# Need help with transformer

I have one transformer. Primary 230V and two secondaries 14V/3A, 15V/3A and ground between them. I am curious can I use that transformer as center tap transformer.

Here is the picture:

I know I can use it as 29V/3A without problem and 15V, 14V alone.

I would appreciate any help, thanks. :)

• Obviously, the white wire is a tap on the secondary, but it isn't centered. (It's hard to imagine why anyone would build a transformer like this.) What are you trying to accomplish? Nov 18, 2018 at 17:40
• I want to test some amplifier boards that need center tap transformer. I will not use it like that for long period of time, only for testing. Nov 18, 2018 at 17:41
• If it is being used to develop a bipolar power supply (e.g., using a bridge rectifier), then the 15V winding will be doing the lion's share of the work, and ripple will be increased (half wave rather than full wave rectification). If you don't try to max out the current, you'll probably be fine. Otherwise, just go get a proper center-tapped transformer. Nov 18, 2018 at 17:44
• I would maybe use around 100-200mA. And yes I plan to build bipolar power supply for amp. with bridge rectifier. Nov 18, 2018 at 17:45

simulate this circuit – Schematic created using CircuitLab

Figure 1. The transformer is tapped but not in the centre.

You can generate a split rail power supply as shown. SW1 represents your choice of connecting to the 14 V or 15 V tap.

• I just want to replace my 12-0-12 transformer, I will not create new circuit. link In link is my rectifier board. Nov 18, 2018 at 18:04
• The transformer would not be rated as shown if the white wire was not connected between the other two. Nov 18, 2018 at 18:08
• @Dave: You've lost me there. Can you clarify? GN = 0V, WH = 14V, BK = 15V. Nov 18, 2018 at 18:11
• First, look at the physical construction -- it's B - W - G, and this usually means that W is the tap. Second, if you can draw 3A from G-W and 3A from W-B at the same time, they must be separate windings. Nov 18, 2018 at 21:50

you cannot use that transformer as a center tapped transformer - these are called tapped transformers.

if you want to confirm do one thing.( actually you have to go with measuring impedance - usually most hobbyists dont have that... so i am giving you a simple way of measurement with multi meter)

1. measure resistance between 0 and 14V
2. measure resistance between 0 and 15V
3. measure resistance between 14V and 15V

if [1 & 3 ≈ half(3)]- it is center tapped

if not - tapped transformer.

• Would it be big problem to put it in place of my 12-0-12 for testing amplifier boards. It is just for short testing not long playing or anything? Nov 18, 2018 at 18:13
• Actually not. it depends on your rectifier design Nov 18, 2018 at 18:22
• This is the board I use, I dont know how to put pictures in comments. [Link] (ebay.com/itm/…) Nov 18, 2018 at 18:26
• you cannot do with that board - its an regular full bridge converter Nov 18, 2018 at 18:30

Here's one idea for dealing with the asymmetry: Connect the center tap of the transformer to the circuit ground through a pair of back-to-back diodes.

simulate this circuit – Schematic created using CircuitLab

The idea is that if the ±20V rails are equally loaded, the two diodes in the center (D3 and D4) will not conduct, and both windings will be working at full efficiency. Only if the load is highly asymmetric will one diode or the other conduct, and then the 15V winding will be supplying the bulk of the power.

• I just want to replace my 12-0-12 transformer, I will not create new circuit. link In link is my rectifier board. Nov 18, 2018 at 18:07
• As I said, you just need two additional diodes. If you're not willing to do that, then either ignore the problem or just get a proper transformer. Nov 18, 2018 at 18:10
• is 1n5408 ok for this situation? Nov 18, 2018 at 18:15
• Yes, 1N5408 would be fine. Nov 18, 2018 at 21:55