At about mile 6, I came to Beacher Spring, which is a great little artisenal well that flows constantly.  Refilling water here, I stoped to chat with some locals on ATVs. After a short break I pressed on. About 2pm it started to rain, and I had to break out rain gear. The rain quit around 3pm, but picked back up around 4pm and continued the rest of the day.

Being cold and wet, I had a choice. I could stop around mile 20 and camp, or press and finish the 26.1 mile trail and go home to a warm shower and soft bed. I chose the latter, and pushed as hard as possible. I finished the trail about 9:15pm, using my headlamp for the last 45 minutes. After returning to My car, I changed into dry clothes, then proceeded to raid my food bag, eating almost everything that didn’t need to be cooked. Overall it was a very long and eventful day/night hike.

The afternoon was back and forth down rock roads and bad trail. About 5pm I hit cedar creek and stopped for a rest. Soon after I found the only decent view on the trail: 

About 7:30pm I’d had enough, and decided to stop. After reviewing my gps while lying here in my tent, I wish I’d pushed down to dry fork campground. So far the only trail traffic I’ve seen all day is a couple on horses shortly after the bugs chased me into the tent. They were pretty rude. Im also not a fan of the damage horses have done on this area either. I don’t think I’ll be coming back here anytime soon. My plan is to get up early and push to leave this trail behind me. After bushwacking through a bunch of crappy “trails” I need a bath badly.

Tldr; skip cedar creek.

Broke camp early and headed back. Return trip was pretty uneventful. Most of the last stint was downhill. The only thing that slowed me down was the abundance of spider webs crossing the path every few feet. I must have destroyed 5 spider-years worth of effort on my way down the mountain.

I only encountered one other hiker on the trail, and he was heading out. I arrived in camp pretty late, taking time to check out the available campsites. All other campsites near the summit were empty. For all intents and purposes, I have a mountain all to myself tonight. Since this campsite has an established fire ring, I may even enjoy a campfire for an hour or so after dark…

There is a problem with rebuilding these battery packs however. Joining the cells with a thin nickel strip is impossible without a spot welder. You might say “What about soldering the pack together?” Well, heat from the soldering process would significantly decrease the life of the cells, if it didn’t kill them outright. “So just buy a spot welder!” Not going to happen. I can justify spending a few hundred dollars on components to build a spot welder, but not the $1200 or so it would take to buy one. “So break out a soldering iron and an oscilloscope and build one!” OK, I think that is doable but its going to be a complex process. This will be the most complex electronics project I have undertaken to date.

The design that I will describe here is derived mainly from these three sources:

• http://www.pittnerovi.com/jiri/hobby/electronics/welder/
• http://www.pocketmagic.net/2012/07/capacitor-discharge-microspot-welder-cutter/#120208
• http://www.turtlesarehere.com/html/cd_welder.html

The heart of the design is a bank of four 330,000 µF 16 V electrolytic capacitors connected in parallel. This should give an equivalent capacitance of 1.32 F and should hopefully lower the capacitors ESR to a usable level for this application. The capacitors shown here were sourced from eBay and are made by Nippon Chemi-Con. The data sheet for these capacitors does not list an ESR but lists a ripple current. Ripple current really is not useful to us, and I do not have an ESR meter, so we will have to evaluate the capacitors in the testing phase.

All of that stored energy has to be delivered to the weld joints with some sort of flexible cable, for which I will cut up an old welding lead I have lying around. I am currently leaning on TIG welder tungsten electrodes for the actual contact tips, but that is currently up in the air. Copper buss bars will connect the capacitors together and will connect to the cable and switching silicone.

Switching this much power is quite tricky and has led to failures in other people’s projects. Simple spot welder designs use a SCR to switch the load, which is certainly durable enough to handle the abuse, but latches in the conducting state until the voltage drops low. A design like this relies on adjusting the capacitor bank voltage to tune the weld. This is OK but does not provide much fine control. More advanced designs use MOSFETs to switch the current on and off in short pulses. This allows for tuning the welder more accurately and allows a “pre-heat” pulse to clean the weld site before the main welding pulse. This is the design I will attempt to use.

The MOSFETs I chose for this design are International Rectifier IRFB7437. This is a fairly beefy device with a low on resistance and a fairly low gate capacitance. I plan on using six of these devices in parallel to provide some safety factor.

I will drive these with a dedicated low side driver IC under microcontroller control. The MOSFETs can be seen in the picture to the left in the tube. I purchased extra in case I destroy parts in testing. Also in the photo are the protection diodes and TVSs I plan on using to protect the capacitor bank, and a 4×20 LCD display to provide a user interface for the microcontroller. I plan on driving this project with a 40 pin PIC micro.

To provide power for the project I plan on using a 1u server power supply. This simplifies things since the PSU provides almost all of the voltages this project requires. It will also allow me to incorporate a  standby feature that may be useful.

I know this is not much in the way of a formal design, but I am back-of-the-napkin designing this project. I did run a few simulations to reassure myself that the expected currents would probably not destroy the MOSFETs. Now all I need to do is build up and test the circuitry and write the control code. I’ll keep you posted.

Interspersed with this project, I have been trying to complete a long-term project for a local company located in an internet dead zone. the companies only direct service options are dial-up which is too slow, satellite with its ridiculous ping times and bandwidth caps, and a fiber optic line which would cost about $20,000 to install and would set them back $500 a month. To get around this I used a long distance wireless link to another site which has a reasonably priced and decently fast cable connection. The link distance is about two and a half miles. For line of site in southern Illinois, this requires elevation at both ends. The company in question happens to have a lightly used 100 foot free-standing tower at its site, and we were able to install 50 feet of Rohn 25 tower at the other end. I currently do not have pictures of the 50 foot end, but may post them later. I also do not have images from the latest climb on the 100 foot side, but I did find some of the photos from the last time I went up to change a router on a different link.  The tower appears to be a Rohn SSV which  in this configuration is rated for about 15 square feet of antenna area in a 70 mph wind zone. This is quite a bit more than what we actually use.

Each section of the antenna that you can see bolted together, is 20 feet long. The router and dish seen here sits at about 80 feet. The new dish sits at about 90 feet. The new link uses a pair of Ubiquiti NanoBridges in the 5 GHz frequency space. The bridges alignment is somewhat critical, with only a few degrees of beam width to play with. Luckily, due to the local geography, the two bridges are at almost the same altitude, which simplifies dish aiming somewhat. The link is capable of 40-50 Mb data rates and is quite stable. Initially the link was unstable, but lowering the output power of the radios seemed to improve stability greatly.

I have done quite a few odd jobs for this company in the past. The great thing about working with them is they only bring me interesting problems to solve, and they have reasonable budget expectations. Here is some of the work I have done in the past:

The image to the left is a network rack wall mounted in a mechanical room. Not terribly interesting, but It worked out well for this installation. On the right is a NEC DSX telephone system. This was not something that I had done before, but was not terribly complex. The two worst parts were terminating all of those cables and programming the system.

Well, that’s it for now. In the near future I will be documenting a battery pack spot welder build and a small software project that a friend and I are working on.

Design Requirements

In order to design our hypothetical spacecraft, we need to start with a realistic set of requirements. The requirements can start broad, and be refined as individual aspects of the design are considered. So, what is the minimum set of requirements for our mission? Since the mission is manned, the vehicle will need to carry a handful of people to Mars and back. Since we like our astronauts, our design must return them all alive. When we get to Mars, we want to do some science stuff, which may or may not require landing. That means carrying some cargo and some way to land and return to Mars orbit. For this discussion I will assume somebody has a good way to get from Mars orbit to the Martian surface and back to orbit. Go talk to Elon Musk about his pet Dragon for doing this.

That’s about it for the absolute requirements, but adding a few goals make the mission more politically feasible. For instance, setting a goal of going to Mars on a reasonable budget will not help with the main mission requirements, but is necessary if Congress is going to fund it. To this end, we should also set a goal of making large portions of our spacecraft reusable. After all, we probably want to go to Mars more than once, right? If we can shave a significant amount of cost off the second trip, that trip is more likely to be funded. We should also set a goal of taking more than just two or three astronauts to Mars. We do want to do science when we get there, so taking several scientists is probably a good idea.

Fast or Slow

The first decision we need to make in our design process is whether we want to drive the Ferrari or the Chevy Van. In order to get to Mars our spacecraft needs to change its solar orbit from one that coincides with Earth, to one that coincides with Mars. To do this efficiently, spacecraft use Hohmann transfer orbits which require the spacecraft to speed up along the orbital path, which causes the vehicle to enter an elliptical solar orbit. When the spacecraft is far enough from the sun, it accelerates some more to circularize its orbit at the new distance. This is, of course, a great simplification of real transfer orbits, but gives us an idea of what is involved. We can talk about orbital transfer more when we discuss propulsion at a later date. The important part, which we have to decide now, is do we want to go to Mars fast or slow. Going fast takes more fuel which means we can haul less cargo. This is offset a little by the decrease in crew needs (food, water, etc) that the shorter trip allows, but it’s not much. I’m not going to Math this out tonight, but we are talking something on the order of doubling fuel use for a 10% decrease in trip time. Carrying all of that fuel means we have to take less stuff for a given total mass. Since our goals are to take along several people instead of two or three, and to do significant  science when we get to Mars, it seems like we are going to need the cargo hauling ability. So no space Ferrari for us, looks like we are taking the interplanetary Chevy Astro van.

Look at that, our first design decision! With that accomplishment, I will sign off for tonight. Stay tuned for more.