Problem found when connecting two grounds

Hi,

I had a PCI digitiser card installed in my PC (https://www.okos.com/digitizers) . This card can count pulses from encoders and do some processing.

Iam powering these encoders from an outside power supply(5V). The problem i found is if i connect three encoders(**broken link removed**) altogether its drawing 200mA. If i connect the fourth encoder (**broken link removed**), it will draw 400 mA.

Everything will work fine if i didnt connect the board ground with the power supply ground. If i connect both grounds together it will start drawing more current(1A) and the voltage drops to 2.1V(Iam restricting the current to 1A in my power supply).

Moreover if i disconnect the fourth encoder from the system(with both grounds connected) it will work fine.

Iam using digitiser card from OKOS (https://www.okos.com/digitizers)

Can anyone tell me what is wrong in the connections?

Thanks

Everything will work fine if [B said:

i didnt connect the board ground with the power supply ground[/B]. If i connect both grounds together it will start drawing more current(1A) and the voltage drops to 2.1V(Iam restricting the current to 1A in my power supply).

Moreover if i disconnect the fourth encoder from the system(with both grounds connected) it will work fine.

Click to expand...

Seems like the 4th encoder is getting shorted possibly because of some lose soldering or component failure etc, you have to check it standalone to verify whether it is drawing same current as per its datasheet and working fine? the condition of voltage dropping and current increasing happens when something is getting shorted.
but i don't understand what you meant when you said Everything will work fine if i didnt connect the board ground with the power supply ground. How is it working fine if the supply ground connection is hanging and not connected to the system?