Digital output does not give 5V when hooked up to a circuit

[rtjoyce]

I have a very simple circuit that I want to operate a solenoid valve. I have an NI USB 6008 with 12 digital I/O ports, a KF0602D solid state relay, a 12V power supply and a solenoid valve.

The idea behind this circuit is fairly simple: use a digital line from the 6008 to close or open the switch in the KF0602D. With the switch closed, current flows from the power supply through the switch, to the valve and the valve opens. When the switch opens no current flows and the valve closes.

When I hook up my 6008 to the KF0602D, however, my digital output "high" drops from 5V to 1.73V. This is a problem because my switch requires at least 3V to close. I am not sure why the voltage is dropping so significantly because the relay is supposed to only need about 3mA of current with a 5V input, well within the range of the 6008's listed specs. What can I do to make this work?

[crossrulz]

Yeah, it looks like the 6008 should be able to handle it.  You still might want to try adding a digital buffer circuit to see what happens.  My only other thought off the top of my head is to check your polarity going to the control of the relays.

[johnsold · Accepted solution]

The USB-6008 digital outputs are open drain only with pull-up resistors of 4.7 kohms. This will NOT drive your SSR input which requires > 2 mA.

Invert the polarity of your control signal. Wire the -input of the SSR to the DO line of the 6008.  Wire the +input of the SSR to +5 V. Then you have the 8.5 mA of drive capability to do what you want.

Lynn

[rtjoyce]

"Invert the polarity of your control signal. Wire the -input of the SSR to the DO line of the 6008.  Wire the +input of the SSR to +5 V. Then you have the 8.5 mA of drive capability to do what you want."

Would this work for running multiple relays? I'm looking to operate 11 on one card.

[crossrulz]

---

@rtjoyce wrote:

"Invert the polarity of your control signal. Wire the -input of the SSR to the DO line of the 6008.  Wire the +input of the SSR to +5 V. Then you have the 8.5 mA of drive capability to do what you want."

Would this work for running multiple relays? I'm looking to operate 11 on one card.

---

Yes it would.  You just have to wire up the 5V to the + side of the control input of all of your relays.  You should also put in a series resistor to make sure your digital inputs don't drag everything down.