Question about center tapped primary transformer

Thread Starter

Roflberry Pwncakes®

Joined May 18, 2021
7
So I just received a transformer I ordered on Amazon to rectify for DC rails to drive a class B push-pull amplifier and I thought it was just a single primary winding when I ordered it, but it turns out that it has 2 primary windings that are center tapped. The box says 115V/230V, and the primary wires are colored black/yellow/red, but according to a customer review on Amazon the red is the center tap, not the yellow. This isn't the first rectifier I've built, but it's the first time I've dealt with multiple primaries. So my question is... for 115V do I connect the hot & neutral mains to one primary winding and the center tap, or do I connect across both windings and leave the center tap open? And can the primaries be wired in parallel to get more power out of it by connecting both outside primary wires to hot and the center tap to neutral and will I see more voltage in the secondary or will it just pull more current and dissipate more wattage? I suspect that by using both primary windings in that fashion, the voltage will double in the primary, but the turns ratio will go from 5:1 to 10:1 and so the secondary voltage will remain at 24. Am I wrong? Like I say, I've never encountered a transformer with multiple primaries, so I've got no idea about how different configurations of connections to the mains will behave. I've checked out the datasheet and it's full of clerical mistakes, so I'm not trusting anything the datasheet contains, but I'm attaching it and a link to the manufacturer's page for the product anyway.

https://www.jameco.com/z/102111-R-J...r-24-VCT-1A-115-230VAC-Wire-Leads_102112.html
 

Attachments

MrChips

Joined Oct 2, 2009
24,458
You can identify the primary windings vs the center tap by measuring the DC resistance with an ohmmeter. The resistance between the two primary leads would be twice that between one primary and the center tap.

For 120V operation, use one primary and the center tap.
If you use both primary leads you will get half the output voltage on the secondaries.
 

Ian0

Joined Aug 7, 2020
3,541
I’ve seen these before. Most European/American dual-voltage transformers have two separate primaries that can be connected in series or parallel depending on the voltage.
The Chinese tend to prefer an auto-wound arrangement, tapped 0-115-230V, so they wind one half with thick wire (rated for the total,power at 115V) and the other with thin (Rated for the total power at 230V)
Connect any two windings to 115V and see what voltage is on the third wire. If you get 57.5V then that wire is the centre tap. If you get 230V you have connected the centre tap and one end.
If you are using it on 115V, then make sure you use the thick primary (with lower resistance) and make sure you properly insulate the other wire as it will have 230V on it.
Two separate primaries would seem like a safer arrangement, as no one working on a 115V product would expect to find a wire with 230V on it.
 

Thread Starter

Roflberry Pwncakes®

Joined May 18, 2021
7
You can identify the primary windings vs the center tap by measuring the DC resistance with an ohmmeter. The resistance between the two primary leads would be twice that between one primary and the center tap.
Yeah, I caught that right after I posted this. I hooked a meter onto it and got 45Ω between the yellow wire and both it's neighbors and 90Ω between the black and red wires, confirming that the yellow wire was the center tap as per the datasheet. The person who left the review on Amazon saying that red was really the center tap must have gotten one that was manufactured incorrectly that somehow made it past QC and got out the door.
 
Top