Specifications

CA10 Course Notes
Sharp Electronics (UK) Limited
CE Technical Support GroupPage 10
Over current control
Over current control is performed by monitoring the amount of current passing though the chopper output
transistor drain earth return resistor, R716. This resistor is an extremely low value, 0.47R, and Ohms Law
dictates that the current flowing through it must be proportional to the voltage across it. If the base
voltage required to turn on Q702 is +0.65V then the voltage across R716 to ‘trip’ the circuit would have
to be approximately 1.2V ( if the volt drop across D713 was 0.6V ). This results in a trip current through
R716 of 2.55A.
The voltage developed across R716 is used to turn Q702 on via D713 and potential divider, R719/20.
If Q702 turns on it will discharge C714 rapidly removing the gate bias and Q701 which will turn off until
the current trough R716 decreases to a point where Q702 turns off and Q701 turns on again ( after C714
has charged up to four an a half volts ). In the case of heavy current over load this will cause the power
supply to whistle as the frequency of the power supply stopping and starting will be within the audio
range.
Figure 7 : Current Regulation Circuit
D713 is used to set the voltage / current (through R716) at which Q702 will initiate an “over current”
action. Therefore, the smaller screen sizes fitted with the CA10 chassis may not have this diode in the
circuit shown above
Snubber circuit
Figure 8 : Snubber Circuit