Panasonic KX-MB1520FX7 Service Manual page 42

KX-MB1520FX7/KX-MB1520FX8/KX-MB1520FXW/KX-MB1530FX7/KX-MB1530FXW
6.7.3.3. O.C.P (Over Current Protection) circuit
1. Function
If motor driver can supply more than 15 watts, FB and ADF motors may become fire hazards.
To prevent the risk of fire, this circuit is provided.
2. Circuit Diagram
3. Circuit explanation
When the current supplied from 24V exceeds 0.44A, the voltage between two registers R657 and R658 becomes more than
0.6V (=0.44A*2.7ohm/2), consequently both Q650 and Q651 turn on, then "STBY" signal (IC502_pin44) becomes "Low" level.
When "STBY" signal becomes "Low" level, IC502 is deactivated and currents of both motors are cut off.
So the currents which IC502 can supply are limited to less than 0.44A and the wattage is also limited to less than 10.7W
(=0.44A*24V).
By limiting the wattage less than 15W, risk of fire hazard is eliminated.
Also "CRM1" signal is provided from IC300_pinB10 to base of Q651.
This signal is designed to become "High" when software can not control motors properly by some accidents.
When base of Q651 becomes "High", "STBY" signal becomes "Low" and motor currents are cut off.
Therefore even in the case of software can not control motors, motors stop automatically for securing safety.
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