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ULN2803A as Relay Driver: two relays in parallel on one channel?

David Bistrovic
Prodigy 50 points
Other Parts Discussed in Thread: ULN2003A

Hello Community!

I have an urgent question on the ULN2803A:

I would like to use the ULN2803A as a relay driver. My question is: can I connect to 1 channel 2 relays in parallel?

The coil current of my relays is 84mA, so two relays would be at 168mA. The ULN2803A allows 20-300mA per channel, so I assume this would be ok, correct?

But can I connect two in parallel as long as I don't exceed the 300mA per channel?

Thank you very much for your help!

David

  • 0
    TI__Mastermind 24485 points
    David,

    First of all, welcome to the forums.

    To answer your question:

    Yes, you should be able to connect 2 relays to a single channel as long as the current through the channel is not exceeded.

    To follow up with an additional design concern that is more important:

    Mind you, and this is a much more important point, that the current through each channel will also be limited by how many channels you are using concurrently and how long they stay on.
    The Table 1 Design Parameters in the ULN2803A describe that you can have 8 channels, 300mA, and 100% duty cycle, but this is not possible all concurrently. One of the primary reasons is self heating of the device.

    Although we don't have the same figure in the ULN2803A datasheet, please refer to Figure 4 of the ULN2003A datasheet.
    You can see that as more channels are utilized, if they are all on at the same time there is a maximum collector current through each channel. For 7 channels all operating at 100% duty cycle, the maximum current per channel is along the lines of 70mA.
    I just wanted to make you aware of some other limitations that might affect your design.

    Does this make sense?

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Prodigy 50 points
    Michael, thank you for the fast reply. Yes, this makes sense. In our application we are normally using only one channel at a time. In some cases we will be using 2 channels at the same time for a duration of 1-3 seconds - one channel with 168mA and the other with 48mA at a duty cycle of about 10-15% - so I should be fine there as well. Thanks again for making me aware of this! Cheers, David
  • 0 in reply to David Bistrovic
    TI__Mastermind 24485 points
    David,

    Very happy to help.
    Please let me know if you have additional questions or concerns.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Prodigy 50 points

    Hello Michael,

    I have one more question:

    In my application I am switching a motor, a transformer, and a few solenoid valves. I have selected high current relays with a 5V-DC coil and a switching rating of 16A for the motor and for the transformer (which are both 120V-AC/16A or 230V-DC/8A; or 24V-DC/0.5A when I switch a contactor to turn them on/off). And 5A relays for the valves (which are 24V-DC, and only 1A).

    Since these are all inductive loads, with high voltage and high current, in some cases DC loads and in some cases AC loads: do I still need a MOV around each relay coil to protect the ULN2803A from spikes and back emf? Or is the diode in the ULN2803A protection enough?

    Also I am thinking about RC snubbers between the relays contacts, which is applicable for both AC and DC loads. Would this also help to protect the coil as well as the ULN?

    Thank you very much for your help!

    David

  • 0 in reply to David Bistrovic
    TI__Mastermind 24485 points
    David,

    For an application like a stepper motor (just as an example), there are negative transients consistently hitting the output transistors and the internal clamp diodes to COM are typically enough for this application.

    When you are using the Relays to control other peripherals, it really depends on the transients from the relays. My understanding from what you are describing is that the relays are isolating these other high current devices from the ULN2803A.
    Is that correct? Please let me know if I am misunderstanding, and if so, please share a schematic.

    You mentioned before the current through the relays would be 168 ma. With this load and a 5V relay the internal diode to COM pin should be enough, but it is always a good idea to test it out.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Prodigy 50 points
    Hello Michael,

    That is correct. I have the relays switching a vacuum pump (motor), a transformer, and solenoid valves. The ULN2803A is driving the coils of the relays. And I use an Atmel 328P to drive the ULN2803A.

    This board is made to run on several machines: one machine has 120V-AC motor/transformer/valves, one has 230V-AC motor/transformer/valves, one has 120V-AC motor/transformer but 24V-DC valves, and one has 24V-DC external contactors (for motor/transformer) and 24V-DC valves.

    The valves are all low current (between 0.25 and 1A, depending on Voltage).
    Pump motor and transformer are high current (each around 15A at 120V-AC, or 8A at 230V-AC).

    Please see the schematic here: www.bistrovic.com/schematic_relay.pdf

    I am wondering if the MOVs around the relay coils are necessary or not?

    Thank you!
    David
  • 0 in reply to David Bistrovic
    TI__Mastermind 24485 points
    David,

    I can't guarantee that it would work without them, but typically the internal diode to COM pin is enough to handle inductive loads.
    At 170mA and 5V, the internal COM diode should be sufficient.

    As I mentioned before, I have run this with stepper motors which have some pretty nasty negative transients on the output pins that happen multiple hundreds of times per second, and this application did not require the external MOVs.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Prodigy 50 points
    Michael, thank you very much for your help!

    -David