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ULN2003LVDR relay driver will not turn off

Steve Varga
Prodigy 95 points
Other Parts Discussed in Thread: ULN2003LV

Hi,

I have a ULN2003LVDR driving a a 5 volt relay with a coil resistance of 130 ohms.

The ULN2003LVDR (driver) input is driven by an MSP 430 running at 3.6 volts. 

The relay is attached to one of the outputs and to  +5 volts.

The driver energizes the relay fine but when the MSP430 sets the driver input low again to .002 volts, the driver continues to conduct and not release the relay. The output of the driver stays at about 1.5 volts. Disconnect the relay momentarily from the circuit and the  driver returns to the non conducting state.

Relays with ~200 ohms of coil resistance work fine and the relay described above will work OK with a 50ohm resister in series.

Thanks,

Steve

  • 0
    TI__Guru**** 164796 points

    Hello Steve,

    The maximum coil current, 5V/130 ohm is just 38mA; which is fine.
    Is pin 9 (COM) of the ULN2003LVDR connected to the 5V supply?
    I would not hurt to check pin 8 (GND) for 0V.

     

  • 0 in reply to Ron Michallick
    Prodigy 95 points

    Ron,

    Common Pin 9 is  tied to +5 and pin 8 is solid to GND.  

    Steve

  • 0 in reply to Steve Varga
    TI__Guru**** 164796 points

    Steve,

    Can you replace the ULN2003LV or try a different channel? A waveform of the output trying to turn off would also be helpful.
    The results are very unusual unless the output is permanently damaged in some way.

    The internal diodes protect the ULN2003LV outputs when the relays turn off from the inputs going low or if the 5V source turns off (or fails).
    However there is the possibility of a spark gap transient to the output pin if the relay connection (relay to ULN2003LV output) is broken while the relay is energized (hot disconnect). When the line is initially broken, the coil becomes an open circuit has no means to pass its current other than to self oscillate. After a quarter cycle of oscillation, the voltage is at a maximum and could jump the gap in the break.