# Multitap primary transformer Wiring

Discussion in 'The Projects Forum' started by ayushextreme, Apr 18, 2014.

1. ### ayushextreme Thread Starter New Member

Apr 10, 2014
19
0
I got a beefy transformer from an old UPS. There are 4 wires on the primary side. Blue Green Yellow and Black. I connected Black to ground and connected the other wires to the mains. Here is the secondary voltage when each wire was connected.

Blue to mains-> 8.5v on secondary
Green->6v
Yellow->7v

I am thinking it's center tapped for working with 110v AC? I want to get above 12v so that I can make a powersupply. I tried connecting other wires like Blue and Green to mains and it was a shot circuit. Also I tried Connecting Blue and Green together and other ones to the mains. It heated up the transformer. And it's taking a long time to cool down! I am attaching a pic.

2. ### ericgibbs Senior Member

Jan 29, 2010
2,422
370
hi,
Use a multimeter to measure the resistances of the 4 wires Yw, Gn, Bl,Bk relative to each other and to the transformer frame, post your results,

Obviously, have it unconnected to any mains source!

3. ### ayushextreme Thread Starter New Member

Apr 10, 2014
19
0
Resistance values

Wrt black,
blue->16ohm
yellow->17ohm
green->19ohm
green blue 4 ohms
green yellow 2 ohms
yellow blue 2 ohms
Wrt frame infinite resistance I think. Or Out of range of my shitty multimeter
. I dont think the 2ohm values are correct? I dont have a constant current so can't do 4 wire measurement.

4. ### ericgibbs Senior Member

Jan 29, 2010
2,422
370
hi,
From those values, based on a 230Vac nominal primary, my best guess would be.

Black = Neutral @ 0R
Blue = 220v @16R
Yellow=230v @ 17R
Green-= 240v @19R

If 120V nominal, tapped at 110v, 120v,130v

E

5. ### ayushextreme Thread Starter New Member

Apr 10, 2014
19
0
Okay, So is there a way to get 12v or above from the transformer? There was a 12v 7.5 AH battery in the UPS. How was it charged? I see a microcontroller in the UPS board, 4 relays, 4 mosfets, a UC3843 current mode pwm controler. No inductor or toroid.

6. ### ericgibbs Senior Member

Jan 29, 2010
2,422
370
If the original UPS was powered from say 110V mains, then the outputs you measured would be double,, suitable for a 12Vdc battery system.??

Blue to mains-> 8.5v on secondary
Green->6v
Yellow->7v

7. ### ayushextreme Thread Starter New Member

Apr 10, 2014
19
0
I hardly think It's 110v. It's made in India, we use 220V here

8. ### ericgibbs Senior Member

Jan 29, 2010
2,422
370
Yes, India is the same as UK..nominal 230Vac

9. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
Why do you think the 4 wires are the primary side?

The two wires on the other side are thicker and look to have thicker insulation (common to have thicker insulation on primary wires).

What ohms does that winding on the other side measure?

10. ### ayushextreme Thread Starter New Member

Apr 10, 2014
19
0
I thought the huge wires were to carry high current on secondary. It's from a ups so it should work in both directions right? It has resistance of 1.2 ohms approx

11. ### ericgibbs Senior Member

Jan 29, 2010
2,422
370
Hi Roman,
The secondary winding of a transformer is normally the outside/last winding, this can be seen in the OP's photo.

Also the 'primary' winding resistances indicate that it is a mains selection tapped primary winding.

E

12. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
Yeah i know I used to work in a transformer winding shop.

I could not see from the photo which wires are to the outside winding, both sides seem to have wires at the periphery. Sometimes they double the wires back under the tape so they all come out at the outer edge.

The other reason I thought the primary may have been the red/blue side, was because the ohms figures given in post #3 are way too low.

A 240v transformer primary (on a transformer of that size) is usually in the region of 60 to 100 ohms.

To the OP; what is the highest ohms reading you can get across any TWO of those four wires? The highest total ohms reading will be the "240v" setting, assuming this is a multitap mains transformer.