03 iunie 2020

Driving one-coil latch relay without H-Bridge

The figure below shows a simple circuit using the MC9S08QE128 microcontroller from Freescale to drive a Finder 40.61.6.005 single-coil latching relay with a standard ULN2003 Darlington driver with open-drain outputs and inductive-kickback protection. Clamping diodes on each ULN2003 output pin catch high-voltage transients that occur when you interrupt the coil current. Because demagnetization uses low-value resistors, you must wire at least two open-drain buffers of the ULN2003 to both endings of the relay coil to ensure enough current when the microcontroller pulls down.
To simpify the schematic, one common reset rail can be used to all relays. The work sequence will be:
    Rel1 ON
     RST
      Rel1 & Rel2 ON
       RST
        Rel1 & Rel3 ON
    RST
... and so on.


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