105C) Is your flesh instantly seared when you accidentally touch one? If you dig around in the data sheets you should find the temperature/life curves. They embrace the chemical dictum of "double the reaction rate for every 10 degree rise". Luckily this pays us dividends in reverse. If you run a 105C rated cap at room temperature its life will be something like 200,000 hrs. (20 years 24/7) So keep your caps at a reasonable temperature and they will greatly exceed the life of the LEDs. And if you keep them cool, 1000hr/85C caps will even serve you well. |
12/24/2009, 03:08 PM | #52 |
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I did some research today. Aparently the lifetime of the capacitor is a well known issue in power electronics. I found a couple of articles:
http://powerelectronics.com/passive_...uction_boosts/ this one basically says what I did, but gives actual details of what, and why it happens. This one is even better. It is right on our topic: http://www.lrc.rpi.edu/programs/soli...E2009-7422.pdf It discusses the lifespan of led drivers, and failures due to aging of the capacitors. I skimmed over, and it seems that commercial drivers are good for 10K-15K hours. We should try to get the DYI drivers close to this, otherwise, it may be cheaper to buy one. Merry Christmas! |
12/24/2009, 03:38 PM | #53 | |
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I understood that this is a bit different when high current/voltage ripples run through a cap, i.e, in power switching applications. These spikes create high temperature, but for very short times within the capacitor and cause failures in small areas (the "hot spots"), while overall, the temperature stays reasonable. Anyway, I don't think we can design the perfect driver, but the rules of thumb I would follow: 1. use the best in/out capacitors I can get, and 2. do not mount the drivers permanently, and use connectors, to allow quick replacement (even with commercial drivers) |
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12/24/2009, 11:51 PM | #54 | |
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Occasionally someone screws up and a bad load of them hit the market. Around the turn of the century this occurred with one cap that was used in millions of pc mother board power supplies. For a while there, 90% of all mother board failures were considered to be from that problem. |
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12/25/2009, 11:47 AM | #55 | |
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Some major films have been filmed using my electronics modules, (and still are). If they can survive harsh environments ranging from deserts through to the antartic (often both!), I reckon they can survive the relatively pleasant environment above an aquarium, with only slightly higher humidity than usual and an ambient temperature. I can see the argument in the paper linked to, but in real life, I've never experienced it. The driver is very cheap, in any event (roughly $3 per 6 leds) and in my case they're all replaceable modules, but of all the things that could go wrong, I don't think capacitor destruction is high on the list.. Simon. |
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12/26/2009, 12:06 AM | #56 |
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der did you ever make a real schematic? the few changes you listed are almost half the schematic.
thanks jeff |
12/28/2009, 09:30 AM | #57 |
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I've been "offline" thanks to the holidays. Once things settle down I'll put something out.
BTW I appreciate the cap discussion, so thanks everyone. I might just build with the longer life parts anyways, since they're not always more expensive (Panasonic's 7k hr caps in this size are actually CHEAPER than the 2k hr caps I used, by a few cents.)
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12/28/2009, 03:10 PM | #58 |
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Are we changing any of the P/N's? I just did an estimate on Digikey and with buying enough stuff for 10 builds, I only need 7, it was less than $40.
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12/28/2009, 04:20 PM | #59 |
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Yes, but I haven't looked up appropriate numbers. I'd just look for similar component values for the caps that were longer life. I'll do a "revised" part list when I put the schematic up.
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12/29/2009, 07:52 AM | #60 |
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I like look of the Nichicon caps. But twice the price, they are still under $0.50.
The only other Panasonic one I found was the P-13459-ND, which is 5K@105*.
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01/02/2010, 04:58 PM | #61 | |
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Having gone through a couple of prototypes now, it's pretty cool (in every sense of the word ) when it finally works, but getting there needs some thought. [sulk] I prefer digital electronics to all this analogue stuff Of course I managed to make things worse by using the Luxeon Rebels I had lying around (3.15v forward voltage drop) in a circuit designed for Cree XRE's (3.6v forward voltage drop) without re-reading the datasheet. With Vin at 19.95 volts (3.15x6 + 0.7v for the diode + 0.35v for the chip), everything runs nice and cool now... When I was pushing 24v through it, I got about 2 minutes before things started going haywire... Just a precautionary tale Simon |
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01/02/2010, 05:18 PM | #62 |
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An updated "real" schematic as promised:
This is just the schematic I posted on the first page with the modifications I mentioned when I posted it (ditching their dimming scheme and condensing R1 into a single resistor, etc.) Also, I corrected some of the part names - their schematic has some silliness (two C2's for example) so these names are a bit wonky, but correspond to the BOM I posted on the first page. I will revise that and post again soon if people are interested.
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01/02/2010, 05:28 PM | #63 | |
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I did go through a few iterations with my driver, but I was lucky enough that things more or less "just worked" other than a few basic adjustments - swapping different Rsense to get different drive currents, etc. Plus one hilarious mistake that had me stumped for nearly a week early on: I misread the spec for the timing cap, and was using a cap that was about 100 times too big. Needless to say, the driver didn't function AT ALL like that. I'm also still on the fence about how to actually build these for my big tank. I'm going to need 25 of them. All my prototypes are on the cheapo protoboards I mentioned earlier in the thread. It feels like a bit of a hacked way to do this, but it works and it's cheaper and quicker than doing a "real" PCB. So I'm just not sure if it's worth the effort to design one and get a board house to make it for me.
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Inconveniencing marine life since 1992 "It is my personal belief that reef aquaria should be thriving communities of biodiversity, representative of their wild counterparts, and not merely collections of pretty specimens growing on tidy clean rock shelves covered in purple coralline algae." (Eric Borneman) Last edited by der_wille_zur_macht; 01/02/2010 at 05:33 PM. |
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01/02/2010, 06:36 PM | #64 |
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just curious how well the pwm curciut worked? did you get any noise when dimming? also it might be worth it if you offered a group buy or something, i might be in for 20. also it might be helpfull if someone did a full picture run through if you did get the boards printed of how to wire them since there are alot of people to who have no idea what a schematic is much less how to read one.
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01/02/2010, 06:52 PM | #65 |
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The PWM dimming works totally fine. I'm using it with an Arduino. Basically, you connect a PWM pin from the Arduino to the PWM pin on the driver. Then, connect the Arduino's ground to the power supply's ground. Program the Arduino to provide whatever dimming profile you want.
There is a TINY bit of noise from the driver itself, but it's VERY quiet. You can't hear it when it's in an enclosure unless you lean right over next to it. Most of these parts have quantity discounts from digikey or other suppliers so a group buy wouldn't save much there. And for the PCBs I'm not really interested in going through the effort of setting something up publicly, but I would definitely share the design if I go that route in case others wanted it. If someone else wants to do a step by step photo tutorial that would be awesome. I'll do one the next time I build some of these (probably a month or more off) if no one else has by then.
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01/02/2010, 07:32 PM | #66 | |||
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Simon. |
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01/02/2010, 08:00 PM | #67 |
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i was thinking more of having a manufacture make a board and have it availiable to buy directly from them. not sure how much that cost since i have only ordered boards from honeywell and those were in the thousands. i would be curious how much small single lam boards cost. depending on price it might comeout that having the boards made now runs about the same as a meanwell that run in the $30 range for 10 or more drivers
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01/02/2010, 08:16 PM | #68 |
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01/02/2010, 08:51 PM | #69 | |
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Simon |
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01/02/2010, 09:04 PM | #70 | |||
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I know people who have used seeedstudio for small board runs. We're talking about the kind of volume where we'd probably be around $3-4 per board. That's for double sided boards. I dunno of any place that does single-sided but would be interested in hearing if someone else knows.
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01/02/2010, 09:10 PM | #71 | |
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Rf = Vfb / Ileds according to the datasheet, with Vfb = 100mV Using Rf = 0.13 ohms, Ileds should be 100mV / 130 mOhms or 769 mA, but the multimeter in series with the LEDs shows only 547mA. Oh well, time to send off for a collection of resistors around that value to see which one *really* gives me something around 900mA... Simon. Last edited by SpacedCowboy; 01/02/2010 at 09:12 PM. Reason: Grammar changes |
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01/03/2010, 12:02 AM | #72 | |
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I have a CNC-router good for about 1.5mils. I'm going to check to see if they just sell the converting software. |
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01/03/2010, 12:33 AM | #73 | |
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The EP2002 quotes an X/Y resolution of 0.005mm (or 0.2 mil) but by the time you've taken account of the mechanics of the process, you're actually at ~4mil in the real world of rotating spindles and milling copper. I can repeatedly do 4 mil circular tracks, and see them as continuous, and line them up on double-sided boards, but getting real results at 1.5mil resolution would be something else! The EP2002 does a surface-scan of the copper-plated FR4 before it mills the copper away, so it can judge the precise depth needed for the Z axis at any given point on the circuit. It does that by making the drill-bit complete a continuity circuit with the copper-plate at regular grid-points before actually drilling/milling. I can't get 4mil accuracy without doing this because the copper-plate surface height varies too much, and the software has to cope with embedding just the correct fraction of the 90-degree or 60-degree head into the copper-plate to reliably get 4mil accuracy. Having said that, PCAM (the EverPrecision s/w) really doesn't do itself any favours. I work at Apple, where there's a huge focus on how the software interacts with the user. I could use PCAM as a worked example of how not to write software for many many reasons... Once you've learnt all the foibles and gotchas, it's actually very powerful under the hood, but it takes some time to find the hidden menus and the same menu giving different options on different screens... Simon. |
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01/03/2010, 10:00 AM | #74 | |
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Also, a note to anyone thinking about building these - I should have said this earlier I suppose, but if you're not planning on using the PWM pin for dimming, you MUST provide a voltage on that pin to get the thing to turn on. Otherwise the driver won't work. This is easy though. Assuming you're using a 24v DC supply, you can just connect the +Vin pin to the PWM pin and it'll run at 100% (as if it was getting a full duty cycle PWM signal). You could just hardwire this on your PCB, but in case you ever decide to add dimming, it might be better to bring the pin out and use a jumper from Vin. In the next week or so I'm going to try some other versions with different caps, based on some of the discussion on this thread. I'll report my findings. I'm going to try with smaller caps, and with no (big) cap on the output side. I'm also going to try two drivers on one PCB with only one (electrolytic) input cap. If these experiments show that you can use smaller caps and/or fewer caps, it would make the circuit simpler/cheaper for people doing several of them. Also going to go ahead and design a "real" PCB in Eagle. I'm a very weak PCB chef so I might be calling on some of the more experienced folks to help me with this.
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01/03/2010, 10:23 AM | #75 |
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Have you seen this? It's for buck pucks, but it should work with these DIY drivers too.
Power Led shield for Arduino |
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