Cracked Unknown PWM Replaced By Universal Module In Pressure Monitor SMPS
One of the technicians brought the SMPS alone, of this pressure monitor, picture of which was shared by him through whatsapp upon my request. The SMD PWM IC had cracked and we could not trace out its number. The only way was to remove it and provide a three wire Universal SMPS module, usage of which has been explained by me in several of my previous articles. For those who are new to this, just have a look of the module here:
After taking out the PWM, and cleaning up the area, I connected the black wire to Hot ground and red wire of this module to B+ return wire in the primary of SMPS transformer, the other end of which goes to 300V B+. The 4.7mfd/450V capacitor was replaced as it was showing low value. (Green wire was cut as it is not being used in this circuit, which did not have a feedback circuit.)
Then when I powered it up momentarily and checked the output, it was showing 17V, even though the output adjusting preset on this module was at minimum. As the relay was that of 12V, the output could only be 12V and not more. So after ensuring that the SMPS is functional, I removed the two caps to know how the secondary wiring was done in this double sided PCB:
The output was going to a regulator IC that provided power to the two SMD ICs you can find on the board and to the relay coil. So, I took a 7812 IC, soldered its ground pin on the place where the two caps were mounted, removed the SMD diode from the secondary of the transformer, fixed FR207 in its place, connected two 1000/35V caps in parallel, and gave this output to 7812.
Connected the output of this 12V regulator to the + on the board. Applied power and found that the display was getting on and the four control buttons also functioned well. Evidently the PS was used only for the relay and ICs and did not draw high current that could blow up the 7812. The required current was very much within its specifications.
Mission accomplished with minimum efforts.
Following is a picture of the defective components, before viewing of which, kindly help me hold the bag to push the satisfaction in! (LOL)
(The two caps removed from the secondary were good and were given to the customer)
This article was prepared for you by Parasuraman Subramanian from India. He is 72 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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You may check on his previous article on ATX Power Supplies Repair