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LED working in unusual way!

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Mrunal Ahirrao

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Hi everyone I have faced unusual problem of LED and I'm curious regarding solution for it. Problem is: I have used ULN2003 for driving relays, and interfaced the LED's cathode on the outputs of ULN2003 and provided +5V via 1K resistor to anode of LED, but LEDs exhibited some unusual voltage on cathode which inhibits the relay to switch off when ON. Experimentally, I am using 12V relay, and on the output of ULN2003(which is connected to LED and relay) I am getting 6.65V which should be 0V when no input is provided to ULN2003 ideally.I have also replaced ULN2003 and even I removed ULN2003( but LEDs connected to ULN pins) but no problem was solved, but when I removed LED's connection with ULN then its working correctly. Did anybody got same problem?
 

I think a schematic would help but I'm guessing the issue here is the 12V relay and LEDs being mixed. For the relay to be used safely, the catch diodes in the ULN2003 should be connected to the relay supply (12V) so when connected to LEDs fed from 5V they forward conduct and leave 7V (12V- 5V) across the LEDs.

The cure is to either disconnect the COM pin (pin 9) completely or connect it to 5V and add a new diode across the relay coil.

Brian.
 

betwixt said:
I think a schematic would help but I'm guessing the issue here is the 12V relay and LEDs being mixed. For the relay to be used safely, the catch diodes in the ULN2003 should be connected to the relay supply (12V) so when connected to LEDs fed from 5V they forward conduct and leave 7V (12V- 5V) across the LEDs.

The cure is to either disconnect the COM pin (pin 9) completely or connect it to 5V and add a new diode across the relay coil.

Brian.
Click to expand...
I have completely disconnected the COM pin9 from 5V as well as 12V. and I also added a separate diode across relay.

- - - Updated - - -

chuckey said:
LEDs have a very low reverse voltage breakdown voltage, this is whats happening, just put a decent diode in series with it to stop it happening.
Frank
Click to expand...

so diode should be placed in series with cathode of LED?
 

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