I should have posted this part first, but to continue from part 1.
The alternator has to spin a bunch to do this. You need about a 3 to 1 increase in speed to do this. This means at 3600 rpm engine speed, the
alternator is doing about 11,000 rpm. Not to worry. Check out a small block Chevy. It depends on the gear ratio, but if you were blasting down
the freeway in the old days (75 mph) the Chevy motor was doing 2500-3000 rpm. The crankshaft pulley is about the size that you are looking for, which
means that the alternator is buzzing along at 9000 or so rpm. Stomp on it and passing gear will get you well above the 11,000 that your welder is running
at. So the engineers at GM have already considered this form of abuse for you.
The existing diodes are too small, so they get canned. You need at least 25 amp 200 volt PIV diodes. Get 3, stud-to-anode and 3 stud-to-cathode.
Fortunately these are fairly cheap, few bucks, bigger is better, but anything over 50 amps is overkill.
You need 2 heat sinks with plenty of fins like you see in large audio amplifiers. You mount 3 of the diodes of one polarity on one of the heat
sinks and the other 3 on the other heat sink. If you don't insulate the diodes from the heat sinks then the heat sinks become a common conductor
for the 3 diodes and one is (+) and the other (-). This assumes that the two heat sinks are electrically insulated from each other. If you follow
this suggestion then you must follow the suggestion to buy the diodes as cathode to stud and anode to stud types. You can do it with all one type
but it is more difficult and messier as a connection must now be made to the stud portions of the diodes and you need to electrically isolate the
diodes from the heat sinks.
Electrically isolate the heat sinks from the cabinet by mounting them on isolation shock mounts. These typically come either with threaded studs,
or threaded inserts. I used ones that were about an inch in diameter and had 1/4-20 threaded studs on them. Connect your welding leads to the heat
sinks. One is positive, the other negative. I bought some female connectors at a welding shop. They sometimes referred to as Miller style. They are
a tapered brass rods that mate with a tapered sockets. I mounted these sockets on the side so that my welding cables were not permanent with
the welder. The sockets come and red and black so that you can keep the polarity correct.
You can get the shock mounts from Grangers. The diodes form factor is a D0-5 shape and most electrical/electronic supply houses carry them.
A 1N1186 or 1N1188 should work fine.
For excitation I found that the self excitation route was a pain, so I obtained a small permanent magnet motor. You need 3 amps at 16 volts.
Burdens surplus catalog has bunches of them. I used a vacuum cleaner belts to drive the motor and made pulleys to give about a 2 to 1 increase
in the crankshaft speed. This ratio will vary depending on what kind of motor you end up buying, but the permanent magnet motor will have to be
driven at least the rated nameplate speed to get anywhere near the maximum nameplate current rating.
A simple power rheostat will suffice to control the field although a simple power transistor circuit out of a Radio Shack book is a nice touch
and it is what I evolved the circuit to.
I made a small box about 12 inches on a side and 6 inches deep. On one of the 12x12 sides I cut a hole to match the shape of the alternator. Make the
other 12x12 a removable door. The older GM alternators have 4 screws to hold them together. I removed the screws and made the hole in the box to fit over
the end of the alternator with appropriate bumps to match the casing. I bored holes in the bumps to match where the screws went. Then I put the screws back
in and now I had an alternator with a box on its back side. I mounted the heat sinks on rubber shock mounts which are electrical insulators and help
protect the diodes from all the engine vibrations. If you drill some holes in the cover and on the bottom under the heat sinks, the fan on the
alternator will now draw air over the heat sinks and keep the whole thing cool.
I constructed a mounting bracket that bolted to the gasoline engine. The alternator sits above the crankshaft, although it could be mounted on the
side. Alternators are not direction sensitive when it comes to generating electricity. However, the cooling fan may not be as efficient when operated
in a reverse direction but the effect would probably not be harmful. Also the brushes are mounted off to one side for the direction of rotation.
There are many approaches to mounting the alternator and it is a matter of preference and what kind of engine you mount it on.
The welder will run 3/32 rod quite nicely and if you push it, 1/8". It is a little hard to get the arc started and an arc stabilizer from Burdens would
I still have mine and I made a second one for my brother. I went so far as to make a foot control for it and used it with a tig torch. Works pretty
good. You do a version of the Miller lift start in that you start at a low current setting make contact with the torch and add current as you lift
the torch away from the metal. That way it doesn't stick as bad. I welded a copper tube once, that was about the limit as copper really drains the
heat away very quickly.
Best thing is I haven't fried any diodes yet.