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Electrical basics
Hi there!
Our team is trying to learn some electrical basics. Up to this point, we've just been trying stuff and hoping they work, but we want to know some basic ways of electrical work. -best methods for wiring; to reduce circuit resistance, loose connections, portability, shorting, damage to wires and connectors, etc -methods for testing electrical components - motors, controllers, switches, etc.$@# to be used during build and troubleshooting -battery management - how do we test batteries for how fast they draw down under load (there are some complex methods and equipment that apply a 100 A load for 30 seconds that tell you a lot about a battery that simply looking at as charged voltage does not).$@# A new battery is not necessarily the best battery. -ability to measure circuit variables (voltage, amperage, resistance).$@# Measuring the resistance of our drivetrain is very difficult and requires special methods that I don't understand (resistance < 0.05 ohms) -explanation of why we get certain results or don't get them.$@# I still don't understand why we see lower amperage on the competition drivetrain than what is predicted.$@# And I don't even know how to figure out why other than just start replacing things. -methods to combat tripping our current limits through the various breakers and controllers Thanks! |
Re: Electrical basics
Hi there,
Definitely a broad set of good questions, so here we go: - Reducing resistance usually amounts to increasing wire gauge. >= 6AWG for battery/breaker, >= 12AWG for high current motors (CIMs). We're going to 4AWG for our breaker connection and 10AWG for our CIMs this year since we found wires getting warm last year. Also, reduce wire length as much as possible by placing connected components near each other. - Creating solid connections: it's overkill, but soldering on crimped terminals is the best way to ensure that they stay on place. We strip off the plastic housing on these terminals, slide on some heat shrink, crimp the terminal, solder over the crimp and then heat shrink. We didn't have a single terminal come loose all season. - Make sure there's electrical tape or (preferably) heat shrink tubing over any bare connections. This reduces chance for shorting and makes things look cleaner. - It's often overlooked, but put thought in to where you position the components. Minimize wire length from battery->breaker->PD board (within reason, allowing for ease of battery removal). Keep your motor controllers close to your motors, PD board and cRIO. Keep cRIO near to the digital breakout and motor controllers. Before drilling any holes, set out all the components as a test on a similar sized board to make sure you have an optimal arrangement. - For testing, provided you guys have enough components to recycle, create a test board with all the electrical fixtures on it. This is a great off-season activity and will save you lots of time during build season when you're not having to kludge together something to test components. - Get a battery beak for battery testing. It's a one-time cost and has saved us more times than you'd think. http://www.andymark.com/product-p/am-0995.htm. If you want to get fancy, try putting them through a real world loading test with a test bot and five minutes of drive time after a full charge. Measure the battery's discharge level and keep record of your "best" batteries. - The new PD board will have current measuring capabilities over the CAN bus, so you can use that for your measurement needs. - You've learned a very important lesson about engineering: theory doesn't always match experimentation -- and that's okay! There are so many variables in one of these systems: battery internal resistance/charge level, motor parameters, electrical parasitics (R/C/L). If you're not getting the current you're expecting, it's most likely that either your computations didn't take all the variables into account or your experimentation is violating some assumption. Oftentimes, replacing components with "known good" ones is the best way to debug a fault. - As we found out in elims at IRI, once you've tripped a breaker, it is more likely to trip again. For a competition bot, you'll want to replace it just to be paranoid. Keep that breaker for a practice bot. - Make sure you have your gear ratios such that you will not pull too much current under the inevitable pushing matches. If you have yourself geared for 20fps with 6 CIMs and you get into a pushing match with a robot (or a wall), you're likely going to pop a breaker. If you've already built the system and there's no time left to fix it mechanically, you can also implement a software limit on how much duty cycle you'll put on the motors. This way, the motor controllers will never actually deliver 100% of the rated power to your motors and it should save your breaker. Let me know if you have further questions! |
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Step 1: locate any post by Al Skierkiewicz
Step 2: click on his name in the post heading and select "Find all posts by ..." (Warning: he has over 9000 posts, and the majority of them are about wiring safety.) Happy reading! 😊 |
Re: Electrical basics
To help you in your search,
Center the PD in the robot, this will keep the wiring short. There is no significant advantage to moving to #4 from #6. If you keep it short, the difference is millivolts. #6 is .0005 ohms per foot and #4 is .0003 ohms per foot. #10 is much better than #12. Uninsulated terminals are available from the same sources and cost less. Use the right crimper. Those meant for insulated terminals will not give you the same crimp force. Breakers that have tripped will not degrade. If they are repeatedly abused over a long period (5-10 matches) the internal contacts will pit and raise the internal resistance. Under those conditions, replace them. However, if they are warm to begin with, they will trip at a lower current than at ambient temperature. For a little more than the cost of the Battery Beak, you can get a West Mountain CBA IV USB analyzer. This will draw current for a much longer time and give you a display that you can save to disk, and overlay in the future. This allows you to track the health of the same battery over years. It also will match the curves used by the manufacturer and calculate amp hour ratings. It will show battery cells that do not match. The Beak is great to put in your pocket and know if the battery you put in the robot is charged. Use the "wire foot" analogy. At 100 amps, 1 foot of wire has a voltage drop that is predictable. #6 will drop 0.05 volts/ft, #10 will drop 0.1 volts/ft and #12 will drop 0.2 volts/ft. The stall current of a single CIM motor is 131 amps under test at 12 volts. On a typical FRC robot this more like 116 amps. That is the current it will draw when starting and anytime you are applying full throttle and the robot is not moving. Yes, 6 CIM drives have the ability to draw over 600 amps from the battery. The internal resistance of the battery is 0.011 ohms/11 wire feet. So at 600 amps, that is 6.6 volts dropped in just the battery. Yes the main breaker can withstand that for short periods but not forever. Loose connections (bad crimps, bad solder jobs, loose battery terminals, loose PD connections, etc.) can amount to several wire feet of loss per connection and can also raise the temperature of the device they are attached to. A loose terminal on the main breaker can raise the internal temperature above 100 degrees. |
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Thanks for all of the great advice! Right now, we are without an electrical engineering mentor to help us out with this, so we are struggling a bit in this area.
If I have any questions, I will post them here. Thanks again! -Joe |
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I've been trying for years to convince my team to solder the wires or use heatshrink or both, since our connections kept coming loose, but, while we were talking to some people at a conference, someone gave us a few of these: https://www.wagobox.com/shop/wago-22...412-20pcs.html
We were a little skeptical, but we tried replacing some of our connections with them, and they worked perfectly. If you're not careful, they can pinch your fingers, but they're easy to use, they don't come loose, and they're reusable. We bought a box of them and used them on our robot, and I haven't brought up soldering or heatshrink to my team since then. If you must crimp, though, make sure the people know how to do it properly. Too many of our connections have come loose because someone didn't crimp the right way. |
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They work well. |
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They are only rated for 32 amps.
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Micahel,
The insulation is rated to withstand 400 volts without arcing through the material. If you were to use this for line voltage it would not cause a safety hazard if handled or attached to metal. (line voltage of 120 volts is the RMS rating not the peak.) The current rating is based on temperature rise using continuous current (usually for 24 hours) If you exceed the current, the device will warm and the max voltage rating will likely fall. Once you exceed the melting point of the plastic, it will deform bringing the conducting parts closer to the surface. While CIM motors in an efficient design won't run at 32 amps continuous, under certain conditions, you may exceed that for several seconds to a minute. In that case, high temperature will result. In addition, the temperature will likely also effect the holding tension device and may just release the wire. Your mileage might vary. |
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The voltage rating of the device is the max voltage before the insulation between conductors or between a conductor and the outside breaks down and allows a current. This is entirely a function of the various dielectric and insulating materials being used between conductors and the electric fields they can withstand before ionizing and carrying a current. The current carried by the insulating conductors has no effect. So these two ratings are unrelated. You should not exceed either of them, even if you are way under the other one. |
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While we are on the subject...
There are various current ratings for wiring so you need to know when looking at charts and breaker sizes. The current robot rules are written using the NEC guide for conductors in an open space (as opposed to conduit and bundles). This table takes into account the resistance of the wire and the expected temperature rise caused by continuous current in the conductor for general insulation. It also adds a little fudge factor by calculating the voltage drop in a typical house wiring run and making sure that at maximum current, there is still sufficient voltage at the load. The insulation used on the CIM motor wire is (or at least was in the past) a higher temperature rating and is therefore rated for higher current then you would expect for it's size. Another set of current tables is established by the aircraft industry. That current table is based on the minimum allowed voltage drop to instruments in the cockpit. Adjustments to those tables account for length (both conductors please), load current, bundled conductors and altitude. You can find more from the FAA here... http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/99c827db9baac81b86256b4500596c4e/$FILE/Chapter%2011.pdf |
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Also the WAGO lever nuts are only sized for 12 AWG and smaller wire -- not really big enough for the 10 AWG many teams (like mine) prefer for drivetrain circuits. We will probably stick with 45A APP connectors for the drivetrain. |
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When possible we connect the motors directly to the speed controller by placing the controllers near the motors they drive. This eliminates another connector and therefore another possible failure point. We use screw mount push on connectors on Victors and Jaguars.
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Thanks! |
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http://www.digikey.com/product-detai...7866-ND/293277
We buy one hundred at a time. When needed we use the APP 35 and 45 amp contacts. Crimper is available from West Mountain Radio (outside Milwaukee) as well as other places. |
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Will you still be doing this with the limited length of leads on the new speed controllers? Or splicing in on the PD board side?
-Ronnie |
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We will likely add APP connectors to the new speed controller output wiring. I don't like that but we will adapt. I hate adding another point of failure.
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Here is a good thread to read if you're having issues with your wiring connections: http://www.chiefdelphi.com/forums/sh...d.php?t=119549
There is absolutely no need to both crimp and solder every electrical connection. Crimping is just fine when it's done properly. I would strongly encourage you to do rudimentary tension testing on a couple crimped fittings. They should be able to sustain far more force than one can every apply by tugging on the connection by hand. Ratcheting crimpers are key to ensuring consistent and strong crimped connections. |
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James,
We crimp then solder for one very good reason. We lost a World Championship to Beatty when one of our crimp connections let loose. We have vowed that will never ever happen again. |
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Final soldered connections for the competition robot are often checked by a mentor, and usually at the request of the student. It's not a lack of trust (on our part) or confidence (on theirs), but a matter of quality assurance by all to ensure we are building the best that we can.
In any case, I would want to be able to say we did everything we could to ensure a good mechanical connection. |
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James, we crimp first and then solder. We have had a stunning run of zero electrical failures.
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Soldering relieves all of the cold-working from crimping making the crimping process pointless. Most solders only begin to melt at around 200C (depends on formulation), and copper will start to stress relieve around 150-200C (depends on alloy). I'm not saying that soldering can't work, or doesn't work; my team and I have used numerous soldered connections with great success. Both soldering and crimping are perfectly valid methods of joining connectors to wires. What I am saying is that it is pointless to do BOTH to the same terminal. It is simply a waste of time because soldering essentially undoes crimping. |
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James,
Loosing a World Champs to four time winner Beatty Machine is not pointless. We would be the only four time winner instead of the other way around. Since that loss we have won three times by crimping and soldering. You are never going to convince us otherwise. Sorry... |
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I didn't say it was unsound, I am just saying don't fix what ain't broken.
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I guess I am not being clear here. Saying "you are never going to convince us otherwise" shows a remarkable amount of close-mindedness, something that I would not have expected. I would think that when presented with a compelling argument and data that anyone in FRC would be open to changing their mind. Perhaps your are not convinced now, and that's fine, but saying that you will never be convinced... that is what disappoints me. |
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James,
I don't want to mislead anyone. NASA does not allow soldered connections for the reasons that you have mentioned. The auto industry swears by them. In my heart I know you are right and I use crimped contacts everyday as often as I use soldered contacts. However, in our case, the team will not accept loosing another Championship to Beatty or any team because a of a failed crimp. It's a choice, and that is all it is. We take a lot of pride in our solder jobs and insure that a minimum of solder is used so as to not wick solder into the wire under the insulation. We also make an effort to correctly strain relieve all wiring and to tie it down so that it can't fall off or be pulled out. It is not always pretty but it always works. |
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Al, I (and perhaps others) would be interested to know if you did an in-depth root-cause analysis to determine why the crimp failed... and why soldering was the accepted solution instead of, for example, a change in the crimping process or better quality control of the crimping process. |
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The wire pulled out during the match. It could have been anything, but we know that soldering following a crimp does produce a more in depth visual check of the terminal. We made soldering standard practice some time ago and when we started using APP terminals, we continued soldering. We use a West Mountain Radio crimper for the APP contacts. All crimps are checked with a tug test prior to soldering. We like backing up the backup.
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I can't remember years, but I can remember standing outside the back of the Einstein Stage at Epcot looking at the bare wire. I don't even know if we were calling it Einstein at that time.
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Some guys wear special backpacks, some guys wear particular jerseys, some guys solder their crimps.
We've all got our superstitions and foibles. (you don't believe me? Pay attention to yourself next time you eat M&Ms) |
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So there's an open question here. |
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We are not replacing the crimp, we are adding solder to a crimped connection.
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What I point out (and what Ether is noting) is that by soldering the crimped connection you are stress-relieving the crimped material, removing the effectiveness of the crimping. Because of this, one would get ostensibly the same result by soldering the terminal without crimping first. Assuming, of course, that what I opine is correct and factual. Edit: this has been my point all along. |
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I disagree that the terminal would no longer be crimped after heating it for solder. I will experiment with that concept at a future time and let you know.
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You never want your solder to be your mechanical connection. So you want to crimp even if you soldier. You might lose some pressure from work hardening lost from solder. I am not going to speculate on that You are not going to lose the deformation though. If look at the automotive industry you will find mostly crimp connections with no solder. Cost could be a factor there. So done right, clearly you don't need solder. But that is with qualified tooling, qualified connectors & testing. Mil-HDBK-217F, a electrical reliability standard has a multiplier of 20 for failure rate calculation for non-qualified crimps to mil-spec crimps. The handbook does not have a base failure rate for crimp+solder. Not really an apples to apples thing, but shows the risks of doing the crimp poorly. The biggest down side to using solder is having the solder wick into the wire causing a stress riser & decreasing flexibility.
As for as I can see: Correctly crimped connectors do not need to be soldered. Solder done right gives you an extra bit of security for when the crimp is not done correctly. In any case you want to properly route your wires so the connection is not under tension & is not carrying the weight of the wires. |
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A bit more on the OPs question. In diagnosing robot failures, it's almost always some sort of mechanical issue. The symptoms you'll have on the field will be jerky, unreliable operation or rebooting of the cRIO/radios.
1. Ensure that terminal screws are tight. When you arrive at the competition -- check to see that they're tight, as the process of shipping/hauling the robot might have loosened the screws. 2. Wires should have strain relief / tie downs near the point of connection to the relay/speed controller. This reduces the chance that movement on the robot causes the screw to come loose. 3. See other discussion in the thread regarding how to fasten terminals to wires. Suffice it to say that there are engineering differences about the best way to do it. For your team, it's most important to select a method that works for you, have a procedure that everyone follows, and inspect the results of every crimp/solder operation. Most robots I see have the correct wire sizes on them -- I'm not sure if it's because the inspectors are catching it before I seen it, or teams are doing a good job. |
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I can see where one might solder a crimped connection on a FIRST robot. It does not have to last all that long. But in a professional environment this is not an acceptable practice. One must use the correct crimp for the wire and insulation type and install with the correct tool and procedure. Curiously the military allows us to solder to braided shields but only with a sleeve that includes heat shrink and glue that stress relieves the solder joint. |
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Keith,
We are aware of those dangers and teach our students to carefully solder being sure to watch for solder to wick under the wire insulation. We also take other precautions that would not be practical in mass installations like ships and consumer products. However, our 2006 robot is still in service as a demo and we haven't had a failure in the electrical connections on that robot yet. It needs some mechanical repairs but we have beat it up pretty bad over the years. |
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I work in custom automation. The only time soldering to a crimp terminal is allowed is when crimping onto a solid wire such as a resistor. I do like the idea of teaching students how to solder and properly done I can see that soldering would not be a problem, I too believe that with the proper tooling it is not needed and not acceptable in manufacturing. I prefer to teach the students the techniques that are acceptable in manufacturing, and those will last just as long.
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For those who say soldering a crimp connector is not allowed. Is that a standard, company policy, or long standing practice? One reason I ask is you can buy Mil-Spec solder connectors. The ones I know of have a mechanical strain relief after the solder joint & no crimp so this really is not an apples to apples comparison.
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We use soldered mil spec connectors on military product at work. The connector backshell has mechanical strain relief to the cable though.
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http://www.alliedelec.com/search/pro...x?SKU=70011029 |
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Do you guys buy the 45 amp or 30 amp APP connectors? Also, what experience do you have with the 15 amp versions for smaller wire? We use 12 gauge wire for all motors so the 30 amp will be fine but was wondering if we should maybe buy the 45 amp ones in case we decided to go up to 10 gauge wire? how do the 45 amp connectors crimp to 12 gauge wire?
-Ronnie EDIT: Do you guys get the ones you can break apart or the ones fused together as one unit? |
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![]() We've used the 15A, 30A and 45A contacts on their respective gauges without a problem. The amperage ratings on the contacts are slightly misleading. All the contacts are rated to 45A, however it's recommended you only pass the listed currents through as that is what APP considers safe for those gauges. eg you can put 45A through the 15A contact, however it's unlikely that the cable it's attached to will survive. That being said, always use an appropriate gauge for each circuit, and sized such that it is legal for the rules. However, if you have 18AWG wire on a 20A circuit (legal per 2014 rules) don't worry too much about using the 15A contact, it's more important you match the contact to the wire gauge than the contact to the current. We also tend to buy the fused connectors. The vast majority of our cables only need the conventional red/black combination, it's easier than roll pins or glue, and the slight extra cost is negligible. However we do keep a few loose ones about for when they're needed. Finally I highly recommend getting the TRIcrimp tool for the PowerPoles, it's very easy to use and will crimp all three sizes (to the appropriate sized cable). They're available from Powerwerx and Andymark. |
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We have found that the 30 amp contacts can be slightly spread open to make inserting #10 easy. They are slightly large for #12 but a little tight for the #10. WE also now stock the 45 amp which is fine for #10. We use the West Mountain Raido crimper which does all three sizes. I am not entirely happy about the job on the 45 amp though.
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Realize the battery plugs are "rated" for 50 amps & is on a 120 amp breaker. 6 gauge wire is rated for 101 amps for chassis wiring. But all these ratings are for continuous service. In FRC you are limited by battery size & match length.
Anyway back to the PP-45s. If you look on the data sheet, the rating comes from the UL rating based on 65 C or largest cable size. We are an edge case where peak current is much greater than average current. After heavy use, you can grab the connector without burning yourself, I wouldn't worry about it much. Build a panel that is going to be inspected by an UL inspector is a different subject. |
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Notice that between the different sizes, the actual contact area between two crimps is roughly the same. What's different is the size of wire they crimp onto, so the current rating is for the wire size, and isn't related to how much current the crimp itself can conduct.
So if a 30A crimp gets a good crimp on 10 or 12 gauge wire, it will certainly carry the current, since it has the same connection contact with another crimp as the 45A crimps do. |
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I kinda answered this question already:
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That being said generally, these properties are simplified into rating tables, for FRC, it's ok to pass 40A through a 12AWG cable since the longest run that you might have is one or two meters, however the usual application for APP may involve runs up through ten meters in more hostile environments, hence the more conservative ratings. TLDR: Current ratings on the PPs don't mater, chose a wire that is appropriate for whatever current you are using, and then select the PP for that gauge. |
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Tim,
The UL ratings are actually based on heat rise for specific currents under continuous duty. So if the contact rises above a certain temperature after having that current flowing for 24 hours or more, then it will receive a lower rating. We use the 30's because we have lot's of them. We purchased 45 last year to give them a try but don't really feel they are needed in our applications for two minute matches. |
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I am very intrigued to see the variety of ways teams work with their connectors. My question to the teams that solider and crimp is, what type of crimping tool do you use?
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Sorry I worded that question badly. I meant to ask whether or not their teams used ratcheting crimpers.
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Munch,
We use a ratcheting crimper and then solder. Our crimper is a West Mountain Radio tool but I am sure it was manufactured by someone else and just has their name on it. It may be a better tool than the one linked above as it has terminal retention to assist in holding the terminal prior to crimp. We use a very small amount of solder. We do not want to add to the rigidity of the connection but the solder does give us a little insurance and a slightly lower series resistance. |
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To anneal to soft you need to heat copper alloys to 370C plus range. Most solder for electronics melt in 183-200C range. Unless you are way over heating your joint, I don't think you are annealing of the crimp.
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As far as I know, which I'll admit isn't that far, most electrical-grade copper parts such as wire terminals are made from C11000 (aka 110 grade, ETP) copper. C11000 stress relieves at 180C, which as you point out is a 'minimum' soldering temperature. My understanding is that the residual stresses in a crimped connection are a significant part of what gives a crimp terminal it's good electrical and mechanical properties, and soldering heats up the joint enough to relieve those stresses. See the google book Copper and Copper Alloys, page 252, Table 4, for stress relieving temperatures for various copper alloys. |
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