FILENAMES: Miller_Time_1.gif Miller_Time_2.jpg Miller_Time_3.jpg Miller_Time_4.jpg DESCRIPTION: These are pictures of a home-made spot welder timer. Posted by Ken Moffett . The following description was provided: ================================================================= When I started at my current job, I had a Miller LMSW-52 (220vac/2.5KVA) spot welder on a foot-operating stand. I needed to weld a cylinder from 0.010 sheet stainless steel. Miller specifies the welding times, in their "Handbook For Resistance Spot Welding" flier, in terms of the number of cycles at 60 Hz. The welding times on light sheet metals are down as low as 1 cycle. This is 0.017 second. The welder frequently burned through the metal, no matter how fast I tried to get on and off the foot pedal. I found that Miller makes a timer for this unit, but it costs $600. The spot welder and stand only cost $550. I located a manual, on Miller's Web site, that contained the wiring diagrams for their timer. It was very simple: a circuit breaker, an electromechanical timer and an electromechanical relay in a box. Electromechanical timers and relays have inherent make-break delay times of around 0.1 second. I think that Miller never intended this size spot welder/timer for very, light metal. I also ran across a homemade spot welder and control at: http://www.5bears.com/welder.htm It looked like a very nice, inexpensive design. This used a zero-crossing, solid-state relay that eliminated the delay of the electromechanical relay. But, he used an electromechanical timer, so there was still the 0.100- second limit. The spot welder was small, so longer times would be necessary. I decided to design my own electronic timer, to control a zero-crossing solid state relay. "Junk-box rules" say you design with what you have, or can get cheap. In the end I purchased the circuit breaker (new: $20), the SSR (surplus: $5) and the counter IC (new: $3). I have a pretty good electronics junk-box! I should probably explain the original circuit. On the far left and far right of the schematic are the parts of the original spot welder. The welder's primary winding and the control switch. The spring loaded, On-Off switch is mounted near the front of the spot-welder transformer. When it is mounted on the Miller stand, a steel cable mechanically links it to the foot pedal. The foot pedal also links to the electrode jaws. Pushing down on the pedal will close the jaws, apply pressure to the metal to be welded, and then activates the switch. The switch was wired in series with the power cord andthe primary winding. As long as the pedal was held all the way down, power is fed to the electrodes. In the schematic (Miller_Time_1_tif), and in the actual wiring, I separated the wiring to the foot switch from the welder's power cable and the primary winding. The 12vdc power for the timer circuit is a generic "transformer/bridge/three terminal regulator" configuration. Because I was concerned about the possibility of electrical noise from the spot welder getting back into the timer, I added an in-line RF filter and MOV's (two 130vac units in series for 260vac) in front of the 12vdc-supply transformer. The 60 Hz clock, for the counter, is derived from the 12vdc-supply secondary. This is made up of a 1N4842 diode, two 100k and one 10k resistors, and a 2N3904 transistor. The signal at the transistor's collector in a 60 Hz square wave. This square wave is continuously fed to the counter's clock input (pin 14) The counter (Maxim ICM7250) is a two-digit, BCD up-counter. When a brief 12v signal in applied to the trigger (pin 11), all of the outputs (pins 1-8) are pulled low. Unless "00" is selected, this pulls the common pins (C) on the two BCD thumb wheel switches low. This does two things. First it pulls the counter's reset (pin 10) low, enabling the counter to count up at a 60 count per second rate. Second the negative terminal of the solid state relay in pulled low, turning on the power to the spot welder primary, secondary and electrodes. As the counter increments, the output bits go high in a two-digit, BCD sequence. When all of the bits selected by the thumb wheel switches have gone high, the reset pin goes high, stopping the count. And the negative terminal of the SSR goes high, turning off the power to the spot welder. Selecting "01" on the thumb wheels will turn on the welder for one 60 Hz cycle or 16.666 milliseconds. Selecting 02" will be 33.333 milliseconds, and so on. This gives a range of 0.016 to 1.5 seconds. This may not be enough for some welds, but will work for all of mine. If longer times were required, two of the timer IC's could be cascaded, and another BCD thumb wheel added, to extend the range to 999 cycles or 16.65 seconds The trigger circuitry: the 1N4841, two 22k resistors and a 0.47 uF capacitor, could probably have been fed directly through the spot welder switch to the +12vdc. However, since the wiring to the switch was in close proximity to the power cord and the welder transformer, I opted to add the reed relay for some noise isolation. It may not be needed. Also the MOV(s) and the RF filter may be overkill, but I decided it was worth the effort, since I had them already. The SSR should have a current rating of twice the spot welder's primary current according to the manufacturer (OPTO22). The circuit breaker was chosen as a high current on-off switch. All of the components were mounted in metal case on the side of the Miller stand. Pictures of the installation are: Miller_Time_2.jpg shows the timer case mounted on the Miller spot welder stand. Miller_Time_3.jpg shows the circuit-breaker/switch and pilot light on the "front" of the case, and the thumb wheel switches on the right side (cover). Miller_Time_4.jpg showes the major component layout. The pilot light, switch, terminal strip and SSR are mounted in the case. The main power cord comed the bottom and the cord to the spot welder goes out the back. The thumb wheels, RF filter and timer circuit board are mounted in the cover. The cable from foot switchcomes in the basc of the case and connects to the timer perf board. All recommendations will be politely ignored as "this works for me!" ;) Seriously, comments or questions are welcome! Ken Moffett Scientific Instrumentation Macalester College 1600 Grand Avenue St Paul MN 55105 651-696-6383 moffett@macalester.edu Home: 612-823-3959 moffett1@mr.net