Not able to turn on a DC output

[rodtak]

I am trying to use a TTL level signal to actuate a 24V DC solenoid valve. Using the most simple, normally open, DC output module (Grayhill 70-ODC5) and a rack w/LED indicator lights, I seem to be able to turn the ssr on and off (based on the status indicators). However, the module will only turn the load off. As long as there is power on the output side of the ssr, I can not switch the load on. If the ssr is in the on state, and then the power is turned on to the load, the solenoid gets power. I can then turn the ssr off, and the solenoid is off.
The load is connected in series with a DC power supply to pins 1 and 2 of the module, w/negative attached to 2.

What am I doing wrong? Any help would be appreciated, and thanks in advance!

 

[itsmoked]

Here's my guess.

Disconnect your TTL source and then jumper (correct polarity) 5V to the control side of the opto. See if things work as expected then...

I believe your TTL is too gutless to pull this off. As the load increases on the SSR module the required voltage/current to the input side increases marginally. That's why it works with NO load and not with a load. TTL is often down around 3V when loaded. The specs state Nominal OPTO voltage '5V'

Measure your TLL ON voltage and tell us what you get when it's driving the opto.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rodtak]

Thanks for the tip! You were right on. After a little testing I found that using a variable power supply set at 5V the module works flawlessley. It continues to work down to nearly 3.5V. Below that, the module will no longer work predictably. Measuring the TTL signal when driving the opto, it reads 3.45V, hence the problem.

Is there a way I can use a 5V Vcc and use the TTL source to pull the opto low?

 

[itsmoked]

"Missed it by that much." As Get Smart would say.

Sure but a lot depends on your physical setup. Can you hang a bunch of additional parts here and there?

Often it is better to use the TTL to PULL DOWN the signal. This reverses your logic though.

You might Mickey Mouse things buy adding a resistor from 5V to the SSR input. Not enough to turn on the SSR but when the TTL goes on you end up with 3.8-3.9V Not a solution I would want to run with...

I am running my living-room aquarium with an ancient XT-PC via an OPTO-22 SSR board. I'm using the PC's parallel port to run the SSRs and have no problems with it. Prior to that I was running my washing machine with a straight IBM PC and also had no problems with those SSRs for 4 years.

What is your TTL stuff?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rodtak]

The ttl is coming from a PCI frame grabber (Matrox Meteor II) which has at least 2 i/o on a daughter board. I need to utilize these primarily because there are not enough PCI slots to support an additional i/o board. Another PCI slot is being used for a quadrature counter card.

Actually, the i/o rack for the ODC5 modules will support either positive or negative logic. I will try to reverse the logic and see if that will work. Perhaps pulling them down would be more reliable.

 

[itsmoked]

Interesting.. Yeah if you can pull down and not suffer from initial conditions that will work better.

My aquarium suffers from initial conditions because a wave making system has an SSR based air bleed to provide cavitation air. It always comes on for the whole time the PC is booting. This is long enough to break several siphon jumps between tanks.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rodtak]

I may have to change from a normally open solenoid valve to a normally closed valve. This should avoid the initial condition during boot and initialiazation of having the valve open until the software can close it. I just had a NO valve on hand.

Reversing the logic works fine. I just need to find an available 5V source for Vcc. I think there is one available on a stepper controller board being used on the same system. As long as they all share a common ground, it should be good to go.

Thanks again for the help! I would have looked a long time before finding the problem. The system will be better for the minor changes as a result.

Hope to return the favor some time.

Rod Takata
Industrial Products Group

http://www.ppsindprod.com

 

[itsmoked]

rodtak; Do be careful with that 5V as just a 5V line from "somewhere" smacks of ground loops. Also stepper supplies are notoriously noisy. Sounds like a fun project!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com