Replacement LED?

On 22 Mar 2016, DaveC wrote


Just measured working light of same model: using power supply @1.5v (measured at flashlight battery terminals), the current from the ps is 400 mA. Estimating 75 percent efficiency that makes about 450 mW.
So a 1/2W replacement LED might be what I?m looking for.
Thanks.
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From what small ammount of playing with the voltage converters, they are almost like a transformer in action. If you go to a higher voltage, the low voltage current will be a lot more than is used by the load, and if going to a lower voltage , the current from the battery will be less than the actual current used by the load. That keeps the total power drawn the same minus the efficency of the converter.
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News to me. In what way are multi-layer ceramic caps and soldering irons incompatible ?
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On Sat, 26 Mar 2016 19:56:12 +0000 (UTC), Andre Majorel

Not a bit.
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Andre Majorel wrote:

Really, before I got my pick and place machine, I hand-soldered about 25,000 .1uF 0805 capacitors. NEVER ONCE had a bad one. I still hand-solder a fair number of low production boards and prototypes, and have never seen a problem with MLCCs.
Jon
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On Sat, 26 Mar 2016 19:56:12 +0000 (UTC), Andre Majorel
Sorry for the delay but I missed the followups to my comment.

Thermal shock easily cracks MLCC caps. I learned that the hard way while fixing several Apple Mac Mini computahs, which feature a collection of MLCC on the bottom of the main board. <https://web.archive.org/web/20130606062903/http://blog.helpmymac.ru/?p585 The original failure mode was shorted MLCC caps caused by either thermal shock or board flex. The bad ones were easy to find with an ESR meter. However, when I tried to install replacements (and guessing the part value because Apple doesn't supply service information to non-authorized repair shops), I managed to crack and short several known good MLCC caps with a soldering iron. Having learned the lesson, I used some solder paste and a hot air SMT reflow gun to do the soldering. I also pre-heated the PCB and let the caps cool down slowly. I don't know if that was necessary, but it worked every time. I'm told that two solding irons used as a tweezer also works, but I haven't tried that yet.
You'll find some more details under: <https://www.google.com/#q=mlcc+capacitor+crack <https://www.google.com/#q=mlcc+capacitor+soldering+iron Lots of articles and guidelines on handling and soldering these caps, some of which warn about using a soldering iron. For example:
<http://www.vishay.com/docs/45034/soldrec.pdf 6. Soldering with a Solder Iron Attachment by soldering iron is not recommended. A heat shock may cause a crack in the MLCC chip capacitors, however, if solder iron is used, the following precautions should be taken: ... (etc)
Damage Prevention When Soldering Ceramic Chip Capacitors <http://www.eptac.com/webinars/presentations/eptac_09_17_14.pdf Hand Soldering - A pencil type soldering of 30 watts maximum and with a diameter of 3 mm maximum should be used. - The soldering iron tip temperature should be less than 300C [572F] and maximum contact time should be 5 seconds. - The soldering iron tip should never come in contact with the component body. Ever try to solder a small MLCC cap without touching the body with the soldering iron tip? Good luck.
<http://www.murata.com/en-us/support/faqs/products/capacitor/mlcc/mnt/0001 In order to prevent damage (cracks) to the component that can be caused by localized rapid heating and heat shock, preheat the chip, for example, to prevent it from being subjected to heat shock.
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Jeff Liebermann wrote:

I've heard these stories a number of times. And, yes, maybe some people use insanely hot irons or in some other way cause this problem. And, some really large caps are prone to this damage. But, as I say, I have hand- soldered over 25,000 0805 MLCC caps of modest value without seeing this problem. I use a very good Weller temperature-controlled iron, and run it at a modest temperature. Much better to use an iron with really good thermal conductivity at a lower temperature than one with poor conductivity at a very high temperature.
Jon
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Jeff Liebermann wrote:

How were the new caps stored? If it is where they can adsorb moisture, you can damage them with an iron that is too hot.
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On Thu, 31 Mar 2016 04:20:59 -0400, "Michael A. Terrell"

Well, I must admit that I didn't take any special precautions. Methinks that the relative humidity in my office runs between 40% and 60% but is not monitored or recorded. Occasionally, it gets low enough to where static electricity becomes a problem, or high enough to where I'm rather uncomfortable, but those are rare. The caps a mix of cut tape and loose bags stored in Ziploc bags (mostly pink anti-static) and in paper coin envelopes. Nothing in hard plastic or metal drawers that might chip or crack them. Although I know that these caps make tolerable hydrometers (and microphones), I don't think they can absorb enough moisture from the air to where a steam explosion would be a problem.
I did some digging to see if humidity might be a problem in storage conditions. There were plenty of notes on how a cracked capacitor might allow water to enter the dielectric. Soft (solder) termination is the recommended fix. Some suggests pre-heating the capacitors before soldering to drive off any moisture. One demands that the caps be used within 12 months. I didn't see humidity as being a problem until AFTER the capacitors had cracked. It would take some time for the moisture to alter the capacitor characteristics. With my hand soldering technique, I managed to instantly produce shorted capacitors, which methinks was more likely due to uneven thermal expansion, than to moisture incursion.
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On Thu, 31 Mar 2016 04:20:59 -0400, "Michael A. Terrell"

I haven't had any problems with cracking but I used to have problems with end caps falling off. I haven't seen the issue for some time, though perhaps it was a problem with the manufacturer. Our purchasing group prefers Murata, so that's what I use (GRM series).
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krw wrote:

We pre baked some boards and other components before assembly and reflow. That eliminated cracked multilayer SMD capacitors, tombstoning of two lead components and losing end caps. That was in N Central Florida which has plenty of humidity problems. The so called HVAC 'engineers' were clueless about how to control the humidity, so we had to resort to baking. LSI SMD ICs were backed and heat sealed into antistatic bags with moisture adsorbing packets.
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On Fri, 01 Apr 2016 17:21:31 -0400, "Michael A. Terrell"

Sure, baking humidity sensitive parts is common before pick-n-place. It's a bit less common when hand (de)soldering, though. ;-)
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krw wrote:

We had a high failure rate on hand soldered FIR chips, before we started baking them. The bottom of the packaging was the thinnest, and it would bow out as it released steam during hand soldering. No one in EE or ME believed me, until I finally got them to try it for themselves.
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Not at all. That was very interesting. Thanks Jeff and everyone.
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