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uln 2803

Nathan Hancock
Prodigy 40 points 


I'm using a T.I. uln2803 chip. If I sink 0.02 amps into input 1B then how much current should output 1C source? I'm using a microcontroller to source the uln2803. The microcontroller's voltage limit is 5.5 volts. Each of its outputs can source no more than
0.02 amps. I'm sourcing both of them with 4.75 volts. When I measure the voltage across pins 9 and 10 it reads the same as source voltage. When I hook one of the microcontroller's outputs to pin 1 on uln2803 I hook a test wire on to pin 18 and a test wire to
common ground the voltage meaures 0.655 volts and the current measures 0 amps. I was expecting to read 0.5 amps. What am I doing wrong?
  • 0
    TI__Guru**** 168356 points

    >>I'm using a T.I. uln2803 chip. If I sink 0.02 amps into input 1B then how much current should output 1C source?

    With a 5V input on B1 the current flowing into pin1 will be about 1.3mA. The micro-controller can do this easily.
    The 1C output will turn on, if the voltage on pin18 is greater than 1V then hundreds of milliamps will flow into the 1C pin.
    A load should placed between a positive voltage and a collector pin.

    >>I'm sourcing both of them with 4.75 volts. When I measure the voltage across pins 9 and 10 it reads the same as source voltage.
    If the loads driven by the ULN2803 are inductive then Pin 10 should be connected to the power supply for the loads.

    >>When I hook one of the micro-controller's outputs to pin 1 on uln2803 I hook a test wire on to pin 18 and a test wire to common ground
    >>the voltage measures 0.655 volts and the current measures 0 amps. I was expecting to read 0.5 amps. What am I doing wrong? 

    The ULN2803 sinks current to ground the load controls the current. If the pin 18 were connected directly to 5V then the current would be much greater than 500mA and the ULN2803 may be damaged.

    Regards,
    Ron Michallick