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Electrical Engineering/volt amps vs amp output ratings

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Question
I am looking for a transformer that has an variable output voltage of 0-24 Volts and a max output amp rating of 3 - 5 amps. Of course, I don't know if I would be able to get exact but I am looking in these ranges.

Many hobby transformers (like for trains) state the DC & AC voltage output and is variable since the lever controls the speed of the train. Theses transformers seem to have a VA rating also, or volt-amp.

Other transformers, for example like the one I plug into the wall for my outlet for my computer modem has an 5.5 DC V output and 2.2 Amp output rating and does not use the VA rating.

Battery chargers are transformer which are rated for Amp output.

I am interested in a hobby train transformer for my project, because the variable voltage regulator and the fact they are found easy on eBay at low price.  However, the VA output is confusing me and I don't want to buy something that cant achieve the 3-5 amp current.

I know the amp or current is dependent on the resistance, which in my project will be water properties plus sea salt quality added for conductivity.

What is the difference between a VA (volt-amp) rating on a transformer vs one that states the A (amperage) output?

Would a toy train transformer be able to put out a 3-5 amp current like a battery charger (assuming both have 110 AC input) and the salt-water bath solution is providing the conductivity?

Why do the train transformer use VA output ratings instead of amp output ratings like other transformers on the market?

Thank you,
MR

Answer
For an AC or DC circuit having a resistive load, power in watts is equal to the voltage multiplied by the current.  In AC circuits that may have capacitance or inductance (i.e. not only resistive), it makes more sense to specify volt-amps, which is also the voltage multiplied by the current.  But in capacitive or inductive circuits, the peak voltage does not occur at the same time as the peak current, so VA is not the same as watts.

In your case, the load is inductive (a motor).  The lever on the transformer controls the voltage.  So for example, if the maximum voltage is 24V and maximum current rating is 5A, that is 120VA.  If you lower the voltage to 10V by moving the lever, the transformer can likely still provide the same 5A current capability so 10V x 5A = 50 VA but the motor will probably draw less current at this lower voltage).

DC power supplies will usually state the voltage (regulated or non-regulated) and also the maximum current that can be drawn.  In DC circuits, there is no power dissipated in capacitances or inductances, other than for a few milliseconds when first turned on, so no VA rating is given.  So for a DC supply, you can multiply the DC voltage by the current to get the watts of power that can be supplied.

Hope this helps!

Dave  

Electrical Engineering

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Dave Nyce

Expertise

I have been an electronics engineer for many years. I can answer questions on analog and digital circuits and my specialty is sensors.

Experience

I am the inventor on 27 US patents, and also some foreign ones. Developed sensors for many years. Licensed private pilot (airplane and rotorcraft), have HAM radio license. I'm not an expert in computer networking.

Organizations
AAAS, Certified Control Engineer, (former UL Advisor for Intrinsic Safety), Benefactor member of NRA. Life member of the following: Experimental Aircraft Assoc., US Parachute Assoc., National Trapper's Assoc., Apex Masonic Lodge #584, Scottish Rite of Raleigh, NC, Academy of Model Aeronautics, Grass Roots North Carolina, NC Rifle & Pistol Assoc. Member of the following: Aircraft Owners & Pilots Assoc., US Hang Gliding & Paragliding Assoc., Tripoli Rocketry Assoc., Apex Historical Society, The Planetary Society, USA Volleyball, Shriners of North America, York Rite Masons, National Space Society, Benevolent and Protective Order of Elks, and the Precision Aerobatics Model Pilots' Assoc.

Publications
Books: "Magnetic Displacement Sensors" section in: Measurement, Instrumentation, and Sensors Handbook, CRC Press, 1999. ISBN 0-8493-8347-1; "Magnetostrictive Sensors", "Hall Effect Position Transducers", & "Strain Gage Accelerometers" in: Instrumentation and Control, a Mechatronics Handbook, John Wiley & Sons, Inc. NYP; “Magnetic Level Gauges” chapter in: Instrument Engineers’ Handbook - Measurement & Analysis, 4’th Edition, CRC Press, 2003; Author of the book “Linear Position Sensors, Theory & Application”, John Wiley & Sons, 2004; “Electronic Transmitters”, “Linear & Angular Positioning of Machinery”, and “Inerting Systems” sections in: “Instrumentation Engineers’ Handbook – Process Control, 4’th edition, (2005); “Hazardous area classification and management”, and “HART Networks” sections in the book: “Instrumentation Engineers’ Handbook – Digital Process Networks and Software, 4’th edition, publication in 2011 Magazines: Magnetostriction-Based Linear Position Sensors, SENSORS magazine, April, 1994: Tank Gauging Advances, Fuel Technology & Management, January, 1997: Magnetostrictive Position Sensors, Measurements and Control, September, 1998; A Moment in Positioning, PTDesign, February, 1999; Magnetostrictive Position Sensors (update), Measurements & Control, September, 1999; Position Sensors for Hydraulic Cylinders, Hydraulics & Pneumatics, November, 2000; Magnetostrictive Linear Position Sensors, Fluid Power Journal, April, 1999; Sizing & Applying Magnetostrictive Linear Position Sensors, Motion Systems, Feb., 2002; Position Sensors in Medical Applications, ECN, May 15, 2002; Featured in “Level Sensors Go Floatless”, Machine Design, May 8, 2003; "Guitar Man" feature article, The Pelican Post, Oak Island Press, Oak Is., NC, Winter 2005; The LVDT: A Simple and Accurate Position Sensor, SENSORS magazine, August, 2005; "Model Airplane Day!", FLYING MODELS magazine, September, 2006; “Kids Having Fun!”, Half-A Flyer magazine, January, 2012

Education/Credentials
BSEE, MBA, Management by Objectives - Honeywell, Total Quality Management - MTS Systems Corporation, Looking Glass Management Workshop - Center for Creative Leadership, Motion Control Systems - Western Michigan University, College of Engineering, Organizational Excellence - University of Cleveland, Finance for Executives - Sloan School of Business, MIT.

Awards and Honors
Vaaler award, EDN magazine, "Inerting for Safety", 1987 Listed in Who's Who in Engineering, in the South, in the World. "Total Quality Management" medal awarded by MTS Systems Corporation 1991 "Best Sequel" award for the video production: "For Engineers Only" at the MTS national sales meeting, Las Vegas, 1998 (written and directed by David S. Nyce) Voted "Most Effective Leader" at Center for Creative Leadership: Looking Glass, Greensboro, NC 1995 Silver Award for New Technology at SENSORS EXPO, in Chicago, 2001 for SEF Liquid Level sensor MTS Circle of Innovators award, 2003 Elected Master of Masonic Lodge #584 , Apex, NC, 2005 "Gold Honour Award" for outstanding service in York Rite Masonry, by the York Rite Sovereign College of North America, August 22. 2007 Board of Directors: WaaRev Sensors, and the Apex Historical Society Maynard Pearson House Plaque hanging in the Masonic Fellowship Hall, for Outstanding Service and Dedication to Apex Masonic Lodge #584, Apex, NC

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