MOSFET Relay Won't Switch Off

Thread Starter


Joined Sep 3, 2009

I am using an OMRON G3VM-S5 MOSFET relay to turn a pump on and off. The relay is wired to a USB board that is controlled by Labview software. When the 4V analog signal is sent from the board to the relay, the relay switches on, and the pump runs. When the analog signal is switched to 0V, the pump switches off but only for about a second. After this, the pump runs continuously. I can't seem to figure out why this is happening. Does anyone have any ideas?

There is a circuit diagram attached along with the datasheet for the relay.

Thanks for your help.


I am a little confused about the circuit diagram... How is the pump powered? Are you driving an input to the pump or are you trying to turn the power on and off by connecting and disconnecting power from the pump?

Thread Starter


Joined Sep 3, 2009
The pump is powered by a 12V dc power supply that is plugged into the wall. Instead of switching the pump on and off by hand, it has the option of using an external start/stop via 2 terminals in the back of pump. Once you close the circuit, the pump runs. When the circuit is open, the pump stops.
mik3's suggestion is definitely the first thing to be tried.

Can you operate the relay manually without the USB board? Does it operate the same without the USB board?

Do you have any details on what the relay is connected to in the pump? That is definitely not my area of expertise...

Thread Starter


Joined Sep 3, 2009
No the board does not send a signal after the one second. Even if the board is completely unplugged from the computer the pump continues to run. The relay is normally open, so for some reason it is being forced shut after that one second. Thanks for your help, I am not an expert on this stuff by any means.
If the usb board is disconnected from the relay and the pump is off. Can you turn the pump on, by applying 4v to the resistor on the usb side of the switch? Can you ground the resistor to turn the pump off? Does it operate the same as when the usb board is connected?

Do you have any details on what the relay is connected to in the pump?


Joined Aug 30, 2007
I I read the spec sheet right the relay is for ac and the pump is dc.
Sounds like it's latching up, like using a triac on dc.


Joined Apr 5, 2008

What is the normal control current for the start/stop input of the motor?
How is the start/stop normaly used?
Is there a specsheet of the motor?


Thread Starter


Joined Sep 3, 2009
StayatHomeElectronics, yes to your first three questions.

I had look inside the pump, and from the looks of it the relay is connected to a transistor that operates a mechanical relay to turn the pump motor on and off.


There is another data sheet that I didn't attach that shows the relay being used with either a dc or ac load. I will look into it though.

Thanks for your help everyone.


Joined Feb 4, 2008
If the relay is for AC then it has a TRIAC inside and it stays latched because the DC current does not fall to zero to switch it off.