ULN2003A: Relay Driving with ULN

Nathan Mathew

Part Number: ULN2003A

Hello Team,

Good Day!

We are facing an issue with ULN2003A.

We are using ULN2003ADR to drive relays in our design.Atmega2560AU is used as the main controller. In that when we are connecting the ULN's 9th Pin (COM) to 12V the relays are working fine but when we lifting the 9th pin some relays are working and other relays are not working but the voltage is proper between the relay coil.What may be the reason for that? Do we need to consider anything in the layout or schematics? Kindly find the attached schematics for reference.

7.1_P Schematic.pdf

over 2 years ago

0 Patrick Shalton over 2 years ago

TI__Mastermind 31560 points

Hi Nathan,

What voltage are you lifting the COM pin to? COM should be the same voltage as the highest relay driving voltage, I think 12V for your schematic.

Also, what do you mean by some relays are not working but the voltage is proper - do you mean that the xC pin is not going low?

Thanks,

Patrick

0 Nathan Mathew over 1 year ago in reply to Patrick Shalton

Prodigy 50 points

Hi Patrick,

The 9th Pin (COM) is not connected to any voltage. The relay is getting 11.4V but the relays are not working. But when we connect the COM pin to 12V it is working.

0 Nathan Mathew over 1 year ago in reply to Patrick Shalton

Prodigy 50 points

Hi Patrick,

The 9th Pin (COM) is not connected to any voltage. The relay is getting 11.4V but the relays are not working. But when we connect the COM pin to 12V it is working.

0 Patrick Shalton over 1 year ago in reply to Nathan Mathew

TI__Mastermind 31560 points

Hi Nathan,

Sorry for the delay on this, due to the holidays in the US a lot of the staff was on leave.

When COM is floating and the relay is getting 11.4V, is the low side of the relay/the ULN xC pin low, or is it also at 11.4V?

What channels are not working? And, do the same ones consistently work/not work?

Thanks,

Patrick

0 Nathan Mathew over 1 year ago in reply to Patrick Shalton

Prodigy 50 points

Hi Patrick,

While we lifted the 9th pin(COM) of ULN, the relay coil is getting a voltage of 11.4V while switching ON and 0V while switching OFF. But the relay is not turning ON. But when we give a voltage of 11.4V externally without ULN the relay is getting turned on.

While we doing the same thing by connecting COM to 12V the relay is operating properly.

0 Patrick Shalton over 1 year ago in reply to Nathan Mathew

TI__Mastermind 31560 points

Hi Nathan,

This is indeed strange, something that would help a lot here is a waveform showing Vrelay+, Vrelay-, and COM of both a working and not working relay when COM is floating, as well as a relay when COM is floating.

Unfortunately, it's hard to diagnose at this point, but the waveforms will greatly help.

Thanks,

Patrick

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ULN2003A: Relay Driving with ULN