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I have two of those little crummy pocket-chargers that I would like to turn into one not as crummy battery pack. Internally, they run off cylindrical lithium ion batteries that appear to be the exact same make. Is it just a matter of adding the batteries from one in parallel to the other, or will that confuse the charging/protection circuit? I'm not familiar with lithium ion at all beyond knowing that its discharge curve is really nifty and it needs a lot of special care on the charger or it will burst into flames.
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Feb 16, 2015 18:10
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Parallel Paraplegic posted:I have two of those little crummy pocket-chargers that I would like to turn into one not as crummy battery pack. Internally, they run off cylindrical lithium ion batteries that appear to be the exact same make. Is it just a matter of adding the batteries from one in parallel to the other, or will that confuse the charging/protection circuit? I'm not familiar with lithium ion at all beyond knowing that its discharge curve is really nifty and it needs a lot of special care on the charger or it will burst into flames. I don't recommend doing this, but here is some info anyway: When packs (especially those with cells in parallel) are made, they are matched to similar cells. At the cell-manufacturing stage, new cells are charged for a fixed amount of time, and then sorted based on what voltage is attained (or so the reps from Panasonic lead me to believe). The voltage they reach gives an indication of combination of their internal resistance and overall maximum capacity. They are graded alphabetically (although S rank is still the best lol), and sold to OEMs as lots of the same grade from the same production run. Device manufacturers may further to testing to sort cells specifically based on internal resistance, which helps ensure that multicell packs won't get dragged down by one "bad" cell. Adding mismatched cells in parallel will probably degrade the overall performance, but probably won't be a safety issue...However, for cheap charger circuits, it may still cause issues---they may not be setup to handle the lower impedance of the new, bigger pack without outputting too much charge current. For better charger circuitry, they may intelligently fail if the pack takes too long to charge (for example). Additionally, the decreased pack impedance means that the pack will be able to supply twice as much short circuit discharge current, which could again be an issue with cheaper circuitry. Finally, if you really want to do this, make sure to preserve the nickle tabs spot welded to the cell terminals. You cannot safely solder to the ends of a lithium ion cell, but only to nickle tabs. These are easy to work with, but sometimes require careful engineering and insulation on the part of the manufacturer to make sure the geometry is correct and that none of the sharp edges can cut into cell insulation. Also, before cells can be connected in parallel, they need to be balanced to the same exact voltage. Otherwise, one cell can charge another with a potentially dangerous amount of current (dangerous to the cells).
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Feb 16, 2015 19:05
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Feb 16, 2015 20:21
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Slanderer posted:I don't recommend doing this, but here is some info anyway: Ah okay, I figured there were lots of hidden traps and complex poo poo behind the scenes if even Boeing couldn't get the fuckers to not catch fire in the 787, glad I asked. I think I'll just figure out a way to hook them together upstream of the charging/protection circuits then.
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Feb 16, 2015 20:49
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Today's project has involved searching for the big handful of Molex connectors I bought the other day. Does anyone remember where I put them? I sure don't.  Tomorrow's project will probably involve some component organization, which will likely last all of ten minutes before poo poo's all over the place again.
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Feb 16, 2015 22:29
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poeticoddity posted:The current equipment has nearly 2 amps running through the LEDs, so I'm going to guess that'd cause a problem. I'm probably going to end up passing on this project unless there's some sort of crazy incentive for me, but I'll let you know if anything develops. Thanks again. Would it be possible to put the LEDs and their drivers in a box someplace far from your fMRI setup and just pipe the light in through some plastic optical fibers with a glass or plastic lens on the output end to expand the beam? That'd give you your bright light inside the MRI field with no magnetic or conductive materials to mess with.
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Feb 17, 2015 02:01
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PDP-1 posted:Would it be possible to put the LEDs and their drivers in a box someplace far from your fMRI setup and just pipe the light in through some plastic optical fibers with a glass or plastic lens on the output end to expand the beam? That'd give you your bright light inside the MRI field with no magnetic or conductive materials to mess with. If I recall correctly, he is using LEDs whose luminescence is pretty deep into the ultraviolet wavelengths. I think one worry there is that at deep enough UV wavelengths, a lot of types of glass and plastic become absorbers. I have been told that the UV-blocking properties of sunglasses are basically had for free if you make the glasses out of polycarbonate, and you are a sucker if you pay for extra UV protection. He already is paying a lot for each UV photon coming out of his LEDs and would maybe be unhappy with the possible optical loss. silence_kit fucked around with this message at 01:57 on Feb 18, 2015 |
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Feb 17, 2015 17:07
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What about minimizing current by driving the LEDs in series with a high-ish voltage?
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Feb 17, 2015 23:35
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PDP-1 posted:Would it be possible to put the LEDs and their drivers in a box someplace far from your fMRI setup and just pipe the light in through some plastic optical fibers with a glass or plastic lens on the output end to expand the beam? That'd give you your bright light inside the MRI field with no magnetic or conductive materials to mess with. If I was using a fiber optic system, I'd most likely redesign the system to use one or two large LEDs instead of the small ones I'd been using. Honestly, in the unlikely event I'd be building this, I'd almost certainly insist on the fiber optic route just to avoid running current inside of the magnet. silence_kit posted:If I recall correctly, he is using LEDs whose luminescence is pretty deep into the ultraviolet wavelengths. I think one worry there is that at deep enough UV wavelengths, a lot of types of glass and plastic become absorbers. Different system. The near-eye glare sources use fairly common cool white LEDs. We eventually got some UV LEDs for the equipment that requires it, but those are metal can packages and there's no way in hell I'd be able to find MRI compatible ones. BattleMaster posted:What about minimizing current by driving the LEDs in series with a high-ish voltage? Not a bad idea. Getting that kind of voltage would be tricky (3.6Vf for 48 LEDs per board) but no part of this sounds easy and hopefully I'll have graduated and left before anyone gets dead-set on having me do it and I won't have to worry about it.
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Feb 18, 2015 06:29
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Just messing around last night, I built this little FM transmitter circuit (though with a 5v regulator and wall wort as the power supply rather than the battery):  It works surprisingly well for something I built out of random BJT's I found lying around and a hand-wound coil, though looking at the signal with my RTL-SDR shows a lot of harmonics doing all sorts of crazy poo poo about 1MHz on either side of the signal. Anyway, there's a pretty loud hum being transmitted with it, sounds like a mains hum to me but I'm not positive. In addition to the 470uF + 223pF filtering in that circuit I have another 1000uF cap on either side of the voltage regulator, and I tried adding a 240uH coil to the positive input (between the regulator and the circuit) which actually reduced the harmonics quite a lot but didn't do anything noticeable about the buzz. I'm going to try running it off a battery tonight to see if it is in fact a mains hum or just the circuit oscillating, but I was just wondering if this is something inherent and unavoidable with such a simple circuit made out of ~10 cents worth of parts, or if I missed something easy when it comes to filtering. Shame Boy fucked around with this message at 16:05 on Feb 18, 2015 |
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Feb 18, 2015 16:02
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Chakan posted:Hey, I'm new to this area, but the OP is really old. Are folks working on one that's updated, or able to point me to a beginner resource? On page one someone linked this and said it was really good, but most of the old links and images are dead. Quoting for a little help, I ordered the book though and it should be in shortly.
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Feb 18, 2015 16:31
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Parallel Paraplegic posted:Just messing around last night, I built this little FM transmitter circuit (though with a 5v regulator and wall wort as the power supply rather than the battery): I made something similar late one night years back. However, I was having trouble tuning it or something, so I ended up breaking the loving thing in half and using what was left to make a newer, better one with a sorta-tuned (or at least buffered?) antenna.   Honestly, I don't even remember which is the new half, and which is the old half. The copper clad board had a few pads cut into it with a utility knife, and the perf board looks like garbage because I think I had to cut a few components off and tack on a bunch of new resistors to make it work right. Also, I recall that it was pretty garbage and could be received on multiple frequencies at the same time.
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Feb 18, 2015 17:49
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Chakan posted:Quoting for a little help, I ordered the book though and it should be in shortly. You're right, the OP is pretty old/busted. Anyone have a free evening and feel like making a new decent one?
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Feb 19, 2015 02:44
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Take a look at this article: http://www.anritsu.com/en-US/Products-Solutions/Solution/Understanding-directivity.aspx How are the curves in Figure 4 calculated? I've found similar figures reproduced in many articles, but the equations are never listed.
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Feb 19, 2015 04:57
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Cyril Sneer posted:Take a look at this article: Those look like bode plots, though the falloff rates look a little weird. http://en.m.wikipedia.org/wiki/Bode_plot The math is really complicated, differential equations and whatnot. Engineers are lazy, so they effectively made all of the scales logarithmic and turned diffy-q into algebra (there are some other transforms in there too). Or at least that is what I remember from my circuits class almost 10 years ago.
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Feb 19, 2015 14:48
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gwammy posted:Those look like bode plots, though the falloff rates look a little weird. uhhhhhhmm did you actually look at it? It's not a bode plot. It's curves for error rates in return loss measurements for given directivities.
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Feb 19, 2015 15:02
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Slanderer posted:I made something similar late one night years back. However, I was having trouble tuning it or something, so I ended up breaking the loving thing in half and using what was left to make a newer, better one with a sorta-tuned (or at least buffered?) antenna. Update on my crummy radio: powered it off a 9v battery with a really stable 5v regulator, this tuned it up to 97mhz from 89mhz for unknown reasons, the harmonics are less but still pretty hilariously broad-band (I managed to blot out every radio station within +- 5mhz of my signal  ) and the hum is still there. Also the signal... uh... wobbles. It wobbles back and forth around its center frequency at a rate of about 2hz, creating some really weird patterns in the waterfall. Plus the capacitance of me being within 5 feet of it is enough to de-tune it by up to 1mhz. Anyway I'm going to start over tonight with a better circuit. Thanks for dealing with all my random questions, electronics thread! Wish I had more to give back 
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Feb 19, 2015 18:37
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Cyril Sneer posted:uhhhhhhmm did you actually look at it? It's not a bode plot. It's curves for error rates in return loss measurements for given directivities. I'll admit that I was a little high at the time...
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Feb 19, 2015 20:46
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Does anyone have schematics for a battery powered mini guitar amp with an 8 ohm speaker? Nonentity had his own thread a while back and has a project where he made one posted on his website, and I want to make one of my own. I'm a beginner in electronics, so I couldn't cobble one together on my own, and a detailed set of schematics would be a big help.
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Feb 21, 2015 18:10
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Akula Raskolnikova posted:Does anyone have schematics for a battery powered mini guitar amp with an 8 ohm speaker? Nonentity had his own thread a while back and has a project where he made one posted on his website, and I want to make one of my own. I'm a beginner in electronics, so I couldn't cobble one together on my own, and a detailed set of schematics would be a big help. For the most part the datasheet reference design for something like the LM2877 or TDA1904 will do the job, couple it with a cheap buck-boost converter from ebay and you should be able to run it from 2 AAs.
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Feb 21, 2015 18:48
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Beavis Audio has a wide selection of tiny amps, the Noisy Cricket he designed is pretty popular, uses the bog-standard LM386 http://www.beavisaudio.com/projects/NoisyCricket/ https://www.youtube.com/watch?v=ERSW7uGtfQ0
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Feb 21, 2015 21:36
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The LM386 is classic, I use those whenever I need to drive a speaker and I always smile when I find an old sound card that uses them (you can find them on sound cards made well into the 90s!)
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Feb 21, 2015 23:40
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There's a huge number of little 1-5 watt class-D modules on eBay that would happily power an 8-ohm speaker quite efficiently. I also hold a special place in my heart for the LM386, but honestly it's objectively not that great compared to more modern solutions (assuming you're just driving a speaker.)
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Feb 22, 2015 04:58
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I'm about to start a project at work where we need a pcb to act as a motherboard, with several addressed cards on one bus. One of our engineers suggested we implement our own parallel communication protocol after I suggested UART or i2c. What are your thoughts on parallel vs. serial communication?
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Feb 23, 2015 05:56
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What are your design requirements? Most modern systems have ditched parallel in favour of serial, because, counter-intuitively, serial is fast. You can do pipelining with serial communication, just dumping bits on the line at many megahertz, and they will arrive at the destination in the same order. Not so with parallel communication. But really, you lost me at "implement our own protocol". don't do that.
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Feb 23, 2015 06:06
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ante posted:What are your design requirements? Then there is PCI Express which is (parallel) groups of serial (bidrectional, differential) links!  quote:But really, you lost me at "implement our own protocol". don't do that. This forever. Look into SPI. It's relatively straightforward and not nearly the pain in the rear end that i2c is. Seriously gently caress i2c. gently caress THAT poo poo.
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Feb 23, 2015 06:11
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ante posted:What are your design requirements? We actually don't have any electrical requirements, other than the amount of cards that we need to support, and that the cards need to be addressed; they're supposed to be able to be plugged into any slot for some reason. Physically it needs to fit inside of a case that has existed for four years. I've worked other places as a contractor where the specs were laid out infinitely better, and I'm confident that the people I'm currently working for have no idea what they actually want. priznat posted:Look into SPI. It's relatively straightforward and not nearly the pain in the rear end that i2c is. Seriously gently caress i2c. gently caress THAT poo poo. I've used SPI once or twice before, why the i2c hate?
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Feb 23, 2015 20:16
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UberVexer posted:I'm about to start a project at work where we need a pcb to act as a motherboard, with several addressed cards on one bus. You don't want UART here I think. I2C makes sense if it's for low-bandwidth, system-health, checking Card IDs type of thing. You can load I2C EEPROMs on each board, and use spare pins on the connector to set the address pins of the EEPROMs such that they are all unique. Or you can use I2C muxes as well to create many parallel universes if cards have their own I2C busses already. SPI could make sense also -- tell us more about your design requirements, though I will echo the 'do NOT roll your own' sentiment.
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Feb 23, 2015 20:28
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Roll your own, don't let the haters get you down. Spec out ridiculous setup & hold times, force some poor layout designer to impedance & length match 4 different signal traces, go hog wild and every single issue that crops up will be HW related, you'll never even get to the point where SW could be blamed!
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Feb 23, 2015 20:46
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UberVexer posted:I'm about to start a project at work where we need a pcb to act as a motherboard, with several addressed cards on one bus. If it's low bandwidth, as others have said, I2C is good. You don't want parallel for sure and point to point buses like SPI or a RS-232 don't scale as easily. Though a lot depends on what's talking to what. As said, if you want things like EEPROM's, temp sensors etc on this bus I2C is ideal. If you have two processors talking it opens up other options such as RS-485 (serial) RS-485 is very robust and simple as a serial implementation. It's also multi-point so you can have the master transmit to multiple slaves. But you can't have multiple slaves transmit back to the master without implementing your own arbitration scheme of some kind - only one driver is allowed at a time. I2C is extremely well supported both with software libraries and IC's. In terms of implementing a backplane architecture there are a number of options which can make I2C bettter suited for going longer distances including: 1) Buffering (See PCA9600 or google "I2C buffer") 2) Isolating (google "I2C isolator") 3) And/or bridging to a differential physical layer like CAN. I designed most of an implementation where we have I2C running to ~20 boards with ~100 devices, which, despite being routed everywhere sometimes over long cables has held up well (We do both 2 and 3). I'd recommend doing at least one of those for any backplane implementation (though all buffers come with some annoying nuances due to the bidirectional nature of I2C lines). Otherwise there are a few other bus protocols although they get more complicated and/or less well supported. Actual CAN protocol is decent although it's complex enough that you need a micro, which makes it not too far off from ethernet or USB which are both well supported and flexible (though USB isn't the most robust thing in the world).
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Feb 24, 2015 01:22
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UberVexer posted:We actually don't have any electrical requirements, other than the amount of cards that we need to support, and that the cards need to be addressed; they're supposed to be able to be plugged into any slot for some reason. Physically it needs to fit inside of a case that has existed for four years. I2C is a pain because it needs pull up resistors and a lot of manufacturers implement it a little differently, and so often ends up having a lot of quirks. SPI is much more straightforward, but for this application, you'd need an independent enable line for every device, so it sounds like you actually want I2C because it's addressable. Or RS485 if you want it to be robust and hate yourself.
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Feb 24, 2015 01:26
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asdf32 posted:Actual CAN protocol is decent although it's complex enough that you need a micro, which makes it not too far off from ethernet or USB which are both well supported and flexible (though USB isn't the most robust thing in the world). I'm hesitant to ask... what's available at the sub-micro level to decode I2C? Is it like cleverly arranged shift registers or some horrendous network of passive components?
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Feb 24, 2015 02:05
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JawnV6 posted:I'm hesitant to ask... what's available at the sub-micro level to decode I2C? Is it like cleverly arranged shift registers or some horrendous network of passive components? Registers and other digital stuff but it doesn't matter because tons and tons of IC manufacturers do it for you. So temp sensors, A/D's, D/A's, GPIO expanders and 100'0s of other cheap chips are I2C compliant. The master needs to be a macro, but slaves do not.
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Feb 24, 2015 02:20
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JawnV6 posted:I'm hesitant to ask... what's available at the sub-micro level to decode I2C? Is it like cleverly arranged shift registers or some horrendous network of passive components? I imagine address decoding would be a pain in the rear end with discretes; aside from that, shift registers (for slaves anyway)?
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Feb 24, 2015 02:39
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asdf32 posted:Registers and other digital stuff but it doesn't matter because tons and tons of IC manufacturers do it for you. movax posted:I imagine address decoding would be a pain in the rear end with discretes; aside from that, shift registers (for slaves anyway)?
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Feb 24, 2015 02:48
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JawnV6 posted:I've worked with a bunch of sensors and implemented a slave on a tiny88, I know how it's consumed. I thought you were speaking about a generic I2C frontend that would decode things and provide a single-bit output or something. Well you can get something like this. 4-bit IO expander. It doesn't get much simpler. http://www.ti.com/product/pca9536
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Feb 24, 2015 02:57
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nRF24L01 die comparison - I'd long suspected the super cheap modules were using clone silicon. Good to finally get some verification.
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Feb 24, 2015 04:05
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Normally I use solder with water-soluble flux and give my board a rinse when I'm done. But if I'm fixing or modifying something that I'd rather not wash with water or anything else, is it safe for me to use rosin-core solder and leave it on?
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Feb 24, 2015 17:31
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Anyone have suggestions for BGA placement services? I'm interested in doing a board with a Freescale K70 chip but they only have BGA. Are there any semi cheap services that do single or small number boards?
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Feb 24, 2015 18:37
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BattleMaster posted:Normally I use solder with water-soluble flux and give my board a rinse when I'm done. But if I'm fixing or modifying something that I'd rather not wash with water or anything else, is it safe for me to use rosin-core solder and leave it on? I learned in school that some fluxes will cause corrosion over time, but I'm not sure how true that is, or what kind of fluxes or anything. Is there any reason you can't give it a quick dab with alcohol and take a toothbrush to it?
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Feb 25, 2015 01:44
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