Center tapped transformer rating and connection

[AxleD]

Hi,

I have recently got my hands on a center tap 15-0-15 VAC, 3A transformer and i was hoping to use it to build a dual polarity power supply. I have a few questions and hopefully someone can assist.

1. I have searched and it is not clear if the center tap on the secondary side of the transformer should be left floating? Some articles suggest it should be connected to mains earth/ground?
2. A few websites have suggested there are potential problems with unbalanced current flow in the secondary side of the transformer between say +15V and the center tap and the center tap and -15V in this case. How big of an issue is this potentially?
3. The transformer has no datasheet except for a sticker that reads input: 240v,50HZ AC, output 15-0-15v AC, 3 AMP Max. I presume this current rating refers to the secondary side of the transformer and the transformer should be able to deliver at least 2 amps of current?

Many thanks

[mariush]

1.  If you want to make a dual polarity power supply, then the center tap becomes your DC ground, and the two other ends become positive and negative .

2. I don't think there's much of an issue.

3. Your transformer says 3A.  Best case scenario, it means 30v AC x 3A = 90 VAC

When you convert AC to DC using a bridge rectifier, the voltage and maximum current can be approximated with formulas : 

Vdc peak = Vac x 1.414  = 30v x 1.414 = 42v DC peak  (minus some voltage lost in the 2 diodes that are always conducting in the bridge rectifier, which would amount to 1-2v)
I dc = ~ 0.62 x I ac = 1.86 A

So each side of the transformer should give you about 21 v peak voltage  and 0.93 A on each side.

Now keep in mind, that's a peak DC voltage, the voltage will be a sinusoidal wave going between 0v and that peak voltage about 100-120 times a second (depending on your mains frequency). To have a true DC output, you need to add a capacitor that would charge up with energy when the voltage coming from transformer is close to peak and deliver this energy to the circuit when the sine wave coming from transfomer is nearing 0v output.
To figure out how much capacitance you need, you can use this formula which approximates the amount:  C  =  Current / (2 x Frequency x Vripple)  where Vripple is how many volts you'd be willing to let the voltage sag from that peak voltage.

So as an example, for 1 A of current and assuming you're in Europe (for 50Hz mains) and you want to always have at least 17v on one tap (to allow for 1-2v needed by a linear regulator to output 15v stable), then your capacitance would be  C = 1A / [2 x 50 Hz x (21v peak - 17v minimum)] = 1 / (2x 50 x 4) = 1 / 400 = 0.0025 Farads or about 2500 uF.
So going with a standard 2700uF or 3300 uF 25v (edit: 35v or more would be safer) rated capacitor would make sure you have 17v-21v coming out of that side of the transformer at all times.

edited for more clarity and as suggested by poster below

[Richard Crowley]

1. Whether the center-tap is grounded (or floating or connected anywhere else) is completely dependent on the specific power supply circuit you decide to use.  There is no generic rule.  There are several basic circuit configurations.

2. Absent any context, sounds like typical internet mis-information.

3. Yes, the rating refers to the secondary/output winding(s) of the power transformer.  Yes. a transformer rated for 3A should work comfortably well for a 2A DC supply circuit.

[PSR B1257]

C = 1A / 2 x 50 Hz x (21v peak - 17v minimum) = 1A / 2x 50 x 4 = 1 / 400 = 0.0025 Farads

Couple brackets and units missing here 

So going with a standard 2700uF or 3300 uF 25v rated capacitor

Well, 25V rated voltage is a little tight. Don't forget that the transformer has a higher open-circuit voltage compared to it's nominal voltage. Also the mains voltage can vary between 0.9 and 1.1 V_nom (at least here in Germay  )
At least a 35V capacitor should be considered.

[dom0]

1. I have searched and it is not clear if the center tap on the secondary side of the transformer should be left floating? Some articles suggest it should be connected to mains earth/ground?

This mainly depends on the transformer and only after that comes if you like supplies with grounded outputs. If you let it float the transformer needs to be rated for use in class II appliances (double / reinforced insulation). You'll find that information sometimes on the nameplate of the transformer or you can see that it's a doubly insulated transformer (separate coil chambers, insulation to core etc.). If unsure, ground it.

2. A few websites have suggested there are potential problems with unbalanced current flow in the secondary side of the transformer between say +15V and the center tap and the center tap and -15V in this case. How big of an issue is this potentially?

Irrelevant.

3. The transformer has no datasheet except for a sticker that reads input: 240v,50HZ AC, output 15-0-15v AC, 3 AMP Max. I presume this current rating refers to the secondary side of the transformer and the transformer should be able to deliver at least 2 amps of current?

Depends on what kind of current you mean. If you mean DC after rectification you need to apply a "correction factor", so for a bridge rectifier you need to divide the current by 1.5 to 2, i.e. you can expect it to deliver about 1.5-2 A DC continuously. You can pull out more current, but that leads to higher losses in the transformer and that might dissipate to much heat in the transformer. Personally I'd see 2 A already on the high side. 1.5 A would be a more conservative approach.


Re. cap rating: Unominal * 1.1 * 1.41 * idle factor = required cap W.V. rating
1.1 => 10 % over voltage
idle factor => up to 1.6 for very small transformers, down to 1.0x for large ones. For a medium sized transformer like yours probably 1.2, maybe 1.3.
I'd use 35 V caps.


Re. filter cap size: 1000 µF / A usually works out nice for a regulated supply, gives ~15-20 % ripple (depends on various things), which is a good trade off between transformer losses and usable current output.


Various other stuff:
- Always use dual-pole switches for mains.
- A MOV (varistor) in parallel with the transformer primary is always a good idea.
- Bridge rectifiers should be over-dimensioned generously.
- Having a small RC snubber across the secondaries (say, 100..220 ? + 100 nF) can help with some strange effects (ringing when the rectifiers switch).
- Some people also prefer to put small caps across the rectifier diodes (again about 100 nF, or smaller) to avoid HF ringing and similar issues.
- Secondary side MOVs or Zeners are sometimes seen, but don't make much sense IMHO.
- Polyfuses on the secondary OTOH are handy.
- Oh, don't forget to fuse mains, preferably with a nice power entry module (IEC socket + fuse holder + optionally a switch and/or EMI filter).
- Isolate all mains-carrying parts nicely (shrink tube).
- Don't use bell wire for mains ; use proper wire rated for 300+ V working voltage.
- Ground all metal parts nicely, and use green/yellow striped wire for that.
- Don't ground case parts and other stuff just be screwing them together ; always use a (preferable crimped) PE wire running from a central "PE distribution screw" to all metal case parts.

[AxleD]

Thank you all.

The supply will be used as a student bench supply - independently power breadboard circuits, op amps, low power amplifier's etc so I will leave the center tap "floating". I presume it is only necessary to "ground" the the center tap if you were say connecting the power supply to an external piece of equipment with potentially a "different ground"?

Thanks, I have several 35v rated caps on hand so will use those.

Finally just to clarify then (assuming the DC is taken from the output caps after a full bridge rectifier),

1. If I am only using the positive output from the supply (the negative rail is not in use) then is it possible to draw more than the estimated 0.93A suggested?
2. I presume i can actually connect V+ and V- to get a full 30V if needed in this configuration?

Thanks

[dom0]

The supply will be used as a student bench supply - independently power breadboard circuits, op amps, low power amplifier's etc so I will leave the center tap "floating". I presume it is only necessary to "ground" the the center tap if you were say connecting the power supply to an external piece of equipment with potentially a "different ground"?

As I wrote above this is not just a consideration of what is handy, but also a safety consideration: If the insulation of the transformer fails and the secondary is floating the secondaries carry lethal potential. That's why in this case a transformer with double / reinforced insulation is required by code.
If the insulation fails, but the secondary is grounded a large current flows an will trip the fuse. So in that case no safety risk exists.

1. If I am only using the positive output from the supply (the negative rail is not in use) then is it possible to draw more than the estimated 0.93A suggested?

Not without changing the circuit behind it. You can draw more current if you use a rectifier with two diodes and the center tap as ground (M2 rectifier).

2. I presume i can actually connect V+ and V- to get a full 30V if needed in this configuration?

Yeah, but only if either the GND of the supply floats or the circuit floats (or both).

[AxleD]

dom0,

Thanks for the response, saw your posts only after i replied.

To clarify, when you talk of grounding are you are referring to connecting the center tap to mains earth (yellow/green wire) or neutral (blue wire here in South Africa) which is the return path of the current in a system? I know they ultimately are connected at the panel but i wonder if there is a recommended approach.

Thanks for the additional information, very useful.

[dom0]

Grounding means connecting something to protective earth (PE). Connecting it to neutral will pose a serious risk of electric shock in electrical systems with unpolarized connectors (half of the world), since you can put the plug both ways, so you can't know which wire is "hot" and which one is "cold".

[Richard Crowley]

PLEASE NOTE!  AxleD  appears to be asking about connection to the SECONDARY (output) center tap.
That has NOTHING TO DO with safety ground!!!!
It has everything to do with what the rest of the power supply circuit needs.
There are some circuits that want the center tap to be grounded, and others which will not tolerate the CT being grounded!

Most certainly the transformer (as well as the rest of the bench supply circuit) should be fully enclosed in a sturdy metal box.
And the box should most certainly be connected to the mains safety ground (green/yellow wire).

But NONE of that has anything to do with where (or IF) the center tap of the secondary is connected!!!
Without seeing the rest of the circuit, the connection of the secondary center-tap simply cannot be answered.

Furthermore, it is customary to "float" the common/reference output of the bench supply because you don't necessarily want it to be connected to earth/mains ground.  Typically bench supplies offer an additional (usually green) terminal which connect to earth/mains ground so that the user has the option of whether to connect circuit common/reference to ground or not.  And, for that matter, whether it is negative or positive that is grounded.

[AxleD]

Thanks for the information.

I have added a picture of the circuit I will most likely use for the supply. I believe the circuit will work fine with a grounded or floating center tap.

Not specifically related to the previous questions, but I am now quite curious and I wanted to confirm my understanding of this circuit/configuration:

1. I understand the safety benefit of having the center tap connected to ground if something went wrong with the transformer or if the hot wire came into contact with the chassis for example. The resultant short will lead to a current surge which will melt the fuse and effectively disconnect the system from the mains - and in the process protect both system and user. On the other hand, are we not simply defeating the purpose of an isolation transformer if we introduce an alternative path to mains ground? If I was for example changing a fuse and touched the hot wire by accident wont I now be the path to ground (completing the circuit and putting myself at risk)?

2. I understand that it doesn't matter if the center tap, positive or negative are grounded - current will not flow to earth ground unless there is a completed path. Why would you practically ground the positive or negative terminal though?

3. In the circuit attached, even if the center tap is grounded, why couldn't I connect the positive and negative terminal to get 30V across the load (why does it need to be floating?)

Great thanks, using a separate socket for mains earth that could be bridged does provide some additional flexibility.

[PSR B1257]

even if the center tap is grounded, why couldn't I connect the positive and negative terminal to get 30V across the load

You can, and you will get 30V.

(why does it need to be floating?)

It is still (kinda) floating, hence it is a single supply. It becomes relevant, if you add a second gear of some description (a function generator for example, which BNC jack is always connectet to protective earth) to your test setting.

Long story short, leave the center tap floating and it will be fine in >99% of any hobby applications.

[AxleD]

Thank you all. I will go with the floating option and the external ground socket just in case.
Cheers