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Transformer Voltage RMS, Peak, or Peak-to-Peak?

R RailGrandsonOGR Forum Member

Hi,

 

I am considering buying a USB device called the myDAQ from National Instruments.  It looks like an inexpensive way to get an oscilloscope, DMM etc. and can be used for circuit analysis.  I don't have alot of experience in electronics and I thought this would be worthwhile.

 

The thing is, the AC input is maximum 20 volts and is further specified as +/- 10 volts.  I asked at the NI forum and got this response about using it with our 18 VAC transformers:

 

>>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.

 

I kind of understand this, but I don't know for sure how our transformers are rates.  From looking around, it seems like they are 18V Peak-to-Peak so I should be OK and won't fry the myDAQ.  I am sure someone here knows for sure.

 

If you are interested, the myDAQ costs $189 ($199 with LabView), connects to the computer and with LabView provides a whole bunch of analysis instruments.  It also can be programmed via LabView to provide PWM etc.  It looks like a good value.

 

http://www.ni.com/mydaq/

 

http://www.studica.com/mydaq#

 

Phil

 

 

 

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LN Lad NagurneyOGR Forum Member DIGITAL SUBSCRIBER

My students use NI's MY-DAQ and the simpler version the USB-6009 daily (at least during the academic year). They are actually pretty well protected, so I doubt if you'd burn one out from a transformer (although connecting to the AC line would be a different matter).

 

A 10:1 voltage divider could be easily constructed from standard value resistors, and even if the resistor divider is not exactly 10:1, Lavbiew or Signal Express could compensate for it.

 

I've use the digital outputs to power switches with no problem. The analog outputs do not have the drive power to directly power a motors, except for the smallest ones.

 

If you have any questions, please let me know.

 

Lad Nagurney - nagurney -at- hartford.edu

R RailGrandsonOGR Forum Member

Thanks for all the helpful replies.

 

I heard about the voltage divider some place else, but I thought there was an issue with "impedance matching" and that pretty much scared me off.

 

I am interested in the myDAQ mostly from a tinkering standpoint.  But also because it seems to provide the function of more expensive equipment at a reasonable price (but it need a computer).  There are also good tutorials available.  I interested in learning more about LabView, so that coupled with the myDaq for use as a variety of meters and the ability of LabView to control the Arduino makes that collection look like a pretty useful group of tools.  I am definitely open to advice on this.  

 

Joe, you might have a point, I guess it could be the type of thing I use once and say, "that was neat."  I also thought some members here might be interested to know that this exists.

 

The reason for the oscilloscope is that I have wanted to try making a controller to provide AC Pulse Width Modulation over the rails to control conventional locomotives.  I just think that being able to measure the pulses and see the waveform would be helpful.  

 

Lad, what do you think of the myDAQ as a tool?  

 

Phil

rrmanrrmanOGR Forum Member
Originally Posted by RailGrandson:

I heard about the voltage divider some place else, but I thought there was an issue with "impedance matching" and that pretty much scared me off.

 

Sams reply:

Impedance matching really comes into play at higher frequencies, say 500KHz or above. At 50 HZ there is no need for impedance matching so just about any 10:1 ratio  will work and keeping the resistor  values in the kilohm ranges won't cause problems with overheating resistors at the train transformer voltages.
rrmanrrmanOGR Forum Member
Originally Posted by rrman:
Originally Posted by RailGrandson:

I heard about the voltage divider some place else, but I thought there was an issue with "impedance matching" and that pretty much scared me off.

 

Sams reply:

Impedance matching really comes into play at higher frequencies, say 500KHz or above. At 60 HZ there is no need for impedance matching so just about any 10:1 ratio  will work and keeping the resistor  values in the kilohm ranges won't cause problems with overheating resistors at the train transformer voltages.

 

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