# Simple transformer\Voltage question

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#### westers

##### New Member
I would like some confirmation with regards to my thinking around a very simple transformer\voltage question, (well simple for a lot of you, but not so simple for me).

I have a toroidal transformer with a single primary and dual secondaries. It's rated as 50VAC primary and 18V secondaries, all at 230V.

Now the clarification: If I join the two secondaries together by creating a centretap, (one wire of each secondary being joined together) do I effectively now have a single secondary rated at 36V ?

I think I do as I've now joined each secondary in series and thus the voltages are now additive because of the effective increase in the number of turns of wire across the secondary.

Thanks

#### Nigel Goodwin

##### Super Moderator
Well your post is full of confusing mistakes?, but yes, assuming you have two identical 18V secondaries you can wire then in series to double the voltage, or parallel to double the current - it's designed for that exact purpose.

#### westers

##### New Member
Thanks, but what confusing mistakes have I made?

#### RODALCO

##### Well-Known Member
The transformer rating is expressed in VA.

voltampères

at 36 volts you should be able to draw 1.38 Ampères from it.

#### JimB

##### Super Moderator
Yes to what Nigel said, however there are a few traps for those who dont know.

When connecting transformer windings in series, it is important to get the phasing correct.

To explain:
Connect one end of one secondary winding to one end of the other secondary winding, this should leave two unconnected ends.
Apply power to the primary winding, and measure the voltage between the two unconnected ends, if there is 36volts (in your case) then the phasing is correct.
If there is 0volts, the phasing is wrong.
To correct this, disconnect the joined secondaries and connect the opposite end of one secondary to the original end of the other secondary, re-test and there should be 36volts between ends.

JimB

On edit:

The rating of the transformer should be 50VA not 50VAC.
VA is the product of voltage and current.
So a 50 VA transformer will give 50/36 = 1.38A.
Just what Rodalco said.

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#### westers

##### New Member
Noted on the VA mistake - thanks.

Thanks Jim, you've explained to me the mistake I've been making which prompted the question in the first place !!

I couldn't figure out last night why I was getting a zero volt reading when I was connecting the secondaries. I'd wrongly assumed that it didn't matter which wire was connected to which.

I've just done what you said and bingo, 42V AC measured (the mains is up at about 240V today).

Thanks.

#### Nigel Goodwin

##### Super Moderator
Thanks, but what confusing mistakes have I made?
You said it had a 50VAC primary.

#### MrAl

##### Well-Known Member
Hi,

One extra little caution here...

As JimB made clear, the 'phasing' is important.
When connecting in series you can 'experiment' by connecting two
leads of the two secondaries together to find out what phasing is
correct (as per his post).

When connecting in parallel, you should first connect two leads
together (as when doing the series connection) and measure the
remaining two leads and make sure that there is zero volts present.
When you have zero volts across the remaining two secondary leads
you can be assured that connecting them also together will form a
parallel connection.
The danger is if the first two leads are connected and there is in
fact voltage across the remaining two leads (twice the secondary
voltage) that *if* the remaining two leads were connected together
that would cause a huge current through the secondary which could
actually blow out the transformer, depending on its series resistance.

Thus, when connecting in series first connect two leads together
and then make sure there is twice the secondary voltage present on
the remaining two leads, but when connecting in parallel make sure
that there is zero (0) volts present on the remaining two leads
before connecting those remaining two leads together.

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#### Nigel Goodwin

##### Super Moderator
Reminds me of a 'funny' story from the other year.

My RCD (earth leakage trip) kept triggering, and I isolated it to the immersion heater element in the hot water tank leaking to earth.

So I purchased a suitable spanner and a replacement element, but was unable to undo it, as the tank deformed excessively, and I was worried it was going to split - I checked with a few electrian friends who have done this before, to be told success is probably under 50%, and mine hadn't been undone for many decades.

As my central heating was pretty crap anyway, I decided to have an entire new central heating system, with a combi boiler and no tanks - which is now done and works wonderfully.

In the meantime though we had no hotwater, but I had a crude plan

At work I've got three identical 1000W isolation transformers in metal boxes, and the heater was 3000W, so I borrowed then from work and connected them in parallel - switched on, BIG bang, fuse disappeared and switch contacts welded together.

This is when I found out that the identical transformers were wired differently!, so I had to find out which phase each one was wired in. I did this by connecting all primaries in parallel, and two secondaries in series - feeding a load consisting of two 60W bulbs in series. If the windings were in phase the bulbs would light, if out of phase they wouldn't. By doing this twice I was able to identify the phase of the outputs and wired them accordingly.

This crude bodge worked perfectly for the few weeks required for the plumbers to come and install the new heating, and I took the transformers out the night before they came.

But I can certainly confirm that you get HUGE currents with two 1000W transformers out of phase

#### Diver300

##### Well-Known Member
So I purchased a suitable spanner and a replacement element, but was unable to undo it, as the tank deformed excessively, and I was worried it was going to split - I checked with a few electrian friends who have done this before, to be told success is probably under 50%, and mine hadn't been undone for many decades.
I've fared better than that, with 3 out of 3 successes. I did heed the advice of the electrician who I bought the first replacement from. He said leave the water turned on when loosening the immersion heater. The pressure keeps the tank stronger.

When you have got the immersion heater to turn, water will start to seep out slowly. That is the time to turn the water off and drain what you need to.

I don't know what that has to do with electronics!