SCXI Power Supply Problems

[amcolosk]

I am designing a system that will allow me to send DC signals to a DUT.  These ten signals go through a couple of relays and get shorted to power, ground, and other signals.  Currently I am using a SCXI 1520 that was left over from a previous project to do the task. The problem is that it doesn't seem to take the pressure of the shorts too well and I don't think it will last long under those conditions.  Since I have a SCXI 1001, I can only use SCXI modules.  The other modules I am currently using are: SCXI 1600, SCXI 1169, SCXI 1520, SCXI 1127, and SCXI 1161.   Any suggestions on a good module to use for this?

Thanks everyone!

[hoffiz]

Hi,

The SCXI-1520 is a “Universal Strain Gage” module. The SCXI-1520 module is an eight-channel module for interfacing to strain-gauge bridges and other Wheatstone-bridge based sensors. It is not intended for analog outputs.  The SCXI-1600 module is a USB data acquisition (DAQ) device that features a 16-bit ADC. The SCXI-1600 receives analog signals from other SCXI modules and amplifies, digitizes, and sends data through a USB port.

With the equipment you have right now I cannot see how you are able to output a DC voltage. But it seems to me that you have done it even though it does not seem the best way.  Would you mind explaining more in depth what have you done so far? What connection have you made? How much current is your DUT going to be sinking/sourcing? The more descriptive you are the easier for me to help.

[amcolosk]

I am using the power out ports of the 1520 that are normally used to power the sensors that are supposed to be attached to it.  The ports on the terminal block are P+ and P-.  I normally have them running at a 5V out to simulate the signals. As for the current out, it is probably only a few mA at tops.  They will get shorted to the main power line, 12-14V, and to ground.  The 1169, 1161, and 1121 are used purely for controling the flow of the signals.  The reason I used the 1520 was due to the fact that I had leftovers and I could possibly rig them to work.

[hoffiz]

Hi,

If you take a look at the SCXI-1520 User Manual page 2-3 it explains the way the excitation operates. Page 4-24 does mentiont: Chassis ground is at the same potential as earth ground when the chassis is plugged into a standard 3-prong AC outlet. If PX– is connected to earth ground, the excitation source does not function properly.

A couple more questions I have are: “These ten signals go through a couple of relays and get shorted to power, ground, and other signals.” My guess is that you have another card doing 2 of these ten signals since the 1520 only has 8 channels and one P+, P- per channel.  Let me check I have everything right here? Each one of the signal coming out from the 1520 (P) goes through a relays then goes to the DUT, right? Why do you want to short the DC output to 12-14 Volts, and to ground? What is the purpose of your test?

One device that you might be looking at would be the NI SCXI-1124 take a look at the specification and let me know how it looks for you.

[amcolosk]

I am using a second 1520, to send the extra two signals.  (Nice Catch!)  The Note would possibly explain why I am having problems. 

The test is an Short/Open Circuit Test as described in a handful GMW specs.  Basically it is to test that if any two signals are shorted together during the course of operation, the DUT will be ok. It also checks to see if the DUT will be ok with out one of it's signals. 

At first glance, the SCXI 1124 looks pretty good, I'll read more into it later tonight. 

Thanks a bunch!

[hoffiz]

Hi Drewsky,

Sorry for the late reply since the SCXI-1520 is not a card design for those tests I will be kind of hesitant to use for such a serious test like yours. I’m not an expert on how does this GMW test are suppose to be perform but I can always help on how to use the SCXI, something that might help me a lot is if you can draw the circuit of your connections.

I think your answer should be here in the SCXI-1520 manual page A-5, where it explains all the specification of the “Excitation”.  The effects when lines P are connected to a 12-24 Volts line; if I’m not mistaken, would be the same as connecting two batteries over the positive poles current will try to flow from the 12-24 Volts to the P+.  We do specify what the maximum current is provided but no how much can flow into the card. A very good question; how would you perform this test without the SCXI?

[amcolosk]

Hi Jaime,

Currently the test is done using some rather bulky equipment that is then manually plugged together to test each signal.  I am not sure of any of the details of that equipment.  Here's the description of the test that my boss gave me:

The purpose of the test is that in the vehicle, a harness could chaff and the wires could touch, also in the same connector a short across the terminals could cause a fault.  We want to make sure that the circuit is protected so that if a short does occur in the vehicle to power, gnd or another signal it does not cause a thermal event, unwanted maneuver, or damage a device.

I am seriously doubting the1520's ability to handle the tests I can give it, but the SCXI 1124 has this quote too:

The output voltage is referenced to the isolated ground reference and is short-circuit protected. Therefore, you will not damage the module if you short the voltage output of a channel to its own ground. However, you must not short the voltage output of a channel to earth ground. If the isolated ground of the channel is at a large common-mode voltage with respect to earth, shorting the voltage output to earth ground damages the module.

So it appears that any tests using the SCXI will limit me to only controlling the signals, and I will have to use an independent outside source that can take the beating it will be put through.

Attached is a schematic of what I am trying to do.

Thanks!

[hoffiz]

Hi

In the case that the DUT ground is not the same as the SCXI chassis, then like the manual states it might damage the cards in the case of a large common mode voltage with respect to ground. I really appreciate the picture it really helped a lot clarifying my doubts. For and automated solution you should be taking a look at this two products: NI PXI-4130 Power SMU and Switch Modules although for specifics on this products you will have to post in the correct discussion forum.

Let me know if it helps

[amcolosk]

Hey Jaime,

Thanks for all of your help!  I have finally fixed the problems I was having.  I am still using the SCXI 1520 to simulate my signals, but I added a resistor to the end of the lines coming from it.  As long as the reisitor is large enough, not enough current will be drawn to damage the SCXI 1520. 

So thanks again!

Andrew