No Schematic? Don’t Worry, Here Is A Way To Solved – PHOENIX Open Care Warmer OCW100
The power supply board of this warmer which also handles the heating of a carbon crystal element, was brought to me with the complaint that the connector portion of the heater burned due to power hike and that portion was found charred. What you see in the second picture above is its AC step down Transformer. Let us have a look of this board:
First duty was cleaning the board and removing the burnt out components. Three fuses were found burnt. I also noticed that the BTA12 Triac, which handles the heater load, was also short. But the heating element was found ok on testing with Analogue multimeter. The board was combed for any more defective components and all defectives were replaced. When I applied the power, the DC outputs of 5V & 8V were steady. In order to study how this circuit works, I tried my best to obtain a circuit diagram. But as these are custom made protected circuits, nothing could be done in this line.
I fetched the mother board, transducer and other connectors in order to connect and check. I found that the switching through the Triac was taking place, but it did not remain steady. Since using a heating element keeping it open on the service table was a risky factor and further because of the brittle nature of the carbon crystal element, I used 200W bulb as a load. The opto-couplers used in the circuit were of special types with darlington transistors inside.
The numbers were: 4N33 & MOC3011, which were General Purpose 6-Pin Photodarlington Optocoupler & 6-PIN DIP Zero-Cross Optoisolators Triac Driver Output. Let us have a look at its uses along with the Relay and Triac:
Though the IC number of MOC is different, the functions are same. Such opto-couplers are used in these types of devices which require precise current adjustment, as these are baby warmers. Following are the pictures of the controller board and details of ICs used in the board:
ATMEGA 8535 – 8-bit Microcontroller with 8K Bytes In-System Programmable Flash ULN2803 – Octal High Voltage, High Current Darlington Transistor Arrays SN74HC245 – Octal Bus Transceivers With 3-State Outputs CD74HC138 – High-Speed CMOS Logic 3- to 8-Line Decoder/Demultiplexer Inverting and Noninverting DS1232 – Micro Monitor Chip – Halts and restarts an out-of-control microprocessor. Holds microprocessor in check during power transients. Automatically restarts microprocessor after power failure.
HD74HC74 – Dual D-type Flip-Flops (with Preset and Clear)
HCF4051B – SINGLE 8-CHANNELANALOG MULTIPLEXER/DEMULTIPLEXER
OP07C, OP07D, OP07Y PRECISION OPERATIONAL AMPLIFIERS
The four wires that come to the PS board were Ground, relay on command and gate control for MOC3011. Control of the driver circuit is to ensure that the heating is done as per selected setting. In order to study the function of the PS board, I took a picture of the bottom side of the PCB, took a print out and marked the components on the paper. Let us have a look at the schematic obtained; which of-course was a result of minute study and markings done and after repeated attempts. First I used pencil, checked and rechecked markings, carried out corrections before overwriting with a black sketch pen.
It was possible only because it was a double sided through-hole PCB. Anyhow, this helped me understand the function very well. The triac passes on 230V AC to the element upon getting triggered. The 3011 is switched on by the uController, upon which the relay gets on to connect to the heating element. The current sensing is done by the 4N33 which works as a pulse generator to boost the voltage for the element to glow. The pulse for triggering this, is from the uController. Having understood this much and after checking that 8V and 5V were present, I connected the board to the uController and applied power. I saw the bulb glowing, but it went off after a few seconds. On checking that after switching on, the pulse required for boosting the voltage was not coming out from the uController. So, in all probability, that port has gone bad. The track upto the pin was found ok and it was not strained or cut.
That means either the IC is defective or the software has got corrupted. I fetched the ATMEGA IC and also bought an ATMEGA programmer but the interfacing software was an issue and I could not do much progress on it, mainly because of other pressing work. So, I stopped further work on the board and sent the entire kit to a firm which handled manufacturing of medical equipment, as they had all the facilities to copy the software, if it was not protected or write a fresh program for its proper function. But as far as I am concerned, my job was over and here is the proof of the damaged board working:
So, a contended satisfaction got collected to the bag!
This article was prepared for you by Parasuraman Subramanian from India. He is 76 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 Relay Rattling Problem Solved In AMREX UPS Model PROVIEW-600
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