Is USB-6008 OK for 18 Volts AC?

[RipRip]

Hi,

I am new to electronics and would like to get the USB-6008 to use with experiments and eventually for school.  I am interested in using the multimeter and oscilloscope functions with a variable voltage source of 0 to 18 Volt AC.  

It seems like the myDAQ would work with this OK, but I want to be sure.  As I understand it, the 18 VAC source would fall within the specified operating range of +/- 10 volts because it should be +/- 8 volts AC.  Is that correct?  I don't want to get this and fry it!

Thanks

[crossrulz]

Is that 18V RMS, Peak, or Peak-to-Peak?  If RMS or Peak, then you will go outside of the +-10V.  If Peak-to-Peak and the 0V is truly ground, then you will be fine since the voltage range will be +-9V.

[Dennis_Knutson]

And the 6008 is different than the myDAQ. The 6008 does not have a multimeter or scope function unless you spend some time writing it yourself.

[RipRip]

I din't know that because they seem to be discussed interchangeably.  It looks like I should make sure to get the myDAQ.  

I asked the manufacturer about the voltage.  By the way, the transformers I will be experimenting with are the Lionel and MTH Trains power supplies.  

Phil

[SnowMule]

You can always build a small voltage divider or use a differential probe to knock the voltage down to an acceptable level. 

In MAX, launch test panels... the analog inputs and digital I/O is pretty easy to work with in there.  Much more than that and you'll need to do some programming.

[johnsold]

Phil,

Another thing to consider is that train transformers are not very good transformers. The result is that you may see voltages significantly higher than the nominal voltages at no load.  You may also see transient peaks much higher than the nominal transformer voltage when switching loads (or derailing a locomotive).  I would use a 24 V range or higher and consider some kind of transient protection for the DAQ inputs.

Lynn

[RipRip]

Lynn,

You are right about the trains tranformers, they can be pretty sloppy.  Also, the models seem to have variable performance.

Here is what I got back from the manufacturer

>> The transformer voltage is measure by RMS.  As for if 0 voltage is truly ground, we are shooting from the hip and saying no.

So will then a voltage divider work?  I read elsewhere on this forum that impedance might be a problem with the USB-6008 (which I thought was similar to the myDAQ) and that an "active buffer" was required.  I wasn't sure if it was OK to reply to a post that has been inactive for a year so asked here.  The original post (short) is here:

http://forums.ni.com/t5/Multifunction-DAQ/Using-the-NI-USB-6008-to-measure-high-voltages/m-p/1668128... 

I could do the voltage divider, but I am not sure what is meant by the active buffer or what the impedance considerations might be.

[SnowMule]

The low input impedance is a problem with small/weak signals... voltage off photodiodes, crystal oscillators, etc... anything that can't pump out sizeable currents.  For measuring a power supply or voltage source it's a non-issue. 

The buffer can be nothing more than an op-amp voltage follower.  Or you can throw some gain on that op-amp and use a higher range on the DAQ (a little more accuracy).

"Good" transformer or not, it's likely going to do what OP needs it to do just fine.  But OP will need some protection on the DAQ because of its limits. 

[johnsold]

Phil,

OK.  18 V rms corresponds to 25.5 V peak for a sine wave. With no load the voltage may increase 20% or more above the rated full load voltage (18). That would produce a peak voltage of 30.6 V.

It also sounds like they do not ground the secondary of the transformers.

If you are testing the transformers alone, you can just ground one side and everything will be OK.  If you are connecting the transformer to other equipment or instruments, then the whole system needs to be considered.

I might use a voltage divider consisting of a 2200 ohm resistor and a 250 ohm resistor.  The output will be 10% of the input  If you run the 6008 on the 10 V range you will be able to tolerate instantaneous voltages at the transformer output of almost 100 V (peak) without any other protection.  Your resolution would be about 0.1 V.  That is not great but probably adequate for an application using model train transformers.

Lynn

[RipRip]

Lynn,

I guess I goofed up the title of this post.  I am really talking about the myDAQ and not the 6008.  I stupidly thought that was the model number of the myDAQ.

It seems what you have said would also be true for the myDAQ.  Is that so?

If I were to use a voltage divider to limit output to say 25%, wouldn't that would provide 2.5 times better resolution?

If that is the case, I guess I could use the voltage follower to provide protection to the myDAQ simply by limiting the voltage input to the op amp?  If I were to do this, I would then need to connect the op amp to a + and - DC supply, right?

Phil

(I may actually be getting this now)