Output current limitation on NI 9264 Analog volutage output module

[john.sheppard]

Hello,

I have a question related to the NI 9264 analog voltage output module.  I am using this module with a compact DAQ interface in LabView 8.2.1, and am trying to output a signal to activate a Solid State Relay (Magnecraft - part number 861SSR115-DD-1).  I understand that the 9264 module has a maximum output current of 1 mA and this is too small for the current required to activate the relay.  The minimum input current for this relay is listed as 12 mA; however we have used a PCI-based DAQ with a 68-pin connector in the past successfully, even though the PCI card was limited to 5 mA output (NI DAQcard 6062E family).  I spoke with a product engineer from Magnecraft and it seems that 5 mA was enough to activate the relay despite being below the lower spec (12 mA). 

The result of all this is that if I output 10V from the output module, I will read 10V with a voltmeter but once connected to the relay, the voltmeter will only read 2.25V, so 2.25V seems to be the maximum voltage without exceeding this limitation on output current.  The lower spec for voltage input is 3.5 V on the relay so I will need to get close to this to get it to work.  I am open to any solutions you may have regarding this issue.  I'm sure the signal could be amplified in some way externally, but I don't know if there is a more direct approach or whether NI carries any relays or other products that might be useful with such a small output current.  At this point I need to use the compact DAQ board with this module if at all possible, rather than relying on the older 6062E type setup. 

Any ideas or help you could provide would be greatly appreciated.

Thanks for your time,

John Sheppard

[Wallace_F]

Hello John,

I looked at the specs for the solid state relay you are using, and it just said that the typical input current was 12mA.  That probably explains why you could operate the SSR with only 5 mA. 

The reason that when you output 10V on your NI-9264, but when it's connected to your SSR you only see 2.25 volts is because it is trying to draw more current than the typical rated current of 4mA per channel on the NI-9264. One thing you might try is to use two channels of the 9264 module in parallel to increase the current that is driving your SSR.

Best wishes,

Wallace F.

[LuisO]

John,

How many SSRs are you trying to control with an NI9264?

If I remember correctly, you get 1mA per channel if using all 16 channels, BUT if you are not using all 16 channels, you can output around 4mA per channel, up to a total of 16mA per module.  Since the outputs are sinking/sourcing (i.e., you can put out +/-10V), I would not recommend paralleling channels, so this still doesn't help you.

Could you instead use a relay module to switch external power on and off to the SSR? -- take a look at the NI 9481 (4 SPST) or NI 9485 (8 SSRs)..  You can't just replace your SSR with these (too little current compared with your SSR's 15A rating), but perhaps you can use these to control your larger SSRs.  This way you can provide enough current to the SSRs and make sure they ALWAYS turn on when you want them to...

Good Luck,

Luis

[john.sheppard]

Hi Wallace and Luis,

Thank you both for your responses - this helps a lot.  I never thought of putting two channels in parallel, and I'm only using two modules so I guess I get more current (up to 4mA) than I originally thought.  I just tried putting two output channels in parallel today and it was a sufficient voltage to trigger the relay - I think this will probably be successful.  I need to configure the LabView program to handle two output channels for the relay - I'll let you know if that works out or not. 

If not, I can also look into those additional modules Luis was talking about.

Thanks again,

John

[john.sheppard]

Hi Wallace,

I had a follow-up question to ask you, as we are looking into getting a new NI module with higher output voltage and current capacity.

First, I just wanted to note that the actual module I was using was a NI 9263, not 64.  The situation is the same - not enough current getting ot the relay.. just wanted to clarify.  Sorry for the mixup.

Although we can successfully trip the relay with the NI 9263 using two parallel output channels, we are looking into getting a more appropriate output module to simplify our setup.  We were considering the NI 9472.  The 9472 specs are as follows: 6 to 30 V range, sourcing digital output, 8-channel, 100 µs I/O relay time, and 750 mA max output current.

We will be using this to activate the SSR relay I mentioned, requiring 3.5-32V input with a lower input current spec of 12 mA (though we can probably get away with lower). 

Based on this, do you think the NI 9472 would be an appropriate module for our purposes, or did you have any concerns or other suggestions?

Thanks,

John

[Nathan_R]

Hi John,

I think the 9472 would work for your application. However I wanted to make sure you were aware that the 9472 is a digital output that requires an external power supply to provide the voltage that the digital output will switch. This is why it is rated for 6-30 volts. The external supply can be in this voltage range and the digital output will be 0 volts when off and the supply voltage when on. A figure of this configuration can be seen on page 11 of the NI 9472/9474 Operating Instructions and Specifications.

Please let me know if you have any questions and take care.

Thanks,

[john.sheppard]

Thanks for the information on the NI 9472, Nathan.  Come to think of it, with this relay module our SSR relay will no longer even be necessary - we are using it to power a small solenoid valve but we could simply trigger the solenoid directly with the increased voltage using the 9472. 

I just wanted to verify that you don't foresee any obvious problems with this setup before placing the order - the solenoid valve needs 24 VDC which we can supply to the NI 9472 through an external supply.  Also the solenoid valve requires 5.4 W of power to operate.  I believe that means the current would be I = P/V, which comes out to 225 mA.  A single channel of the NI 9472 should easily be able to handle this current load.

Does that sound ok, or are there any other issues I am forgeting?

Thanks,

John

[john.sheppard]

I actually just spoke with an NI representative over the phone and it sounded like everything should work out fine - sorry, couldn't find an option to delete my last post.

Thanks for all the help,

John

[AnalogKid2DigitalMan]

I'm not familiar with the PXI equipment, but it may be wise to put reverse biased diodes or snubbers right across the solenoid coils to protect the output stage from transient voltage spikes when power to the solenoid is turned off. You may want to refer to the manual, it may mention protection schemes for inductive loads.

-AK2DM