Schleicher PROMODUL-U PLC Rack

A customer sent us a Schleicher PROMODUL-U PLC Rack from a Burkle Hot Press. The UNG 230 Power Supply was inoperable. We disassembled the power supply and found two burnt resistors in the Switch Mode Power Supply MOSFET circuit. The resistors had burnt open and the color code was charred. So determining the resistor value had to be found another way. I also found that the two MOSFET’s in the switching circuit were shorted. I reverse engineered (drew a schematic) of this circuit so that we could determine what the purpose and Ohmic value of the burnt resistors were. This would allow us to replace them with the proper resistance values. And in the process of reverse engineering, I tested the other components involved in the Switch Mode Power Supply circuit. I found that the Current Mode PWM IC was inoperable. I replaced the bad IC and shorted MOSFET’s. And through the reverse engineering process found that the burnt resistors were in the source to bus-ground connection of the MOSFET, and used for current sense feed back to the Current Mode PWM IC. From this knowledge we were able to replace the resistors with the correct values.

I performed the initial power up with the UNG 230 removed from the PROMODUL-U PLC Rack so that I could test the +5VDC and +24VDC output voltages at its edge connector. I gradually loaded the +5VDC output with an external resistor, eventually drawing 5 Amps from the UNG 230 Power Supply Unit. The +5VDC and +24VDC outputs maintained excellent regulation with no voltage ripple.

The UNG 230 Power Supply Unit performed excellently in the PROMODUL-U PLC Rack.