# Combining dual secondaries of a toroidal transformer

Discussion in 'Electronic Basics' started by [email protected], Feb 18, 2007.

1. ### Guest

Hi, I have a fairly good sized toroidal transformer that has two pairs
of primaries and two pairs of secondaries. The inputs are both 115V
and the outputs are both 33V, at 7A. I'd like to combine the outputs
in parallel, to give me 33V at 14A.

I have some questions.

1. The label on the tranny says PRI1 = blue/white PRI2 = black/grey
SEC1 = orange/yellow SEC2 = red/green

So...do I assume that to keep the secondaries in phase, that "orange"
should be tied to "red" and "yellow" should be tied to "green" --
since that's the way they are ordered, on the label? (i.e., do I
trust the label to indicate phasing?)

If I shouldn't trust the label, is there any sort of test I can do? I
don't have an oscilloscope, unfortunately.

2. What's the math equation to figure out what my ultimate DC voltage
will be after my 33VAC goes through full-wave rectification and
filtering?

It's been a long time since high school electric shop, as you can

-Chris

2. ### John PopelishGuest

That is what I would try first, assuming that the first
color was the start of each winding, and the second color
was the end of each winding.
All you need is a volt meter. First connect the the assumed
end of the two primaries together (lets sat, white and gray)
but connect 120 volts to only one of them (blue to white).
Then check that the voltage between the remaining open end
(black) to the one you expect to connect it to (blue), (to
parallel the primaries, later) has very little voltage
between them. I would think that this would be less than a
volt. If you measure 240 volts, the two wires you connected
together are a start and an end.

Once you get this low difference voltage condition, it is
safe to connect the second pair wires together to complete
the primary parallel.

Now you can perform the same test on the secondaries.

Say you connect orange and red together, and measure voltage
between yellow and green. If you see very low voltage, it
is safe to connect yellow and green. If you see about 70
volts, you will have to repeat the test the other way.

Even if you get this all right, it is possible that there is
a slight turns miscount on one or more of the windings, and
the transformer will circulate current between windings of
slightly different voltages, even with no load. If this
happens (you can insert an AC amp meter in the loop formed
by the two windings to check) you may have to add a turn or
two to the low winding to get a perfect match. But this is
rare. At least it is easy to do with a toroid.
The peak of a sine wave is the square root of two times the
RMS value, so neglecting diode drops, I would expect the
unloaded DC to be about 33*1.414=47 volts. It may be a bit
higher since the transformer's rated voltage applies at

3. ### Homer J SimpsonGuest

Connect the two secondary commons together. Now check across the other two
leads with an ammeter for difference current. If it's only a few mA you are
probably OK.

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4. ### The PhantomGuest

If he only gets a few mA, then the secondaries are phased properly. But what
if they're *not* phased properly. Whay do you suppose is going to happen when
he connects an ammeter across the other two leads?

5. ### Jon SlaughterGuest

Nothing much should happen if you get them backwards. The MMF is just going
to push the currents in such a way that they oppose each other and you get
no current. It might heat up but if you don't push much through it then you
should be able to test. What you can do is hook up a fairly large load and a
volt meter and then check both phases and see which one gives you the larger
voltage.
33VAC usually means RMS but your filtering will start at the peak so you'll
need to convert this into peak value and that will be your DC(So you'll get
more). Ofcourse it will not be a true DC as there will be ripple but if you
have good filtering then this would be approximately frequency independent.
You can simply use an volt meter to measure if you like. I get about 45VDC
that you'll get (minus a few volts because of the diode drops and stuff).

6. ### John FieldsGuest

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Absolutely, positively, definitely NOT.

In the first place, the secondaries don't have "commons", they only
have starts and ends. Two leads from the secondaries should be
twisted together and one of the primaries energized (with the leads
of the other kept separate and, preferably, taped up to avoid danger
of shock or short) then the voltage across the unconnected
secondaries should be measured with a _voltmeter_. If the reading
on the voltmeter is 66 volts, then the start of one secondary
winding is connected to the end of the other and they're wired in
series. In order to connect them in parallel they should be
disconnected and each connected to the other secondary lead.

Once that's done, the primary leads should be twisted together in
pairs and those pairs connected to the mains while monitoring the
secondary voltage. If the voltage is zero the primaries are wired
out of phase and should be connected in reverse. That should cause
the secondary voltage to rise to 16V.

8. ### JamieGuest

but then again, one could read the wire tags looking for the
H1, H2, H3, H4 and X1, X2 , X3 and X4 leads.

H1+H3, H2+H4 for the primary.

X1+X3, X2+X4 for the secondary.
But i know your aware of this, this is only for other readers.

9. ### Homer J SimpsonGuest

If they are not phased properly he doesn't know what he is doing and
shouldn't be doing this.
The current will be high.

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10. ### Peter BennettGuest

NO!!!

If the secondaries are parallelled incorrectly, he will have a short
circuit, and huge currents will flow through the windings.

As others have said, connect one end of SEC1 to one end of SEC2 (say,
yellow to green), and measure the voltage between the free ends - if
the voltage is near zero, the windings are correctly phased, and the
two free ends may also be connected together. If the voltage between
the free ends is about 66 volts, the phasing is wrong, and one winding
must be reversed - connect yellow and red instead of yellow and green.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter

11. ### Peter BennettGuest

This may work for "electrical" transformers (ie: those intended to be
installed by electricians), but I have rarely seen an "electronics"
transformer (one an electronics tech would install inside equipment)
labelled like that.

The best way, regardless of the transformer marking or colour code, is
to get a datasheet for the transformer - it should indicate which
wires or terminals are the start of the winding, and may even indicate
the corrects connections for serial or parallel operation.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter

12. ### Jon SlaughterGuest

hmm, thats why I said to add a load. If the two secondaries are out of phase
and have exactly the same reactance then there will be no current flow with

If you take the two secondaries and put them in series the same effect will
happen but you will need a load incase they are in phase. (or you could just
measure the voltage, you will either get twice the voltage of one
secondary(assuming they are equal) or 0.)

If he doesn't think they are equal then its still easy to tell. You either
get +-(V1 + V2) or +-(V1 - V2). He can tell quite easily if they are in the
correct phase by using series or not. (because if they are in the same phase
then they can only add there voltages together. Ofcourse this assumes that
one of the secondaries are not to much greater than the other)

If he wants to use test using parallel then he should add a load to both
secondaries and then, say, use a current meter to measure the current. The
same stuff applies above but with current. (or he could measure the voltage
on the resistor).

The fact is that either they will work with each other or against each
other. As always you have to be careful about shorting things out.

13. ### Jon SlaughterGuest

Actually no current will flow if these are perfectly identical. The current
in one of the secondaries will attempt to flow the opposite way and there
will ultimately be no flow because of cancellation. Obviously if one
secondary has more turns than the other it will matter but adding a load to
it will prevent any larger currents from circulating. The magnetic flux will
induce a current in both secondaries equal and opposite direction.
"shorting" the secondaries together will not result in any current flow in

14. ### Michael A. TerrellGuest

A 60 watt light bulb at your line voltage, wired in series with the
primary winding(s) will prevent any damage to the transformer during
testing. If it is wired properly, the lamp will not light. If it
lights at close to full brightness, something is out of phase. It is
referred to as a "Dim bulb" tester, and has been in use on a lot of

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

15. ### Michael A. TerrellGuest

A load across a short circuit? Its back to "Electricity 101" for
you.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

16. ### Michael A. TerrellGuest

You still don't get it, do you?

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

17. ### John PopelishGuest

The lesson would stick a lot better if he took his own
advice and started his own fire.

18. ### jasenGuest

if you're lucky the transformer will survive the short-circuit created
if you get the primaries connected out of phase.

better to measure with a voltmeter, possbily while applying a low voltage AC
signal to the secondary or to use a current-limited supply.

Bye.
Jasen

20. ### Jon SlaughterGuest

ITS NOT A FUCKING SHORT IF THEY ARE OUT OF PHASE IN PARALLEL OR

Jesus christ. What happens when you hook up two identical batteries up in
series out of phase? in phase? (ABSOUTELY NOTHING BECAUSE THERE IS NO
CIRCUIT) What happens when you hook them up in parallel out of phase?
ABSOLUTELY FUCKING NOTHING! (but there is a circuit)

What happens when you hook them up in parallel in phase? THEN YOU GET A
SHORT!!! AND THIS IS WHY I SAID IN MY ORIGINAL POST YOU SHOULD ADD A LOAD.
Ofcourse you fucking neglect that part because you want to start some shit.
I guess your life is so boring that the only way you can have some excitment
is to get on sci.electronics.basics and start arguments by being
illogical(ignoring half the sentence so that the other have then becomes
wrong).

Did you guys actually ever goto school or just pretend? Cause I can't
imagine that your professors would pass you if you only did the parts of the
tests that you wanted.