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I have a 240v to 12v transformer even bigger than a microwave transformer and i want to control how long it is switched on for in the range of 50-300 milliseconds to spot weld things.

I was thinking of a big momentary push button switch same as thet have on factory walls to control robots, that i can hit a bit like swatting a mosquito at different speeds in order to get different contact durations.

Perhaps i could make a switch to control the transformer input, out of wood, copper and some silicone rubber springs? to finely adjust the physics of the spring to change it's rebound depending on how it is wedged?

Is it a feasable DIY experiment? would a switch on the AC side be too imprecise, seeing as the AC phase is 20ms and i need to let through 5-10 periods of AC into the transformer, is it not logical?

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    \$\begingroup\$ Use phototriac-triac with zero crossing detector to switch the primary winding. \$\endgroup\$
    – venny
    Jun 27, 2015 at 18:04
  • \$\begingroup\$ Have you considered AC relay which can be controller in the ms range? \$\endgroup\$
    – User323693
    Jun 27, 2015 at 18:37
  • \$\begingroup\$ Thanks for the suggestions! There are some zero crossing relays available, I Don't know very well how they work. ebay.co.uk/itm/… would it be responsive enough to send 100ms AC waves through it? \$\endgroup\$ Jun 28, 2015 at 3:24

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Using a switch to directly control a large current like this is extremely hazardous if the switch is not specifically designed for this application — if the current is large enough, it will strike an arc when the switch opens. This will rapidly destroy the switch, and may end up fusing it closed.

A homemade switch like the one you're describing ("wood, copper, and some silicone rubber springs") is likely to be particularly prone to this issue, as it is not specifically designed to extinguish an arc, and its contact surfaces are not designed to carry high currents. Do not attempt to build your own switches for high-voltage, high-current devices.

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  • \$\begingroup\$ Thanks for the warning. I would have researched and tested it if it had been suggested it was feasible. \$\endgroup\$ Jun 28, 2015 at 9:18
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If it were me, I'd use a nice fast relay (A local NZ company sells a solid state one I really like, check your local dealers), and control that with a microcontroller. Then the sky's the limit, you could have preset times for different materials, use a potentiometer to set exact times on the fly, use an LCD to give information about your settings, even integrate it into some kind of automated welding machine.

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  • \$\begingroup\$ Thanks for the suggestion. i have found a relay with 10ms max on/off time. I'm not sure what kind of microcontroller to use, i would like to find a ready made 555 timer with 50-500ms range, i don't know where to find them, it seems 555 timers are mostly home built. \$\endgroup\$ Jun 28, 2015 at 9:21
  • \$\begingroup\$ @ufomorace I've never heard of pre-made 555 circuits, but don't sweat that, they're easy to whack together, Google search for "monostable 555 calculator", and play around with standard component values until you find a pulse duration that you like. If you're worried about soldering up your own circuit, you don't even need to, at this time scale you don't need to worry about just using a bread board. You could also just use an arduino or similar, that way it can all be done in software. \$\endgroup\$ Jun 28, 2015 at 9:28

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