Covered Topics

Please see the list of the topics I've covered. It's located near the bottom of the page. Thanks for stopping in!!

Saturday, September 12, 2015

Portable Generator Repair

Some friends of mine own this Homelite LR4400 portable generator, seen in the somewhat blurry cell phone pictures herein. It was stored in a garage for several years and they asked me to check it out and do whatever repairs were needed. Using some fresh gasoline and some spray ether (starting fluid), I managed to get the engine to start and run. The main problem, besides a somewhat dirty carburetor, is there is no power output.

With considerable effort, I found a PDF of the service manual with electrical schematic online. The manual contains a complete test procedure as well as coil winding resistance specs, disassembly instructions, etc. With that, I proceeded to take the end cover off the machine and check the electrical components. In the photo below one can see the rotor, stator coils and some of the wiring.

A Quick Word About Generators In General

All generators have what's called a field coil (a stator) and a moving coil (rotor). The rotor spins inside the stator. The stator often consists of 2 or more coils - one goes to the electrical outlets located on the end cap of the machine; the other serves as an "excitation winding" which feeds power to the rotor via a pair of carbon brushes which ride on copper slip rings. As power is fed to the rotor, it becomes magnetized. As the magnetized rotor is spun by the engine, it induces electrical current into the main stator winding, which powers the above-mentioned electrical outlets. In order to control the output of the generator under varying load conditions, either an electronic regulator or other circuitry must modulate the flow of power from the excitation winding to the rotor. There is usually a rectifier consisting of 2 or 4 diodes to convert the AC supplied by the excitation winding to DC to feed the rotor windings. Many generators also have a capacitor connected to the output of this rectifier to filter, or smooth, the rectified DC and thus provide the rotor with a "cleaner" signal. You may be thinking "Well, that's all great, Karl. Why are you telling me all this?" Quite simply - MANY electrical issues with portable generators are traceable to rectifier diodes, capacitors, or electronic regulator boards that have failed. Find and fix that problem and voila - your generator is back up and working again.

The photo on the left shows the parts mentioned above. Notice I am holding the filter capacitor in my hand - this component had an internal short circuit, which effectively prevented the rotor from getting any of the excitation current. With no current getting to the rotor coils, there is insufficient magnetic field to produce any current in the stator winding, and thus no power to deliver at the outlet.

I tested the rotor coils, stator coils, rectifier and filter capacitor with a Klein brand Model MM200 multimeter purchased from a big box home improvement store for $49. All components except for the capacitor tested within acceptable range.

A new capacitor is on order - we'll see what happens when I get it installed.

Some Things To Consider

If you are keeping a portable generator for any kind of grid-down emergency, extended power outages, etc., it might be a good idea to keep a repair manual and the spare proprietary electrical parts on hand for it. Since most small engines built after the 1970s have a solid state ignition, one should keep spares of those parts, too. Even if YOU can't or don't intend to do the repairs yourself, if you have a shop manual and parts, SOMEONE ELSE can for you.

Generators should be periodically run and tested under load. An emergency or an important job is NOT the time or place to discover that the capacitor in your generator has deteriorated in storage and failed. Also, many generators rely upon weak residual magnetism in the rotor's laminated iron core to trigger the excitation winding - this residual magnetism fades over time with non use. If this process goes too far, a repair technician must "flash" the system using a battery to renew the magnetic field. This is NOT something you want to be doing in an emergency - and different generators have different procedures for doing it. Do it wrong, and you may ruin your generator.

After use and BEFORE storing a generator, the fuel should be run out of the engine/tank/carburetor. If the gas tank doesn't have a drain cock, run the thing until it runs dry. Gasoline deteriorates and rapidly becomes such that small engines won't run on it. Also, gasoline as it deteriorates forms a gummy substance that will clog the carburetor's fuel and air passages. Once this happens, the only sure way to resolve this is to disassemble and clean the carburetor. So, as I'm running my gas tank dry, I go one step farther: Once the engine starts to sputter due to lack of fuel, I squirt some starting ether (spray carburetor cleaner works, too) into the air intake, which will enrich the fuel/air mixture and cause the engine to speed back up. Doing this a few times for a couple minutes will ensure that the carburetor float bowl and all its internal fuel passages are thoroughly cleaned and purged of gas.

Sunday, June 7, 2015

New Blowgun Video

Regular readers of my blog will remember my doing a post or two about blowguns. The "native blowgun" has fascinated me since I was a boy. As an adult I have played with both manufactured as well as home made ones off and on over the years. Within the last couple years, not only have I made improvements to my home made ones, but have also found a new use for them as a teaching aid.

Each semester, I do a graphic demonstration of pneumatics principles for my students in Hydraulics and Pneumatics lab. I shoot darts into a board from across the room using a home made blowgun. It's quite amusing to see the "yeah, right - whatever" looks on students' faces as I take the blowgun out and load it with a dart. It's even more entertaining to see the looks on their faces and hear a few of them gasp when they see and hear the darts slam into the board. Most are left with no further doubts regarding the power even a few PSI of air pressure can deliver.

To follow is a link to my latest YouTube video. This was shot at a high frame rate and slowed down so the flight and impact of the dart can be observed.

InKarlsLab - Blowgun Video

Enjoy :)

P.S. One of the most memorable remarks a student made to me in class early on one semester was "I wanna know when the darts come out." He had obviously heard about it from the prior semester's students.

Thursday, June 4, 2015

Handy DIY Tip - Ordinary Rubbing Alcohol As An Assembly Lubricant

Ever tried to slide some tight-fitting shrink tubing or "spaghetti" tubing onto a wire or other object and had it bind about 1/3 to 1/2 of the way way on? This can be a real problem where one does NOT want to leave oil or soap residue on the objects being assembled. My solution for these situations: Ordinary rubbing alcohol from the drug store or grocery store. Rubbing alcohol will wet the inside of the tubing and the object itself, allowing the parts to slide more easily, but unlike oil or soap it will NOT leave a permanent residue. The alcohol will dry much faster than water, lessening the chances of corrosion or oxidation.

I came up with this several years ago and have used it repeatedly since then. This works on tight-fitting hoses and fittings, heat shrink tubing or insulating sleeves for electrical cables, etc. Obviously one wants to be careful about getting too much into electrical wiring where it might short things out or cause a shock hazard.

My boss today at the company I work at part-time was struggling with a home-made handle for an angle grinder. He had a piece of rubber insulation he had taken off a piece of scrap battery cable and was trying to slide this onto a large bolt that would form the new handle for the grinder. The rubber tubing had slid part way onto the bolt and jammed like a Chinese finger lock; he could not go any farther forward, nor could he even pull the thing back off the bolt! I suggested the rubbing alcohol trick, which worked quite well. The rubber tubing slid onto the bolt with little further adieu.

Hope this helps someone.

Monday, June 1, 2015

Off-Grid Mobile Power Supply - a.k.a. "The Battery Cart"

My last post on October 4, 2014 shows a used, but still working, Deka deep cycle marine/RV battery that was given to me by my boss at the battery company where I work part-time. In that post I discussed plans to build a power supply using that battery. Herein I show what I did, and the rationale for doing so.

The photo above shows the 90% completed unit - it just needs the lid and side mounts painted and some closure hardware installed.

First of all - why this took so long:
It has been nearly eight months since I first obtained the battery and formulated plans to build this power system. This project has been in the works nearly that whole time due to time constraints posed by a busy work schedule, family issues, a relative nearly dying, as well as a medical scare of my own - which fortunately was a false alarm. During that time the various parts and pieces sat around for weeks, unassembled.

I wanted a semi-portable off-grid power system to run my amateur (ham) radio equipment and other items such as lighting, small power tools, security systems and to recharge cell phones and tablet PCs or laptops during power outage emergencies, amateur (ham) radio Field Day events or camping trips. To fulfill these needs, it needed to have the following:
1) Be self-contained.
2) Have wheels, since this battery is heavy - weighing in at around 45 lbs. Two of them would of course weigh 90 lbs.
3) Supply 12 volts DC at up to 200 ampere-hours capacity; the unit can hold two of these 100 ampere-hour batteries.
4) Safety - be properly fused with any terminals or connections protected from casual contact with wiring or other objects. Also contain the battery(s) in such a way that any rupture of the case and resultant leakage of acid is contained and does not damage floors, car interiors, or pose a hazard to people or pets.
5) Provide low voltage shut-off protection for the battery in the event it is left unattended with a load connected, thereby preventing damage to the battery through over discharge. This circuit shuts EVERYTHING down when the battery voltage falls to 10.8 volts.
6) Provide standard Anderson "Power-pole" type connectors for interoperability with emergency crews from CERT, ARES and RACES. These connectors, located on the left-hand front of the unit are in the standard configuration for quick match-up with other people's equipment in a field situation.
7) Provide standard 12 volt automotive "cigar" lighter receptacles for convenient use with commonly available 12 volt accessories as well as a pair of 5-way binding posts for use with alligator clips or even stripped wires.
8) Be rechargeable with a standard automotive battery charger, car electrical system, solar panels, wind power, etc.
9) Provide 120 volts AC for running small appliances, small power tools, soldering irons or powering "wall-warts" for laptop computers, etc.

This system achieves all these requirements and provides convenient operation.

The "battery cart" measures 21" w X 24" deep X 28" tall. It is on wheels removed from a moving dolley bought from Harbor Freight tools for $8 during one of their sales. This was far less expensive than buying the same wheels individually at a hardware store. It provides room for 2 Deka DC31DT 12 volt/100 ampere-hour deep cycle batteries to be connected in parallel for a total of 200 ampere-hours of capacity. Automotive "blade" type fuses protect the separate power inverter and 12 volt outlet circuits. The blue plastic boxes seen in the next photo contain screw-on post type battery terminals for rapid connection to an automotive type battery charger with standard charging clamps. Since the terminals are recessed deep within the blue boxes, which are standard electrical wall outlet boxes purchased at the local home improvement store, they CANNOT be accidentally touched or shorted against anything else. The battery in the third photo is enclosed in a standard ABS battery box designed for the purpose; this case has vents for the hydrogen gas to escape from the battery so it doesn't become trapped and pose an explosion hazard. The Anderson power-pole connectors are on the far left-hand side of the cart and are mounted on a Radio Shack black plastic project box. To the right of that are the control box with its 5-way binding posts, a "Harbor Freight Tools special" 400 watt power inverter for supplying 120 volts AC, and two 12 volt cigar lighter sockets purchased from an auto parts store.

The photo below shows the innards with the cover removed. Note the USB charging devices plugged into the cigar lighter sockets. I routinely charge my cell phone and my pocket MP3 player this way. A relative of mine recently was quite impressed at being able to plug her cell phone directly into the battery cart, using a USB cable borrowed from me, after leaving her charger at home :)

The photo below this one shows the battery box - located in the compartment below the wiring, outlets, inverter, etc.

A Word About The Control Box:
The control box looks a bit lame right now. Originally I had intended to have the low voltage cutout board, its relay, and power ON/OFF switch on one metal "dual-gang" electrical outlet box. When I finally got around to building the low voltage cutout board and got an appropriate 30 amp relay (salvaged from a defunct air conditioner electronic board), I realized that everything would NOT fit into one box. These boxes are normally designed such that they can be stacked and screwed together with the screws that normally hold the covers on. Meanwhile the home improvement center in my area had completely changed its product mix and I could no longer buy the original type I had started with. In the interest of expedience, I bought the current issue item and vowed to eventually re-spin the whole design for the control box. So there it sits looking really funky - for now.

The low voltage cut-out board is of my own design and uses an op-amp comparator circuit with a zener diode as a voltage reference. There is some hysteresis built in to prevent chatter, or oscillation, at the 10.8 volt trip point as well as to provide some electrical noise immunity. The board has a 30 amp relay as mentioned earlier; there is another 40 amp relay, salvaged from the same air conditioner the other relay came from, which is used to operate the power inverter. This additional relay is mounted in a gray single-gang box which can be seen behind the inverter in the "cover off" photo.

The low voltage cut-out board is connected with a 5-pin "Molex" plug connector - making it an easy, solderless field replaceable module.

The top blue light is the main power ON indicator. The bottom green lamp and toggle switch are for the power inverter. The inverter is controlled by the toggle switch to prevent it being powered when not needed; it draws close to 0.8 amps even in standby mode. The top left button is the main "power ON" button; the upper right-hand RED button turns the main power off. Since the inverter relay's coil circuit is interlocked through the main power/low voltage cutout relay circuit, the inverter CANNOT run if the main switch is OFF and/or the cutout board has tripped.

What Remains To Be Done:

1) Paint the cover
2) Attach the side rails the cover mounts to; put cap screws or wing nuts to hold the cover to the side rails
3) Add a solar charge controller and an Anderson Power-pole connector on the rear of the unit for solar panel hookup
4) Re-spin the design of the control box - and the cutout board itself. I'm thinking of going to surface mount components on the next version. The current one is a through-hole "prototype" board. A properly designed printed circuit board could replace a considerable amount of the wiring in the box.
5) Eventually upgrade the wheels on the cart - these work but tend to bog down in heavy carpet.
6) Add the second battery.
7) Add volt/ammeters on the front panel - this should be easier with a re-spun control box.
8) Use an Anderson SB50 type plug connector to quickly connect/disconnect the battery for removal or service. Right now I have to unscrew the battery terminal nuts to disconnect the wires.
9) Replace the "blade" fuses with self-resettable thermal breakers. The low voltage cut-out board, inherent in its design, provides the added benefit of locking out the system after a breaker trip until somebody comes along and manually turns the system back on. Therefore the system could NOT just sit there with a short or overload oscillating - turning on and off ad infinitum.

What I've Learned/What I'd Do Differently:

1) This thing is big and heavy - IF I had it to do all over again I'd build it for ONE battery and have any additional batteries as separate plug-in "modules". My two battery system would then be in two pieces, but each would be MUCH easier to carry and store. From the photos you can see it takes up a lot of floor space; when done it will go under a work table. But after months of stepping around this thing during its construction, I like the idea of being able to maybe keep the main unit available in the room and the second battery tucked away somewhere - but connected by a suitable cable to the main unit. A modular system might also lend itself better to carry in a smaller vehicle - the two pieces could be squeezed into whatever nooks and crannies were available, whereas a one-piece cart like I have requires a decent sized cargo space in the vehicle to carry. This CURRENTLY is NOT a problem for me since I drive a pickup truck, BUT if that changes it could be an issue later.
2) Go with a 700 watt inverter - I went with the smaller 400 watt one to discourage using too much power, as even a 200 ampere-hour system will be quickly depleted using a 700 watt inverter. That said, it would be useful to be able to run a larger tool - like my heat gun - for SHORT PERIODS of time.

I hope this write-up is helpful to someone and gives folks some creative ideas for designing their own off-grid electrical systems.