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Toroidal transformer Primary/Secondary turns combination?

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uengin

New Member
Hi,
I would usually be able to work this out but due to some pre-occupations I have to depend on your help.

A toroidal transformer has:
- 2 primary coils rated at 115V each
- 2 secondary coils rated 25V each

What I am trying to do is find a toroid transformer with the right combination such that I can achieve 230V to 115V step down. Now for this to happen, very simply put, the primary coil must be half the number of turns as the secondary right. Meaning, if I connect the secondary coils in the above example in series and use only one primary, or both primaries in parallel, but instead of feeding 115V in to it I feed 230V, I should (given the numbers above) get 100V out right?

The ratio = 115/(25+25) = 2.3
Therefore Vout = 230/2.3 = 100V

Or will I burn out the primary coils because the current will be double what it was designed for?

Thanks in advance everyone.
 

bychon

New Member
If you double the intended voltage on the primary, it will smoke. However, you can connect the 2 (115 volt) primaries in series, connect the 230 volts to them, and find 115 volts available at the center tap of the primaries. (Do not use the secondaries at all.) You will not have isolation of the grounds in this case.
 

JimB

Super Moderator
Most Helpful Member
Or will I burn out the primary coils because the current will be double what it was designed for?

The current will be much more than double, this is not a simple Ohms Law situation.
The transformer core will go into magnetic saturation and the reactance will be drasitcally reduced.
There will be a big flash and a bang, if you are lucky you can just replace the fuse, more likely you will be replacing the transformer.

As Bychon said, just use the primaries as an auto transformer, connect them in series* and take 115v from one end and the centre tap.

* Beware connecting the windings in series, the phase relationship must be correct or you get another flash and a bang! and a dead transformer.

JimB
 

uengin

New Member
Thanks guys, that makes a whole lot of sense. See, I didnt even think about the magnetic field aspects, magnetising current etc. It's been a while since I looked in to power electronics, so I am definitely rusty not mention the state of mind, thanks heaps.

Just to be clear and make sure of my understanding, please confirm that my artistic :))) representative diagram reflects what you have said:
**broken link removed**

Thanks again guys.
 
Last edited:

bychon

New Member
Ummm...yep. You even got the phase right, but since you can't measure phase easily, put a fuse in series with the power b4 you hook it up. 1 amp should do.
 

uengin

New Member
Ummm...yep. You even got the phase right, but since you can't measure phase easily, put a fuse in series with the power b4 you hook it up. 1 amp should do.

Thanks for the quick reply Bychon.

Just another question. I will be using this transformer for a 31W device. So, does this mean that the transformer I imploy for this application must be 31VA or greater?

Thanks in advance.
 

bychon

New Member
I actually don't know, and I don't know how to calculate it.

Next helper, please step up.
 

Ubergeek63

Well-Known Member
I actually don't know, and I don't know how to calculate it.

Next helper, please step up.

OK the transformer is rated at 31VA RMS, but if you go through a rectifier to a capacitor for a DC output the RMS is higher than the DC load by about 20% so you would need more like 40VA.

While an ideal transformer would be fine if you connected the 115V winding to the 220V line, in real life the core would saturate followed by the prompt smoking of the 115V winding.

Transformers are actually rated at full load assuming 20% "regulation", give or take, which means when you remove the load the transformer output voltage goes up 20%.

BTW the easy way to determine if you are connected properly if you do not have the transformer spec is to connect only one winding to the supply and see if you get double the voltage when you connect them in series.

Dan
 

mneary

New Member
As you have drawn two 115V windings in series, it should be good for 230V. It is common for transformers to be designed with this in mind.

Power rating: I ran some simulations in LTSpice to confirm my recollection, that a properly connected autotransformer does not need to be as large as one with separate primary and secondary windings. This is because the I2R losses in the secondary are eliminated and the currents in the primary are reduced, for a given load. As Ubergeek63 mentioned, a "31W" load may not be 31VA.
 

bychon

New Member
Let me clarify. Are you asking, "What wattage should the transformers be rated at in order to deliver 31 watts of power at the center-tap?"

It depends on how close you want the voltage to be to 115 volts.

OK, What wattage should the transformers be in order to deliver 31 watts at the center tap and only have the voltage sag by 5%?

Is that the question?
 

uengin

New Member
Let me clarify. Are you asking, "What wattage should the transformers be rated at in order to deliver 31 watts of power at the center-tap?"

It depends on how close you want the voltage to be to 115 volts.

OK, What wattage should the transformers be in order to deliver 31 watts at the center tap and only have the voltage sag by 5%?

Is that the question?

Well yes, sorry my apology, I should have been more clearer. I have not yet bought or selected the transformer yet. So the question is exactly that, what power should the transformer I choose and buy be rated at so that the it will be able to deliver 31W, with say a variation of 5% on the output voltage?

Please state a short explanation of the reasoning for this as I would like to learn at the same time. Thanks.
 

bychon

New Member
I still don't know the answer, but I helped clarify the question. I hope someone else can respond with a good method to figure this out.
 

crutschow

Well-Known Member
Most Helpful Member
Both windings will carry 1/2 the load current on the tap. If you think about it a bit that will become clear. Remember the power in equals power out (assuming 100% transformer efficiency). Also since the windings are identical, they both must carry the same current, but in opposite directions. Thus half the load current is from the top winding and half the load current is from the bottom winding.
 

mneary

New Member
To be sure, a 31W transformer is big enough.

It might be twice as big as necessary, but it is nice to have a safety factor.... especially since we haven't mentioned 50 vs 60 Hz.
If this will be mass produced, then you need to consult the manufacturer, who will probably be willing to remove the secondary windings to save even more money.
 

bychon

New Member
Hang on. In post 11, you say you haven't purchased a transformer. Why are you trying to use two "wrong" transformers instead of buying one "right" transformer?
 

uengin

New Member
Hi guys, Thanks for all your efforts.

Firstly, crutschow what you say makes perfect sense. However, in the industry, when a toroidal transformer is said to be 50VA, what does that actually mean; each winding is capable of supplying 50VA or, collectively, the primary can handle 50VA and collectively the secondary can handle 50VA?
Actually, the latter sounds more logical and probable than the former. Please advise if otherwise.

Bychon, i think theres been a misunderstanding. I am not trying to use 2 transformers. There just isn't a transformer with the 230V to 115V stepdown, in the 30-50W range. Because the power rating of these transformers are substantially higher, so are their costs. Hence the reason why, I am trying to do what I am, with a cost effective manner.
 

bychon

New Member
I just poked around at www.mouser for transformers. They have a pretty good sorting program, if that's helpful. Still, transformers are expensive. Hope ya find a good one (or two).
 

crutschow

Well-Known Member
Most Helpful Member
Firstly, crutschow what you say makes perfect sense. However, in the industry, when a toroidal transformer is said to be 50VA, what does that actually mean; each winding is capable of supplying 50VA or, collectively, the primary can handle 50VA and collectively the secondary can handle 50VA?
Actually, the latter sounds more logical and probable than the former. Please advise if otherwise.
It means the VA collectively transformed by the transformer is 50VA (50VA in and 50VA out total). The limit is determined by the transformer heat rise caused by the power dissipated in the winding resistances due to the input and output currents.

Since in the autotransformer mode, the output windings would not be used, you could theoretically increase the VA rating by √2 based upon the heat rise (since winding resistance power is proportional to the current squared).
 
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