Issue with ULN2003A for relays

Thread Starter

idir93

Joined Jan 10, 2018
12
Dear guys,
I am using a ULN2003A to drive 5 relays. The inputs come from a 74hc595. The free wheeling diode is connected to Vcc.
It used to work, I could trigger any relay I wish using my Arduino (Through the 74hc595). but now, only two channels are operational.
Attached you will find the PCB. Should I replace my ULN2003?
Regards,
 

Attachments

AlbertHall

Joined Jun 4, 2014
11,506
Can you measure the voltage on the input and output of the ULN2003A on a channel which doesn't work while the relay for that channel should be on and post the voltages here.
 

Thread Starter

idir93

Joined Jan 10, 2018
12
Thank you for your reply.
Well yes, I have measured them already.
for a logic 1 input ( 5v) ==> I find 20v ( which in fact should be 0v, because the relays are all sharing 20v)
 

Thread Starter

idir93

Joined Jan 10, 2018
12
I just measured them. All of them are 900 ohm. I wanted to test them by energizing them all at once (It has worked in the past), but now, only two of them are getting energized ( R1 and R3 only - look at above attachment).
 

dl324

Joined Mar 30, 2015
13,141
Replace the IC.

If the COM connection was connected to the relay coil supply, don't know what could have caused the drivers to fail. It's being operated well below maximum current, voltage, and power dissipation specs.
 

AlbertHall

Joined Jun 4, 2014
11,506
If those voltages were measured on the actual pins of the ULN2003A (eliminating the possibility of bad joints or tracks) then it certainly looks like the chip has died.
 

Thread Starter

idir93

Joined Jan 10, 2018
12
Yes, I have measured exactly at the pins, not the tracks and nor the relay connectors. Right on the pins of the ULN2003.
But if the chip is dead, how come does it have two channels fully operational ( out of the 7).
Best,
 

AlbertHall

Joined Jun 4, 2014
11,506
Yes, I have measured exactly at the pins, not the tracks and nor the relay connectors. Right on the pins of the ULN2003.
But if the chip is dead, how come does it have two channels fully operational ( out of the 7).
Best,
The question really should be why some of the channels die.
Do you have pin 9 (COM) connected to the +20V relay supply?
 

Lyonspride

Joined Jan 6, 2014
137
It's not overly clear, but have you got flyback diodes across the relay coils? Lack of might explain the destruction of the chip as the collapsing EM field can generate up to 10x the coils rated voltage.
 

AlbertHall

Joined Jun 4, 2014
11,506
It's not overly clear, but have you got flyback diodes across the relay coils? Lack of might explain the destruction of the chip as the collapsing EM field can generate up to 10x the coils rated voltage.
That's what the COM pin does, connects diodes across the relay coils, and TS confirms that is connected.
 

ericgibbs

Joined Jan 29, 2010
13,652
Yes.. But I don’t trust those internal diodes...I put diodes across the coils anyway.
hi,
I do the same, especially when the relay coils are some distance from the ULN, reduces the wiring radiation path length.
E
 
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