Dear Forum Members,

I have been using "Linear power supplies" in my Resistance Welding Controllers and weld checkers. My controllers have been working at several factories without any problem. Since this product series needs 5V @ 750mA DC, +12V @ 250mA DC, -12V @ 250mA DC, 20V @ 1.5A DC, 24V @ 250mA DC & a separate 6-0-6 AC output. Due to so many voltages the transformers used in linear power supplies are bulky and heavy too making my controllers and weld checkers heavy and larger in dimensions.

To get rid of the weight and size problems we decided to use SMPS in our products. We got three SMPS from a local SMPS vendor for trial purpose. We never had any kind of problem with linear supplies but with SMPS the system shows weired behaviour.

1. The weld schedule is for 6 cycles but it fires random number of cycles sometimes 10 sometimes 15, 18, 56 ...
2. At the comletion of welding the system shows the number of cycles fired this number also neither match with scheduled cycles nor with fired cycles.

I am using P89C668 microcontroller in these devices and MAX812 power supervisor. With Linear supplies I never had any problem the same board works exactly fine with linear power supplies but shows strange results with SMPS. What could be wrong? Please help me friends. If more information is required I will provide.

Thanks & Regards,
Prahlad Purohit