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I have a need to connect an ARM micro-controller with 3.3V digital output pins to a few 12V solenoid water valves. I figured that I can use a ULN2803A for this task.

The internal circuit for each input/output is depicted below:

enter image description here

Couple of questions:

  1. Is the attached schematic correct?
  2. Do I need anything else in the circuit to protect the micro-controller?
  3. Anything else needed to protect the ULN2803A?
  4. And do I tie the +12V and +5V grounds together?

enter image description here

EDIT: The load I'm driving is rated at about 400mA, 100mA shy of what this part is rated for ( each output ). The datasheet says the ULN2803A can be put in parallel to handle more current. I'm mot sure how that circuit would look.

Would I just logically connect them as if they are stacked one atop the other?

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  • \$\begingroup\$ @IgnacioVazquez-Abrams Thank you. What changes would you recommend? \$\endgroup\$
    – Chimera
    Commented Sep 21, 2013 at 23:16
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    \$\begingroup\$ @IgnacioVazquez-Abrams: the ULN2803 and related parts have diodes between each output and the common terminal, so there is no need to add additional dioces, if the common terminal is connected to the relay supply as shonw in the sketch. \$\endgroup\$
    – Peter Bennett
    Commented Sep 21, 2013 at 23:23
  • \$\begingroup\$ @PeterBennett Thank you. I've edited my post to show the schematic for each darlington pair. \$\endgroup\$
    – Chimera
    Commented Sep 21, 2013 at 23:28
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    \$\begingroup\$ Yes, it looks OK to me (but I haven't checked the ULN2803 pinout). the 5V and 12V grounds must be connected together. What is that symbol between the coontroller and the ULN2803 pin 1? \$\endgroup\$
    – Peter Bennett
    Commented Sep 22, 2013 at 17:18
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    \$\begingroup\$ Regarding protection of ULN2803: it depends on the environment. In case you expect powerful transients, current rushes, dumb users switching polarity during installation... then I would definitely not use ULN2803. This comes from the experience of a product with some 15 years of lifetime by now, which is using ULN2803. It is simply not rugged enough to survive such environments. \$\endgroup\$
    – Lundin
    Commented Sep 23, 2013 at 6:56

1 Answer 1

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The basic connection scheme for ULN2803 is like

ULN2803 connection relay

and for ULN2003 is like

ULN2003 connection relay

where each box can be a relay or solenoid etc, ground side of the relays/solenoids connects to the ULN outputs

BUT the 500mA current per output doesn't tell the whole story, when you use more than one output and the outputs conduct simultaneously then the max current per output for ULB2803 is as shown in

ULN2803 max current

so it hugely depends on the duty cycle and the number of outputs.

Note that ULN2003 has not the same characteristics, it seems to be able to provide less current per output

ULN2003 max current

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