Electronic – ULN 2003 open collector

open-collectorrelayuln2003

I have searched for troubleshooting the ULN and did not find any solution.

I use a ULN chip in a motor inverter for controlling precharge relay and main relay.
The problem is when the motor inverter is powered off, and for instance the charging is on (battery charging) the relay will kick in getting negative trough the unpowered ULN??? Sounds strange, but It's happening.
And I don't want the motor inverter ON all time (uses power and draining the battery)

I can not disconnect GND from inveter.
I can not disconnect POS from relays.

Is there a "easy" solution to this??

enter image description here

The relay is connected to +12V and one output pin on the ULN. The ralay gets some "creepage" current from the ULN, enough to energize the relay. Happens when inverter is off, but other devices are on (like charging)

schematic

simulate this circuit – Schematic created using CircuitLab

Best Answer

enter image description here

Figure 1. Internal diagram of the ULN2003. Source: Wikipedia ULN2003A.

Note the internal snubber diodes. This makes your circuit look like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. The actual schematic showing the internals of the ULN2003.

SW1 represents your power supply. When up, as shown, the snubbers are connected to the switched +12 V. When SW1 is reversed, representing the PSU being switched off, a path to ground exists through all the components in the rest of the circuit. It's obviously low enough to allow your relay to pick up.

The solution:

schematic

simulate this circuit

Figure 3. Connect the snubber COM to the always-on +12 V.

This will eliminate the sneak path to ground. More importantly, it will return the relay current to the correct power source - the "always-on" +12 V.

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