Circuit Function Description.
The basic function of the ISO
The transformer is designed to have a low leakage flux value which allows it to be mounted close to the CRT. To accomplish the low leakage flux, the transformer has two sets of primary and secondary coils mounted on a modified toroid type core. The ideal transformer would be a toroid but this type transformer is expensive because it is difficult to wind.
A relay is used to connect the two primary coils in series or parallel to accommodate 240VAC or 120VAC line voltages. Before power is applied, the relay connects the transformer primary windings in series to avoid excessive primary current for the 240VAC case. The control circuit energizes the relay when the line voltage is 120VAC.
240VAC
or
120VAC
50 or 60 Hz
Input
240VAC
or
120VAC
Relay Control
Output
Circuit Description.
The fuse T506 protects the mains wires and the control PCB.
The power transformer has two internal 2 amp temperature sensitive fuses. Each primary half has one, built in, series connected fuse.
A resettable fuse T507 is connected to the relay driver power supply.
This fuse protects the relay control circuit from square wave input which would occur if an inverter is used as the power source. Capacitor T512 supplies current from the line to capacitor T513 through diode T510 which forms the, relay control, power supply. Diode T511 charges capacitor T512 during the negative going part of the line wave. Transient Voltage Suppressor T516 regulates the 24V power supply and protects the relay coil from over heating.
The Mos Fet T514 shorts out the 24 volt power supply when the input line voltage is 240VAC.
The input line voltage, at which the Mos Fet turns on, is set by the Mos Fet turn on voltage (about 4V), the voltage drops across resistors T509 , T505 , T504 , and the zener diode T503 . 154VAC is the approximate line voltage at which the relay T518 switches. Capacitor T508 and diode T502 keep the Mos Fet turned on for the complete AC cycle to eliminate ripple current in the capacitor T513 . Resistor T504 limits the peak current to capacitor T508 to avoid relay switching due to line transients. The zener diode T515 which is connected from the Mos Fet source to gate protects the Mos Fet gate against over voltage. Resistor T519 is needed to limit the mains current when the relay arcs across both sets of contacts. Capacitors T517 and T520 reduce the relay T518 contact noise which may be generated when switching.
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