Microwave Oven Control Board

This project is designed to replace the defective control board with a new Control Board in Microwave Oven irrespective of brand and capacity. Microwave ovens are dumped as e-waste due to unserviceable control boards. Spare parts such as magnetron, HV transformer, HV diode and drive mechanism, except control board, are readily available in the market. Block diagram of the microwave oven is shown in Fig. 1

Circuit and working

The circuit diagram of the microwave oven is shown in Fig. 2. It is built around 5V voltage regulator 7805 (IC1), microcontroller (MCU) AT89C52 (IC2), four 12V relays (RL1 through RL4), five pnp transistors (BC557), 14 tactile switches, one 16×2 line LCD (LCD1) and a few discrete components.

The circuit is powered with 230V AC. Transformer X1 reduces voltage to 12 volts. Diodes D1 and D2 act as rectifier diodes. Capacitor C1 is connected as a filter. The rectified, filtered and regulated DC voltage is fed to the power section of the circuit, and 5V is fed to the control circuit via 7805.

AT89C52 MCU, running at a clock frequency of 11.0592MHz, is interfaced with 14 controlling switches, 16×2 LCD and four relays. Use Zener diodes ZD1 through ZD4 to connect 12V relay through a pnp transistor to 5V MCU. Diode D5 prevents the 12V supply to the MCU when the door switch is opened.

Relays RL1, RL2, RL3 and RL4 are connected to the MCU through four relay driver transistors T1, T2, T3 and T4, respectively. Transistor T5 is used to drive the piezo buzzer. Pnp transistors are used as active low output from the controller. The LCD is connected at port P2. Potmeter VR1 can be used to adjust the contrast of LCD1. D4 through D7 are protection diodes for relay driver transistors. AT89C52 runs with a crystal frequency of 11.0592MHz.

Software

The software is written in C language and compiled with Kiel µvision 4 software. Logic behind the software is described briefly for easy understanding and editing. In the main part of the software, the MCU scans continuously the condition of the seven input switches, comprising vegetable (VEG), meat, rice, frozen (FRZ), bake, up and defrost (DFR). If any these switches are found de-pressed by the program, the software jumps to the corresponding sub-routine. On auto sub-routine, each branch sets the total time of cooking and on/off delay timing for magnetron and grill heater. Total delay and on/off delay against each cooking styles are shown in the table.

On de-pressing Defrost, the program executes defrost sub-routine. Defrosting technique provided in the microwave oven is for cooking or heating refrigerated food. Normal cooking of refrigerated food in an oven results in rapid burning of outer parts, resulting in damaged food.

For compensating this at defrost mode, the system initially switches on the magnetron just for a few seconds. It slowly increases on time and decreases off time. This technique helps penetrate the heat throughout the food evenly.

Grill option is provided for meat only. Combi option is provided for rice, meat and cake only. On de-pressing Up, the software executes manual mode sub-routine. At manual mode, total cooking time can be selected by pressing Up or Down. With each press, time increments or decrements by one second.

By holding Up or Down, time increments or decrements by 10 seconds. After setting the time, any cook mode can be selected (Micro Veg or Combi). On/off delay is according to the preset sequence, according to the type of cooking selected. Accurate delay time is achieved by enabling Timer0 of the MCU (IC2).

Unlike in conventional ovens, double protection is provided for door opening while cooking, both in software and hardware. When the door is opened, coil ground connection to magnetron relay disconnects. At the same time, software detects this condition and switches off the magnetron.

LCD1 shows all parameters. It is connected in four-wire method or 4-bit mode of operation. Corresponding software is simple and self explanatory.

At EFY Lab, Topview programmer was used for programming the IC.

Download Source Code

Construction and testing

A PCB layout of the microwave oven is shown in Fig. 3 and its components layout in Fig. 4.

Download PCB and Component Layout PDFs: click here

Place all components on the PCB and solder carefully. Load the program (hex file) of the software to the MCU IC. Use IC base on the PCB. Solder the tactile switches on a separate PCB and connect the same to the main PCB, as per the circuit diagram.

After assembling the circuit, enclose it in a suitable box. Fix the switches and relays on the front and back sides of the cabinet. Connect relay coils on the PCB using 2-wire cables. Connect grill heater, HV transformer, stir motor and light using external high current-carrying wires via relay contacts.

Connect the wires to CON1, CON2, relays, CON3 and CON4 as shown in Fig. 2, except HV transformer and heater connections. Connect two 100W lamps instead of HV transformer and grill heater. Switch on the oven, and ensure all controls are working properly.

Check for minimum time durations, and ensure that the lamp is on/ off as the per sequence shown in the table. If the working and time duration is found satisfactory for all cook styles, switch off the unit and connect the HV transformer and heater. Close the covers and prepare any food item as per your choice.

Operational instructions

Auto mode

1. Switch on the power. Display will show Standby.

2. Select one option (frozen, defrost, bake, veg, rice or meat).

Say, for meat:

(a) Press Meat. Buzzer will beep.
(b) Press Micro. (You can select Micro, Grill or Combi).
(c) Press Start. Display will show Meat/Auto/Micro. Time will start decreasing from 15:00 minutes. On completion of time, the buzzer will beep. After the beep, display will show Cooking Complete.

3. You can select any one of the options: Frozen, Defrost, Bake, Veg or Rice, and repeat the above steps (a, b and c).

Manual mode

1. Switch on the power. Display will show Standby.
2. Press Up key. Display will show Set Time.
3. Press Up/Down key to increment/decrement time.
4. Hold Up/Down to increase the speed of count.

For example, to select Veg for 3.00 minutes,

(a) Press Veg. Buzzer will beep.
(b) Press Micro. (You can select Micro, Grill or Combi)
(c) Press Start. Display will show Manual/Veg/Micro. Time will start decreasing from 3:00 minutes. On completion of time, buzzer will beep. After the beep, display will show Cooking Complete.

5. You can select any option: Frozen, Defrost, Bake, Rice or Meat, and repeat the above steps (a, b and c).

Caution

Microwave ovens operate at high frequency and voltage waves (microwaves), which is highly dangerous. Take extreme care while working with microwave ovens. Always close all coverings before operation.

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A. Asokan Ambali is senior instrument mechanic at Naval Aircraft Yard, Kochi