Finding the primary VA when winding your own power transformer?

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

nano1981

Member
Joined
Jun 24, 2018
Messages
9
I have been researching how to build power transformers but I am stuck on finding the proper wire gauge for winding the primary. In order to find the correct wire size I have to know the current in the primary coil. I have two equations from different sources that say different things.

METHOD 1 The first is an article written back in 1947. 
Which says "the primary circuit VA is 1.4 times the total secondary VA"

https://drive.google.com/file/d/1TmXE1M61NKkp3F1en7v4RFJDyNcuRxQl/view?usp=drivesdk

METHOD 2 the other formula is from a transformer calculator which is
Vp x Ip = Vs x Is. Then divide primary VA / Vp (115)

https://drive.google.com/file/d/1fgc_GNVBv5uNKF0Bls47dZQ4nm4ZqUdM/view?usp=drivesdk

My example problem is this

  Vp (primary voltage) = 115
  Ip (primary current) = ???
  Vs (secondary voltage) = 19.2 volts
  Is (secondary current) = 1.5 amps

When using Method 1 we get Ip = 0.35 amps. This seems to makes sense because the primary should be more current than the secondary to make up for any losses.

When using Method 2 we get Ip = 0.25 amps. This has the primary and secondary VA being equal. Which contradicts the first method which says primary VA is 1.4 times the secondary VA.

So which one is correct?
 
Also note that AC VA is much more than DC Watts.

However neither 0% or 40% for losses can't be right. Split the difference and wind for 20% loss on AC VA. This will seem small after you account for AC-DC conversion.
 
Back
Top