Welder foot switch

Foot switch from scrap

I put the foot switch together entirely from scrap.  I found 2 microswitches on a little bracket, a scrap of paneling, and old hinge, a block of wood and some closed-cell foam for a spring.  The wire is a scrap of stranded Cat 5 from a patch cable thrown away at work because the connector was broken.  A little Gorilla glue (and solder) and it was done!

Stepped switch detail

The original plan was to have significantly different spring tensions for the two switches so I could feel when the supply was off and then push harder when I wanted to fire the SCR.  But this was so simple I couldn’t resist.  I just bent the lever on one of the switches so it was guaranteed to operate before the other and called it done.  I really like the ‘elegance from scrap’ implementation.

The plan for how to shut off the supply has changed, too.  I was going to use a solid-state relay on the input to the transformer with a little always-on 12V supply driving the isolation LED through a resistor.  That would default to having the transformer on whenever it was plugged in.  The “shut off the supply” switch would just short out the LED, turning power to the transformer off.  That would leave me free to “wire-OR” some more circuitry in to sense the voltage on the cap and use the same mechanism to shut the supply off when it reached some preset value.

That would have worked fine, but I shifted to the simpler approach of just putting a power MOSFET transistor in series between the supply and the cap, arranged with a resistor from + to turn it on by default.  Again, the shut-off switch would just short the gate to ground, turning the transistor off.  That leaves me with the same wire-OR options for a voltage control, but a much simpler design.

I didn’t have a 25A MOSFET, and there’s no place local to get stuff like that except Fry’s.  They carry NTE stuff, so I found an NTE part that would work and thought for a trip to Fry’s I’d be done.  But the dumb part was $10.50!  I’ll pay for special parts when I need them, but this was just a silly pass transistor.  Electronic Goldmine had suitable parts for $1.29 or less but I’d just made an order there – grumble grumble…

Ah – how about Ebay?  You can get almost anything on Ebay, usually cheap.  The interesting lesson in this was from the fact that I didn’t want to wait a couple of weeks for nice cheap parts from China.  Most places in this country advertise their advantage over Chinese merchants by saying their stuff “ships from USA”.  So adding “USA” to the search term limits the hits to stuff you can get in a few days.  My MOSFET is on its way.  🙂

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