With continuous development of medical reform and population aging, medical equipments have been applied gradually in rural hospital, like ECG is sgnificant example. Knowing about the theory of ECG Operation is very important for its promotion in country.

By methods of transformation at the switching power source featuring broad range and high resolution, the 220V, 50Hz (110V, 60Hz) AC power supply becomes a 20V DC voltage. This ECG's voltage --- the list of the new Clinic equipment  ---works to charge the built-in rechargeable battery at stable voltage and limited current, In addition, it generates a +5V voltage following procession at the AC/DC switchover circuitry and the switching voltage stabilizer.

The +5V voltage supports the digital part of the circuitry with the earth as reference. By ways of width modulation and wave chopping, it offers a +3.5V voltage for supporting the LCD back light generator. By a negative power supply generator, it supplys a -20V voltage for negative voltage of the LCD. Comparing with its 2A capacity and 2.5A upper limit, normal loading of this +5V voltage is about 1.25A.

The self- stimulation switching power source  the ECG machine --- the list of the new Clinic equipment  --- and isolating pulse transformer transform output from the AC/DC switchover circuitry to generate voltages for supporting the isolated circuitry. A –5V voltage is used to support similar part of the circuitry with the earth as reference. The +5V voltage needed by the isolated digital circuit is the direct output from the switching power supply. Its normal loading is about 150Ma in contrast with the 300Ma capacity. The ±12V voltage for the isolated analogous circuit is the non stabilized output from the switching power source. Its normal loading is about 60Ma in contrast with the 100Ma capacity.

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