Diese Seite ist leider nur in Englisch verfügbar. Power supply with over-current protection of prescribed current-time characteristic

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The sane-backend for the EPSON PERFECTION 1260 PHOTO now works fine. Thank you, Gerhard Jäger!

Power supply with over-current protection of prescribed current-time characteristic

The power supply is designed for an output voltage of 15.2volt and a duration current of 1.25amps as required by the Epson Perfection 1260 scanner. The switch-off-time-current characteristic of the over-current protection is similar to that one of a super fast chip fuse used in the Epson Perfection 1260. If the load draws an over current longer than the corresponding switch-off time then the power supply switches off. It can be switched on again by removing the load and unpluggin the main-supply cable for ca. 30 seconds.

Circuit diagram

The operational amplifiers employed with the power supply are all open collector types. Therefore, they can work on a common load. Thereby, the common load is the current source built up with the transistor Q1 (SF117) and the base configuration of the power Darlington transistor.

The TAA861 serves as the voltage regulator. The resistor R10 serves for the fine adjustment of the output voltage.

The twin-opamp TAA2761 realizes the over current protection. The opamp IC2b works as an integrator the opamp IC2a as a Schmidt-trigger (see below for some more words about that).

The duration current can be adjusted by the 470Ohms potentiometer R21 and the switch-off-fime characteristic can be adjusted by the 100kOhms potentiometer R23.

The output impedance of the power supply is approximately 0.005 Ohms at 1 Ampere output current.

  • Circuit diagram of the power supply (file size: 46kb).

    Some pictures of the mechanical construction

    To enlarge the view of one of the pictures just click onto the corresponding thumbnail.

    Over-current protection

    The output current is measured indirectly by the voltage over the resistor R20.