Epson 2070 Service Manual - Page 67

LQ-2070 Service Manual 3. +35 V Constant Voltage Control Circuit The +35 V constant voltage control circuit is illustrated below. Operating Principles 11 C15 Q1 C13 R11 10+ R20 R21 Q2 R15 Q3 R19 R16 R13 R14 C12 + IC1 9+ 81 72 R56 D81 R57 +35V Line ZD51 ZD81 ZD82 ZD83 ZD84 ZD85 GND Line Figure 2-25 +35 V Line Constant Voltage Control Circuit The constant voltage control circuit operates to keep the 35 V line at 35 V ± 6 %. When the voltage between ZD51 and ZD85 becomes 32.7 V ± 2.75 %, PC1 turns on, and then Q2 also turns on. Consequently, switching FET Q1 shuts off. When the voltage between ZD51 and ZD85 becomes less than 32.7 ± 2.75 V, PC1 turns off, and then Q2 also turns off. Consequently, switching FET Q1 operates again. Repeating the above operation keeps the +35 V line at 35 V ± 6%. 4. +35V Line Overload Detection Circuit The +35 V line voltage drop protection circuit is shown in the figure below. +35V Line ZD51 ZD81 ZD82 ZD83 ZD84 ZD85 +5V Line PC1 R56 D81 81 R57 Q31 72 GND Line R83 R20 R84 R86 5 8+ 64 - 7 R87 PWDN GND Figure 2-26 +35 V Line Overload Detection Circuit When the +35 V line is overloaded, it means that constant voltage control is not being maintained. In this condition, the forward current of PC1 drops to 0 A. Consequently, voltage Vf between PC1 and D81 also drops. On this circuit, when the Vf voltage drops below 1.3 V (+35 V line: 33.1 V), IC528 detects the overload and outputs the PWDN signal (+5 V: HIGH active) to port 20 of the CPU. When the CPU receives this PWDN signal, printing stops. When the +35 V line becomes normal again, the voltage between PC1 and D81 also becomes normal. When the Vf voltage goes above 1.6 V (+35 V line: 33.4 V), the PWDN signal is removed. Rev.A 2-19