| 摘要 |
PURPOSE:To prevent overvoltage safely, by adding a resistance in parallel to the partial pressure resistance to detect output voltage. CONSTITUTION:A overvoltage protection circuit of a switching power unit performs switching of DC current with a switching transistor (Tr) Q on the primary side of a transformer T and feeds the current to the load through diodes D1-D2 on the secondary side, a choke coil L and a smoothing capacitor C. The output voltage is detected with resistances R1 & R2 and a variable resistance VR and fed back to a driver circuit B of a Tr Q through a feedback amplifier A, so that the overvoltage of output is prevented. On this occasion, resistances R3 and R4 are added to connect in parallel to partial pressures R1, R2 and VR. The midpoint of these parallel additional resistances R3 and R4 is connected to a contact P of the variable resistance VR. The condition under which the feedback becomes zero can thereby be prevented, even if the contact P is opened.
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