# CENTER TAPPED TRANSFORMERS/SMPS

Hello, i just started learning how a center tapped transformers/smps works and i did some test but now i am confused about the dual power distribution, i mean if a device input power rating is a centered tapped, say +12_0_-12 VDC, when we measure the +12 and -12 the voltage is 24VDC but we can't use the 24VDC on a single 12v:0v device it short circuits the transformer/smps. When we measure the +12 and the GND the voltage is 12VDC and we can even use the +12v:GND with a single 12VDC device, so my question is, does the center tapped device actually uses the 24VDC or the device splits the 24VDC into 12VDC/12VDC.

Thank you.

Sorry for my beautiful English, there is nothing to admire nu(:

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7 months ago

Switch mode power supplies have a common ground but the +12 volts and the - 12 volts should be looked at as separate power supplies unless they are connected to a circuit with a +12 volts, common ground, and -12 volts like an Op-amp.

A center taped transformer in a SMPS can be putting 5 volts out one line and 12 out the second line and common out the third, so you just can't assume it is putting out +12 v_0 v_- 12 v.

I know this is not what you described but it does impress how you must think of the outputs as separate circuits.

Lima79 (author)  Josehf Murchison7 months ago

I thought if it is a center tapped it uses the entire 24 etc volts. Thanks much it helped solved one of my biggest problem.

Lima79 (author) 6 months ago

Thank you all very much appreciated for all your help and support BTW
i am very busy this days got to work nu(: anyway thanks again.

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MERRY CHRISTMAS
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-max-6 months ago

You need to realize that voltage is a differential measurement, between 2 points, always. The concept of a "ground" or the "common" is entirely artificial, a conceptual human construct, a point in a circuit for which we define the voltage to be 0 volts, and also the point whereby all voltages are referenced to. (that's why the black multimeter should typically be connected to ground, and why it's called the common.)

The -12V is measured relative to GND, and the positive 12V rail is +12v relative to GND. You can also take the differential measurement of the +12 and -12V rail to get a delta voltage of 24V, just as much as you can connect a load between the rails to get 24v.

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By simply defining the -12V rail as ground, then you instantly have a 0 -- 12v --- 24v transformer or smps. Just be careful that the point you define to be ground is universal. You can't have one circuit "talking" to another circuit if they have different ground potentials, unless they are galvanically isolated.

6 months ago

Oh, and I should mention that last trick probably has a few "traps for young players" when using real-world regulated power supplies. The outputs are only designed and specified when delivering current and might not tolerate current going the other way.

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Say you have a power supply with a ground, 6v, and 12v output, you should not allow current to sink into the 6V rail, as it can can only source current. So connecting things between the 12v and 6v rail is a big no-no. It in many cases will cause the 6v supply to "fall out of regulation" and the voltage may become erratic.

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But when using batteries, that is fine, and is in fact a very common technique to get cheap and easy dual-rail functionality. The only problem is that the negative rail battery and positive rail battery might discharge at different rates and have different voltage magnitudes.

Downunder35m7 months ago

You are getting something badly wrong here: A transformer gives you AC but not DC, no matter if center tapped or not.

Lima79 (author)  Downunder35m7 months ago

Thank you.

7 months ago

If you look up center tapped transformers on Wiki or through Google you will also find transformers with multiple outputs in a lot of variations.
As it is all AC and to make the thinking easier see the different taps as "batteries".
Each battery has a different voltage, or a similar one but they are all connected in series.
So from one end of the winding to the other you get the max voltage, the taps between provide less - in either direction so to speak.

Back in the day and even today ceter tapped transformers are prefered for certain application as the part count to get a good and stable DC voltage is less compared to normal transformers.
Now they are mostly limited to audio applications or special electronics, like op-amps as stated above.

Lima79 (author)  Downunder35m7 months ago

Thanks for the info. So the center tapped transformer/SMPS is better than a single rail transformer/SMPS.

7 months ago

No, its just different. Lots of things need bipolar supplies.

Lima79 (author)  steveastrouk7 months ago

Thank you.